The GRAVITY+ project consists of instrumental upgrades to the Very Large Telescope Interferometer (VLTI) for faint-science, high-contrast, milliarcsecond interferometric imaging. As an integral part of the GRAVITY+ Adaptive Optics (AO) architecture, the Wavefront Sensor (WFS) subsystem corrects image distortions caused by the turbulence of Earth's atmosphere. We present the opto-mechanical design of the WFS subsystem and the design strategies used to implement two payloads positioned diagonally opposite each other - Natural Guide Star (NGS) and Laser Guide Star (LGS) - within a single compact design structure. We discuss the implementation of relative motions of the two payloads covering their respective patrol fields and a nested motion within the LGS Payload covering the complete Sodium layer profile in the Earth's atmosphere.
In the GRAVITY+ project, GRAVITY is presently undergoing a series of upgrades to enhance its performance, add wide field capability and thereby expand its sky coverage. Some aspects of these improvements have already been implemented and commissioned by the end of 2021, making them accessible to the community. The augmentation of sky coverage involves increasing the maximum angular separation between the celestial science object and the fringe tracking object from the previous 2 arcseconds (limited by the field of view of the VLTI) to 20 – 30 arcseconds (constrained by atmospheric conditions during observation). Phase 1 of GRAVITY+ Wide utilizes the earlier PRIMA Differential Delay Lines to compensate for the optical path length variation between the science and fringe tracking beams throughout an observation. In phase 2, we are upgrading the existing beam compressors (BC) to integrate optical path length difference compensation directly into the BC. This modification eliminates five optical reflections per beam, thereby enhancing the optical throughput of the VLTI–GRAVITY system and the bandwidth of the vibrational control. We will present the implementation of phase 2 and share preliminary results from our testing activities for GRAVITY+ Wide.
We present the Wavefront Sensor units of the Gravity Plus Adaptive Optics (GPAO) system, which will equip all 8m class telescopes of the VLTI and is an instrumental part of the GRAVITY+ project. It includes two modules for each Wavefront Sensor unit: a Natural Guide Star sensor with high-order 40×40 Shack-Hartmann and a Laser Guide Star 30×30 sensor. The state-of-the-art AO correction will considerably improve the performance for interferometry, in particular high-contrast observations for NGS observations and all-sky coverage with LGS, which will be implemented for the first time on VLTI instruments. In the following, we give an overview of the Wavefront Sensor units system after completion of their integration and characterization.
We present latest results and further development of the image reconstruction tool GRAVITY-RESOLVE (G^R), which is specifically designed for Galactic Center observations with the near-infrared phase-referencing VLTI instrument GRAVITY. We show deep images of the Galactic Center in recent years and movies in which the stellar motion around the central massive black hole SgrA* becomes apparent on yet unmatched scales. Moreover, we present a first result of the newly implemented mosaicing method in G^R to jointly reconstruct multiple datasets which have been separately recorded on sky.
The GRAVITY instrument has transformed the field of near-infrared interferometry and is redefining the limits of ground-based observations. In Galactic Center observations, this is shown by routinely achieving below 50 μas uncertainty on astrometric measurements within a 5-minute exposure and detecting stars fainter than 19th magnitude. Nevertheless, systematic effects are still limiting the instrument's performance. In this talk, I will introduce two observing modes to overcome these limitations: Pupil modulation to improve the astrometry and metrology attenuation to overcome SNR limitations. I will detail these two modes and show how significant the improvements are on examples of on-sky data.
Performances of an adaptive optics (AO) system are directly linked with the quality of its alignment. During the instrument calibration, having open loop fast tools with a large capture range are necessary to quickly assess the system misalignment and to drive it towards a state allowing to close the AO loop. During operation, complex systems are prone to misalignments (mechanical flexions, rotation of optical elements, etc.) that potentially degrade the AO performances, creating a need for a monitoring tool to tackle their driftage. In this work, we first present an improved perturbative method to quickly assess large lateral errors in open loop. It uses the spatial correlation of the measured interaction matrix of a limited number of 2D spatial modes with a synthetic model. Then, we introduce a novel solution to finely measure and correct these lateral errors via the closed loop telemetry. Non-perturbative, this method consequently does not impact the science output of the instrument. It is based on the temporal correlation of 2D spatial frequencies in the deformable mirror commands. It is model-free (no need of an interaction matrix model) and sparse in the Fourier space, making it fast and easily scalable to complex systems such as future extremely large telescopes. Finally, we present some results obtained on the development bench of the GRAVITY+ extreme AO system (Cartesian grid, 1432 actuators). In addition, we show with on-sky results gathered with CHARA and GRAVITY/CIAO that the method is adaptable to non-conventional AO geometries (hexagonal grids, 60 actuators).
In the context of the GRAVITY+ upgrade, the adaptive optics (AO) systems of the GRAVITY interferometer are undergoing a major lifting. The current CILAS deformable mirrors (DM, 90 actuators) will be replaced by ALPAO kilo-DMs (43×43, 1432 actuators). On top of the already existing 9×9 Shack-Hartmann wavefront sensors (SH-WFS) for infrared (IR) natural guide star (NGS), new 40×40 SH-WFSs for visible (VIS) NGS will be deployed. Lasers will also be installed on the four units of the Very Large Telescope to provide a laser guide star (LGS) option with 30×30 SH-WFSs and with the choice to either use the 9×9 IR-WFSs or 2×2 VIS-WFSs for low order sensing. Thus, four modes will be available for the GRAVITY+ AO system (GPAO): IR-NGS, IR-LGS, VIS-NGS and VIS-LGS. To prepare the instrument commissioning and help the observers to plan their observations, a tool is needed to predict the performances of the different modes and for different observing conditions (NGS magnitude, science object magnitude, turbulence conditions...) We developed models based on a Maréchal approximation to predict the Strehl ratio of the four GPAO modes in order to feed the already existing tool that simulates the GRAVITY performances. Waiting for commissioning data, our model was validated and calibrated using the TIPTOP toolbox, a Point Spread Function simulator based on the computation of Power Spectrum Densities. In this work, we present our models of the NGS modes of GPAO and their calibration with TIPTOP.
METIS is one of the first-generation instruments currently in development for the Extremely Large Telescope (ELT). As one of the partners in the project consortium, the team at University of Cologne is responsible for the development of the Warm Calibration Unit (WCU) subsystem. This subsystem is designed to deliver a selection of broadband and single wavelength sources for the calibration of the METIS instrument along with pupil/focal plane imaging optics serving as feedback for the internal alignment of the subsystems of METIS during the integration phase foreseen in Leiden. After the satisfactory Final Design Review (FDR) towards the end of 2022, the development of the WCU subsystem is currently in the manufacturing/procurement phase along with the integration of sub-assemblies. We provide an overview of the development of the WCU subsystem at this phase of the project including the progress on the manufacturing of custom components and the details on the compliance with the design principles with a primary focus on the mechanical aspects. These components include as an example: (1) a lightweight optical bench made of carbon-fiber reinforced polymer (CFRP) and with dimensions of roughly 2.5mx3m and a thickness of 20cm, satisfying compliancy both with high loads and tight tolerances (on the level of several tens of microns), (2) custom-designed six adjustable supporting links responsible for carrying the subsystem (roughly 1 tons) and its alignment to METIS focal/pupil planes within desired accuracies 50 microns with the help of laser sensors, (3) kinematic optical Aluminum mounts with a thermal invariant design (i.e. positioning of the optics are not affected by temperature). In the light of the progress, we discuss further the perspectives and the planning towards the full integration and testing of the full subsystem, foreseen to start within the second quarter of 2025.
METIS is a sophisticated mid-infrared imager and spectrograph covering wavelengths from 2.9 to 13.5 μm (astronomical L, M, and N bands), stands as one of the three primary science instruments at the Extremely Large Telescope (ELT). METIS will provide diffraction-limited imaging, coronagraphy, high-resolution integral field spectroscopy, and low/medium-resolution slit spectroscopy. Within the international METIS consortium, the University of Cologne is responsible for designing, manufacturing, integrating, and testing the Warm Calibration Unit (WCU). Among the other calibration functionalities, the WCU also facilitates alignment checks during Assembly, Integration, and Verification (AIT & AIV) through a visible light channel utilizing a CMOS camera, which can capture images of internal focal planes, pupil planes of the WCU, and various METIS sub-systems. This paper details the final optical design, preliminary opto-mechanical design, and analysis of alignment optics in the WCU. A laboratory test setup has been discussed, showcasing the estimation of SNR for the AIV mode of the WCU using an off-the-shelf CMOS camera and interating sphere. This paper also introduces the novel design concept of aluminum spherical mirrors employed in the main optical train of the WCU. We also present a brief overview of alignment verification procedures of the offner relay optics of the WCU sub-system. Furthermore, a detailed overview of the thermal analysis of the whole WCU will be discussed, which ensures the temperature difference between the WCU and the ELT dome will be within specified limits.
METIS, the mid-infrared imager and spectrograph for the wavelength range 2.9 -13.5 µm (astronomical L-, MN- band), will be one of the three science instruments at the Extremely Large Telescope (ELT). It will provide diffraction-limited imaging, coronagraphy, high-resolution integral field spectroscopy, and low and medium resolution slit spectroscopy. Within the international METIS consortium, the University of Cologne is responsible for the design, manufacturing, integration, and qualification of the Warm Calibration Unit (WCU) of the instrument. In this contribution, we present the current status of the optical design and principle of optical operation of the WCU. The main train of the WCU optics is based on a modified F/17.75 Offner relay, with the optical output parameters matching the plate scale, F-number, as well as the exit-pupil position and size of that of the ELT. We discuss the optical design, and tolerance analysis of the WCU relay optics as part of the Optics FDR review by ESO. In addition, we present the concept and design of the Invar mechanical mounts for the WCU Zerodur mirrors, which are expected to undergo thermal and mechanical stresses. Finally, we present the optical design and analysis of the visible channel of the WCU that is aimed at alignment verification, as well as visualization of the METIS focal and pupil planes.
The warm calibration unit (WCU) is one of the subsystems of the future METIS instrument on the Extremely Large Telescope (ELT). Operating at room temperature, the WCU is mounted above the main cryostat of METIS. It will be employed as a calibration reference for science observations, as well as for verification and alignment purposes during the AIT phase. The WCU is designed and constructed at the University of Cologne, one of the partner in the METIS consortium. WCU recently went through a successful Optics Long Lead Items Review by ESO. Now, the WCU is entering the last phase of the project, the Final Design Review (FDR). In this paper, we present the current status of the WCU design and summarize the mechanical and system engineering work. We describe the design of the hexapod formed by six manually adjustable links and its interfaces with the METIS cryostat together with the CFRP-based optical bench and Invar-based optical mounts. Lab prototyping results of one actuator under a nominal load of 5 kN confirms the achievable high linear resolution (20 µm). We present the status of the WCU laser cabinet. We discuss the lastest progress in the laboratory testing of some WCU functionalities, such as the fibre-fed monochromatic sources for the spectral calibration of the LM-Spectrograph of METIS, and the spatial calibration sources using the integrating sphere. We detail the activities foreseen until FDR together with the preparation of the sub-system MAIT work.
We present the testbench aimed at integrating the GRAVITY+ adaptive optics GPAO. It consists of two independent elements, one reproducing the Coudé focus of the telescope, including the telescope deformable mirror mount (with its surface facing down), and one reproducing the Coudé room opto-mechanical environment, including a downwards-propagating beam, and the telescope mechanical interfaces in order to fit in the new GPAO wavefront sensor. We discuss in this paper the design of this bench and the solutions we adopted to keep the cost low, keep the design compact (allowing it to be fully contained in a 20 sqm clean room), and align the bench independently from the adaptive optics. We also discuss the features we have set in this bench.
As part of the GRAVITY+ project, the near-infrared beam combiner GRAVITY and the VLTI are currently undergoing a series of significant upgrades to further improve the performance and sky coverage. The instrumental changes will be transformational, and for instance uniquely position GRAVITY to observe the broad line region of hundreds of Active Galactic Nuclei (AGN) at a redshift of two and higher. The increased sky coverage is achieved by enlarging the maximum angular separation between the celestial science object (SC) and the off-axis fringe tracking (FT) star from currently 2 arcseconds (arcsec) up to unprecedented 30 arcsec, limited by the atmospheric conditions. This was successfully demonstrated at the VLTI for the first time.
With the upgrade from GRAVITY to GRAVITY+ the instrument will evolve to an all-sky interferometer that can observe faint targets, such as high redshift AGN. Observing the faintest targets requires reducing the noise sources in GRAVITY as much as possible. The dominant noise source, especially in the blue part of the spectrum, is the backscattering of the metrology laser light onto the detector. To reduce this noise we introduce two new metrology modes. With a combination of small hardware changes and software adaptations, we can dim the metrology laser during the observation without losing the phase referencing. For single beam targets, we can even turn off the metrology laser for the maximum SNR on the detector. These changes lead to a SNR improvement of over a factor of two averaged over the whole spectrum and up to a factor of eight in the part of the spectrum currently dominated by laser noise.
During the past years, the VLTI-instrument GRAVITY has made spectacular discoveries with phase-referenced interferometric imaging with milliarcsecond resolution and ten microarcsecond astrometry. Here, we report on the upgrade of the GRAVITY science spectrometer with two new grisms in October 2019, increasing the instrument throughput by a factor > 2. This improvement was made possible by using a high refractive index Germanium substrate, which reduces the grism and groove angles, and by successfully applying an anti-reflection coating to the ruled surface to overcome Fresnel losses. We present the design, manufacturing, and laboratory testing of the new grisms, as well as the results from the re-commissioning on sky.
Combining adaptive optics and interferometric observations results in a considerable contrast gain compared to single-telescope, extreme AO systems. Taking advantage of this, the ExoGRAVITY project is a survey of known young giant exoplanets located in the range of 0.1” to 2” from their stars. The observations provide astrometric data of unprecedented accuracy, being crucial for refining the orbital parameters of planets and illuminating their dynamical histories. Furthermore, GRAVITY will measure non-Keplerian perturbations due to planet-planet interactions in multi-planet systems and measure dynamical masses. Over time, repetitive observations of the exoplanets at medium resolution (R = 500) will provide a catalogue of K-band spectra of unprecedented quality, for a number of exoplanets. The K-band has the unique properties that it contains many molecular signatures (CO, H2O, CH4, CO2). This allows constraining precisely surface gravity, metallicity, and temperature, if used in conjunction with self-consistent models like Exo-REM. Further, we will use the parameter-retrieval algorithm petitRADTRANS to constrain the C/O ratio of the planets. Ultimately, we plan to produce the first C/O survey of exoplanets, kick-starting the difficult process of linking planetary formation with measured atomic abundances.
The GRAVITY instrument has revolutionized optical/IR interferometry: fringe-tracking and phase-referencing allow for 30 micro-arcsecond astrometry in a dual beam mode, and for spectro-differential astrometry better than 10 micro-arcseconds. The control of systematic effects is essential to fully exploit this technological advancement. Among those systematics are static phase aberrations, introduced along the instrument's optical path, which in particular affect the inferred separation of two unresolved objects within the same FOV. Here, we present how the aberrations can be measured, characterized by low-order Zernike polynomials and, most importantly, how their impact on the astrometry is corrected. The resulting astrometry corrections are verified with calibration observations of a binary before we discuss how they affect GRAVITY's measurement of the galactic center distance.
We present the successful demonstration of world's first large-separation ~30" off-axis fringe tracking with four telescopes in October 2019. With this technique we increase the sky-coverage for optical interferometry by orders of magnitude compared to current technology. Following the early work at the Palomar Testbed Interferometer, the first demonstration of off-axis fringe tracking at the Keck Interferometer and with PRIMA at the ESO Very Large Telescope Interferometer, and the breakthrough with the GRAVITY Galactic Center observations, we enhanced the VLTI infrastructure for GRAVITY to take advantage of the PRIMA Star separators and Differential Delay Lines for off-axis fringe tracking. In our presentation we give an introduction to the subject, present the enhancements of the VLTI, and present our results from the first on-sky operation in October 2019, with observations of the Orion Trapezium Cluster, a field brown dwarf, and a high redshift quasar.
METIS, the mid-infrared imager and spectrograph for the wavelength range 2.9-14 µm (astronomical L-, M- and N bands), will be equipped with a calibration unit, developed at the University of Cologne, which task is to deliver simulated sources for the test and calibration of the main imaging and spectral functionalities of METIS. Our subsystem, as the full METIS instrument, is currently in the Phase C of the project, which leads to the Final Design Review expected by the end of 2021. In this contribution, we first briefly introduce the general concepts chosen for the Warm Calibration Unit (WCU) and then detail the laboratory work that is undertaken in Cologne to validate most of the concepts presented at the Preliminary Design Review. A core unit of the WCU is the integrating sphere combined with the black body, which is the hub delivering the calibration functionalities. We first report the measured spatial uniformity of the output port of the integrating sphere when fed with the black body source radiation. The measurement made using our uncooled thermal camera, evidences a spatial uniformity below 1% RMS. Longer integration times will further improve the final accuracy on this important parameter. We also take a closer look at the black body source and report on its flux temporal stability, which is found to be better than 1% over a 2h duration. We characterize time windows for different settings of the main WCU light source, which is the black body and stability and repeatability of the detected signal. Through different experiments we investigated the best options to manufacture the aperture mask that will be used to generate artificial point sources.
Instrumental polarization can have large effects on measurements with the VLTI, as it can alter measured polarization and introduce uncertainties. To understand these effects we measured and simulated the instrumental polarization of the VLTI and of GRAVITY. We are able to provide a calibration model for GRAVITY observations and quantify systematic uncertainties due to instrumental polarization. This work has shown to be crucial to measure the polarization of the galactic center black hole Sgr A* where we detect a swing in the polarization angle during flare events. While the analysis was done for GRAVITY, it also gives an important basis for the design of future near-infrared instruments at the VLTI.
The warm calibration unit (WCU) is one subsystem of the future METIS instrument on the European Extremely Large Telescope (E-ELT). Operating at daytime temperature, the WCU is mounted above the main cryostat of METIS and will be employed as calibration reference for science observations, as well as for verification and alignment purposes during the AIT phase. The WCU is designed and constructed at the University of Cologne, partner in the METIS consortium. The WCU, together with the full METIS instrument, went recently through a successful preliminary design review (PDR) phase at ESO and is entering now the Phase C of the project. In this paper, we present the current status of the WCU and summarize the mostly mechanical and optical engineering work. We adopted a hexapod unit to interface with the METIS cryostat and a CFRP-based optical bench to optimally cope with alignment flexure. We develop the case for fiber-fed laser sources feeding the integrating sphere for spectral calibration of the LM-Spectrograph of METIS. We detail the activity foreseen for Phase C including the optical tolerances analysis, the eigenfrequency and earthquake analysis and a preparation of the sub-system MAIT work, finishing the paper with a short overview of the WCU future plans.
We present the preliminary design of the calibration unit of the future E-ELT instrument METIS. This independent subunit is mounted externally to the main cryostat of METIS and will function both as calibration reference for science observations, as well as verification and alignment tool during the AIT phase. In this paper, we focus on describing its preliminary layout and foreseen functionalities, based on the performance requirements defined at system level and the constraints imposed by warm IR background. We discuss the advantage of employing an integrating sphere as common radiation emitter, leading to a novel and versatile design, where the source’s spatio-spectral properties can be varied with high fidelity and repeatability. By combining only few tuneable sources and mechanisms we show how a large instrument such as METIS can be calibrated and tested, without the need of a complex cold calibration unit.
We present the preliminary optical design of METIS, the Mid-infrared E-ELT Imager and Spectrograph, and study the end-to-end performance regarding wavefront errors and non-common path aberrations. We discuss the results of the Monte Carlo simulations that contain the manufacturing and alignment errors of the opto-mechanical system. We elaborate on the wavefront error budget of the instrument detailing all contributors. We investigate the mid and high spatial frequency errors of the optical surfaces, which we model using simulated surface height errors maps of one dimensional Power Spectral Density (PSD) functions.
The new VLTI/GRAVITY instrument is a four telescope beam combiner installed at the VLT Interferometer. The principal novelty of this instrument is the availability of a dual field mode enabling narrow-angle relative astrometry at micro-arcsecond accuracy between two objects separated by several arcseconds. The fringe tracker (FT) stabilizes the interference fringes at up to 1 kHz frequency, allowing for long exposures with the science combiner (SC) as well as phase referenced imaging and differential astrometry (in dual field mode). The FT and SC beam combiners are integrated optics (IO) components, whose 24 outputs are (optionally) polarization-split and spectrally dispersed.
The processing of the photometric signals from the IO components is based on the pixel-to-visibility matrix (P2VM) formalism, that translates them into complex visibilities. The retrieval of the relative phase of the two objects subsequently relies on the combination of the phases measured from the FT, SC and the laser metrology. We will present the adopted algorithms, and an overview of the structure of the developed software. The calibration of the wavelength scales of the FT and SC at the required accuracy presents specific difficulties that we will briefly discuss. Examples of the reduction of on-sky data obtained during the commissioning will also be presented.
METIS, a mid-infrared imager and spectrograph for the wavelength range 2.9–19μm (astronomical L-, M-, N- and Q-band), will be one of the first three science instruments at the European Extremely Large Telescope (E-ELT). It will provide diffraction limited imaging, coronagraphy, high resolution integral field spectroscopy and low and medium resolution slit spectroscopy. Within the international METIS consortium, the 1st Institute of Physics of the University of Cologne in Germany is responsible for the design, manufacturing, integration and qualification of the Warm Calibration Unit (WCU) of the instrument. The WCU will be a self-contained unit operating at ambient temperature outside of the voluminous METIS dewar, feeding a variety of optical calibration and alignment signals into the optical path of METIS. The functionalities of the WCU will be used for routine daily daytime calibrations after astronomical observing nights and verification of the internal alignment of METIS during assembly, integration and verification (AIV). In this contribution we present the preliminary optical design and principle of operation of the WCU in its current state of the preliminary design phase of METIS.
Since its first light at the Very Large Telescope Interferometer (VLTI), GRAVITY has reached new regimes in optical interferometry, in terms of accuracy as well as sensitivity.1 GRAVITY is routinely doing phase referenced interferometry of objects fainter than K > 17 mag, which makes for example the galactic center black hole Sagittarius A*2 detectable 90 % of the times. However from SNR calculations we are confident that even a sensitivity limit of K ~ 19 mag is possible. We therefore try to push the limits of GRAVITY by improving the observations as well as the calibration and the data reduction. This has further improved the sensitivity limit to K > 18 mag in the beginning of this year. Here we present some work we are currently doing in order to reach the best possible sensitivity.
The GRAVITY instrument installed at VLTI uses differential fibered delay lines to spatially filter the incoming wavefronts and accurately control the optical path difference between the Fringe Tracker (FT) and Scientific Detector (SC) parts of the instrument. On top of the differential dispersion occurring in the air, the chromatic dispersion introduced by these fibers impacts the real time performances of the fringe tracker by generating a second-order chromatic phase shift. Moreover, differential dispersion also affects GRAVITY dual-feed measurements that require a length adjustment of both FT and SC fibers. In this contribution, we show how chromatic dispersion can be corrected both in the fringe tracker real-time control as well as in the astrometric data reduction.
The use of optical fibers in astronomical instrumentation has been becoming more and more common. High transmission, polarization control, compact and easy routing are just a few of the advantages in this respect. But fibers also bring new challenges for the development of systems. During the assembly of the VLTI beam combiner GRAVITY different side effects of the fiber implementation had to be taken into account. In this work we summarize the corresponding phenomena ranging from the external factors influencing the fiber performance, like mechanical and temperature effects, to inelastic scattering within the fiber material.
After the first year of observations with the GRAVITY fringe tracker, we compute correlations between the optical path residuals and atmospheric and astronomical parameters. The median residuals of the optical path residuals are 180nm on the ATs and 270nm on the UTs. The residuals are uncorrelated with the target magnitudes for Kmag below 5.5 on ATs (9 on UTs). The correlation with the coherence time is however extremely clear, with a drop-off in fringe tracking performance below 3 ms.
GRAVITY acquisition camera implements four optical functions to track multiple beams of Very Large Telescope Interferometer (VLTI): a) pupil tracker: a 2×2 lenslet images four pupil reference lasers mounted on the spiders of telescope secondary mirror; b) field tracker: images science object; c) pupil imager: reimages telescope pupil; d) aberration tracker: images a Shack-Hartmann. The estimation of beam stabilization parameters from the acquisition camera detector image is carried out, for every 0.7 s, with a dedicated data reduction software. The measured parameters are used in: a) alignment of GRAVITY with the VLTI; b) active pupil and field stabilization; c) defocus correction and engineering purposes. The instrument is now successfully operational on-sky in closed loop. The relevant data reduction and on-sky characterization results are reported.
The VLTI instrument GRAVITY combines the beams from four telescopes and provides phase-referenced imaging as well as precision-astrometry of order 10 μas by observing two celestial objects in dual-field mode. Their angular separation can be determined from their differential OPD (dOPD) when the internal dOPDs in the interferometer are known. Here, we present the general overview of the novel metrology system which performs these measurements. The metrology consists of a three-beam laser system and a homodyne detection scheme for three-beam interference using phase-shifting interferometry in combination with lock-in amplifiers. Via this approach the metrology system measures dOPDs on a nanometer-level.
GRAVITY is a near-infrared interferometric instrument that allows astronomers to combine the light of the four unit or four auxiliary telescopes of the ESO Very Large Telescope in Paranal, Chile. GRAVITY will deliver extremely precise relative astrometry and spatially resolved spectra. In order to study objects in regions of high extinction (e.g. the Galactic Center, or star forming regions), GRAVITY will use infrared wavefront sensors. The suite of four wavefront sensors located in the Coudé room of each of the unit telescopes are known as the Coudé Integrated Adaptive Optics (CIAO). The CIAO wavefront sensors are being constructed by the Max Planck Institute for Astronomy (MPIA) and are being installed and commissioned at Paranal between February and September of 2016. This presentation will focus on system tests performed in the MPIA adaptive optics laboratory in Heidelberg, Germany in preparation for shipment to Paranal, as well as on-sky data from the commissioning of the first instrument. We will discuss the CIAO instruments, control strategy, optimizations, and performance at the telescope.
GRAVITY, a second generation instrument for the Very Large Telescope Interferometer (VLTI), will provide an astrometric precision of order 10 micro-arcseconds, an imaging resolution of 4 milli-arcseconds, and low/medium resolution spectro-interferometry. These improvements to the VLTI represent a major upgrade to its current infrared interferometric capabilities, allowing detailed study of obscured environments (e.g. the Galactic Center, young dusty planet-forming disks, dense stellar cores, AGN, etc...). Crucial to the final performance of GRAVITY, the Coudé IR Adaptive Optics (CIAO) system will correct for the effects of the atmosphere at each of the VLT Unit Telescopes. CIAO consists of four new infrared Shack-Hartmann wavefront sensors (WFS) and associated real-time computers/software which will provide infrared wavefront sensing from 1.45-2.45 microns, allowing AO corrections even in regions where optically bright reference sources are scarce. We present here the latest progress on the GRAVITY wavefront sensors. We describe the adaptation and testing of a light-weight version of the ESO Standard Platform for Adaptive optics Real Time Applications (SPARTA-Light) software architecture to the needs of GRAVITY. We also describe the latest integration and test milestones for construction of the initial wave front sensor.
The work package of the University of Cologne within the GRAVITY consortium included the development and
manufacturing of two spectrometers for the beam combiner instrument. Both spectrometers are optimized for
different tasks. The science spectrometer provides 3 different spectral resolutions. In the highest resolution the
length of the spectral lines is close to the borders of the imaging area of the detector. Also the integration time
of these high resolution images is relative long. Therefor the optical pathes have to be controlled by the feedback
of a faster spectrometer. The fringe tracking spectrometer has only one low resolution to allow much shorter
integration times. This spectrometer provides a feedback for the control loops which stabilize the optical pathes
of the light from the telescope to the instrument. This is a new key feature of the whole GRAVITY instrument.
Based on the optical layout my work was the design of the mechanical structure, mountings, passive and
active adjustment mechanisms. This paper gives a short review about the active mechanisms and the compliant
lens mounts. They are used similarly in both spectrometers. Due to the observation and analysis of near-infrared
light the mechanisms have to run at cryogenic temperatures and in a high vacuum. Except the linear stages, the
motorized mechanisms will get used for several times per observation.
We focus on the main algorithms of the data reduction software for the second generation VLTI instrument GRAVITY. From the interferometric data and the metrology signal, the pipeline recovers the complex visibility of the science target with an absolute phase with respect to the fringe tracker target. Visibilities are then calibrated and the relative astrometry is eventually computed when possible.
We present in this paper the design and characterisation of a new sub-system of the VLTI 2nd generation instrument GRAVITY: the Calibration Unit. The Calibration Unit provides all functions to test and calibrate the beam combiner instrument: it creates two artificial stars on four beams, and dispose of four delay lines with an internal metrology. It also includes artificial stars for the tip-tilt and pupil guiding systems, as well as four metrology pick-up diodes, for tests and calibration of the corresponding sub-systems. The calibration unit also hosts the reference targets to align GRAVITY to the VLTI, and the safety shutters to avoid the metrology light to propagate in the VLTI-lab. We present the results of the characterisation and validtion of these differrent sub-units.
The VLTI instrument GRAVITY will provide very powerful astrometry by combining the light from four tele- scopes for two objects simultaneously. It will measure the angular separation between the two astronomical objects to a precision of 10 μas. This corresponds to a differential optical path difference (dOPD) between the targets of few nanometers and the paths within the interferometer have to be maintained stable to that level. For this purpose, the novel metrology system of GRAVITY will monitor the internal dOPDs by means of phase- shifting interferometry. We present the four-step phase-shifting concept of the metrology with emphasis on the method used for calibrating the phase shifts. The latter is based on a phase-step insensitive algorithm which unambiguously extracts phases in contrast to other methods that are strongly limited by non-linearities of the phase-shifting device. The main constraint of this algorithm is to introduce a robust ellipse fitting routine. Via this approach we are able to measure phase shifts in the laboratory with a typical accuracy of λ=2000 or 1 nm of the metrology wavelength.
The GRAVITY Acquisition Camera was designed to monitor and evaluate the optical beam properties of the four ESO/VLT telescopes simultaneously. The data is used as part of the GRAVITY beam stabilization strategy. Internally the Acquisition Camera has four channels each with: several relay mirrors, imaging lens, H-band filter, a single custom made silica bulk optics (i.e. Beam Analyzer) and an IR detector (HAWAII2-RG). The camera operates in vacuum with operational temperature of: 240k for the folding optics and enclosure, 100K for the Beam Analyzer optics and 80K for the detector. The beam analysis is carried out by the Beam Analyzer, which is a compact assembly of fused silica prisms and lenses that are glued together into a single optical block. The beam analyzer handles the four telescope beams and splits the light from the field mode into the pupil imager, the aberration sensor and the pupil tracker modes. The complex optical alignment and focusing was carried out first at room temperature with visible light, using an optical theodolite/alignment telescope, cross hairs, beam splitter mirrors and optical path compensator. The alignment was validated at cryogenic temperatures. High Strehl ratios were achieved at the first cooldown. In the paper we present the Acquisition Camera as manufactured, focusing key sub-systems and key technical challenges, the room temperature (with visible light) alignment and first IR images acquired in cryogenic operation.
Operating on 6 interferometric baselines, i.e. using all 4 unit telescopes (UTs) of the Very Large Telescope Interferometer (VLTI) simultaneously, the 2nd generation VLTI instrument GRAVITY will deliver narrow-angle astrometry with 10μas accuracy at the infrared K-band. At this angular resolution, GRAVITY will e.g. be able to detect the positional shift of the photo-center of a flare at the Galactic Center within its orbital timescale of about 20 minutes, using the observed motion of the flares as dynamical probes of the gravitational field around the supermassive black hole Sgr A*. Within the international GRAVITY consortium, the 1. Physikalische Institut of the University of Cologne is responsible for the development and construction of the two spectrometers of the camera system: one for the science object, and one for the fringe tracking object, both being operated in cryo-vacuum conditions. In this contribution we describe the basic functionality of the two units and present the final optical design of the two spectrometers as it got realised successfully until end of 2013 with minor changes to the Final Design Review (FDR) of October 2011. In addition we present some of the first light images of the two spectrometers, taken at the laboratory of the Cologne institute between Dec. 2012 and Oct. 2013 respectively. By the end of 2013 both spectrometers got transferred to the PI institute of GRAVITY, the Max-Planck-Institute for Extraterrestrial Physics, where at the time of writing they are undergoing system-level testing in combination with the other sub-systems of GRAVITY.
We present the installed and fully operational beam stabilization and fiber injection subsystem feeding the 2nd generation VLTI instrument GRAVITY. The interferometer GRAVITY requires an unprecedented stability of the VLTI optical train to achieve micro-arcsecond astrometry. For this purpose, GRAVITY contains four fiber coupler units, one per telescope. Each unit is equipped with actuators to stabilize the telescope beam in terms of tilt and lateral pupil displacement, to rotate the field, to adjust the polarization and to compensate atmospheric piston. A special roof-prism offers the possibility of on-axis as well as off-axis fringe tracking without changing the optical train. We describe the assembly, integration and alignment and the resulting optical quality and performance of the individual units. Finally, we present the closed-loop performance of the tip-tilt and pupil tracking achieved with the final systems in the lab.
GRAVITY is a 2nd generation VLTI Instrument o which operates on 6 interferometric baselines by using all 4 Unit Telescopes. It will deliver narrow angle astrometry with 10μas accuracy at the infrared K-band. At the 1. Physikalische Institut of the University of Cologne, which is part of the international GRAVITY consortium, two spectrometers, one for the sciene object, and one for the fringe tracking object, have been designed, manufactured and tested. These spectrometers are two individual devices, each with own housing and interfaces. For a minimized thermal background, the spectrometers are actively cooled down to an operating temperature of 80K in the ambient temperature environment of the Beam Combiner Instrument (BCI) cryostat. The outer casings are mounted thermal isolated to the base plate by glass fiber reinforced plastic (GRP) stands, copper cooling structures conduct the cold inside the spectrometers where it is routed to components via Cu cooling stripes. The spectrometers are covered with shells made of multi insulation foil. There will be shown and compared 3 cooling installations: setups in the Cologne test dewar, in the BCI dewar and in a mock-up cad model. There are some striking differences between the setup in the 2 different dewars. In the Cologne Test dewar the spectrometers are connected to the coldplate (80K); a Cu cooling structure and the thermal isolating GRP stands are bolted to the coldplate. In the BCI dewer Cu cooling structure is connected to the bottom of the nitrogen tank (80K), the GRP stands are bolted to the base plate (240K). The period of time during the cooldown process will be analyzed.
GRAVITY is the second generation VLT Interferometer (VLTI) instrument for high-precision narrow-angle astrometry and phase-referenced interferometric imaging. The laser metrology system of GRAVITY is at the heart of its astrometric mode, which must measure the distance of 2 stars with a precision of 10 micro-arcseconds. This means the metrology has to measure the optical path difference between the two beam combiners of GRAVITY to a level of 5 nm. The metrology design presents some non-common paths that have consequently to be stable at a level of 1 nm. Otherwise they would impact the performance of GRAVITY. The various tests we made in the past on the prototype give us hints on the components responsible for this error, and on their respective contribution to the total error. It is however difficult to assess their exact origin from only OPD measurements, and therefore, to propose a solution to this problem. In this paper, we present the results of a semi-empirical modeling of the fibered metrology system, relying on theoretical basis, as well as on characterisations of key components. The modeling of the metrology system regarding various effects, e.g., temperature, waveguide heating or mechanical stress, will help us to understand how the metrology behave. The goals of this modeling are to 1) model the test set-ups and reproduce the measurements (as a validation of the modeling), 2) determine the origin of the non-common path errors, and 3) propose modifications to the current metrology design to reach the required 1nm stability.
The laser metrology system in the GRAVITY instrument plays a crucial role in an attempt at high-precision narrow-angle astrometry. With a design goal of achieving 10 microarcseconds precision in astrometry, the system must measure the optical path difference between two beam combiners within GRAVITY to an accuracy of better than 5nm. However in its current design, some parts of the optical paths of the metrology system are not common to the optical paths of starlight (the science path) which it must measure with high accuracy. This state of the design is true for most but not all the baselines which will be used by the GRAVITY instrument. The additional non-common optical paths could produce inaccurate path length measurements and consequently inaccurate measurements of the differential phase between fringe packets of two nearby celestial objects, which is the main astrometric observable of the instrument. With reference to the stability and the sensitivity of the non-common paths, this paper describes the impact of a biased differential phase measurement on the narrowangle astrometry and the image reconstruction performance of the GRAVITY instrument. Several alternative designs are also discussed.
The LINC-NIRVANA Fringe and Flexure Tracking System has nearly completed assembly in the lab in Cologne, and will soon be ready for shipment and integration into the full LINC-NIRVANA system at MPIA Heidelberg. This paper provides an overview of the final assembly and testing phase in Cologne, concentrating on those aspects that directly affect instrument performance, including the detector performance and stability of the detector positioning system.
KEYWORDS: Sensors, Process control, Control systems, Cameras, Telescopes, Signal detection, Data acquisition, Computing systems, Interferometers, Interfaces
GRAVITY is the four-beam, near-infrared, AO-assisted, fringe tracking, astrometric and imaging instrument for the Very Large Telescope Interferometer (VLTI). It is requiring the development of one of the most complex instrument software systems ever built for an ESO instrument. Apart from its many interfaces and interdependencies, one of the most challenging aspects is the overall performance and stability of this complex system. The three infrared detectors and the fast reflective memory network (RMN) recorder contribute a total data rate of up to 20 MiB/s accumulating to a maximum of 250 GiB of data per night. The detectors, the two instrument Local Control Units (LCUs) as well as the five LCUs running applications under TAC (Tools for Advanced Control) architecture, are interconnected with fast Ethernet, RMN fibers and dedicated fiber connections as well as signals for the time synchronization. Here we give a simplified overview of all subsystems of GRAVITY and their interfaces and discuss two examples of high-level applications during observations: the acquisition procedure and the gathering and merging of data to the final FITS file.
KEYWORDS: Sensors, Lamps, Camera shutters, Control systems, Analog electronics, Metrology, Detection and tracking algorithms, Optical fibers, Fiber lasers, Laser metrology
The GRAVITY Instrument Software (INS) is based on the common VLT Software Environment. In addition to the basic Instrument Control Software (ICS) which handles Motors, Shutters, Lamps, etc., it also includes three detector subsystems, several special devices, field bus devices, and various real time algorithms. The latter are implemented using ESO TAC (Tools for Advanced Control) and run at a frequency of up to 4 kHz. In total, the instrument has more than 100 ICS devices and runs on five workstations and seven vxWorks LCUs.
Gravity is one of the second-generation instruments of the Very Large Telescope Interferometer that operates in the near infrared range and that is designed for precision narrow-angle astrometry and interferometric imaging. With its infrared wavefront sensors, pupil stabilization, fringe tracker, and metrology, the instrument is tailored to provide a high sensitivity, imaging with 4-millisecond resolution, and astrometry with a 10μarcsec precision. It will probe physics close to the event horizon of the Galactic Centre black hole, and allow to study mass accretion and jets in young stellar objects and active galactic nuclei, planet formation in circumstellar discs, or detect and measure the masses of black holes in massive star clusters throughout the Milky Way. As the instrument required an outstanding level of precision and stability, integrated optics has been chosen to collect and combine the four VLTI beams in the K band. A dedicated integrated optics chip glued to a fiber array has been developed. Technology breakthroughs have been mandatory to fulfill all the specifications. This paper is focused on the interferometric beam combination system of Gravity. Once the combiner concept described, the paper details the developments that have been led, the integration and the performance of the assemblies.
GRAVITY is a new generation beam combination instrument for the VLTI. Its goal is to achieve microarsecond astrometric accuracy between objects separated by a few arcsec. This 106 accuracy on astrometric measurements is the most important challenge of the instrument, and careful error budget have been paramount during the technical design of the instrument. In this poster, we will focus on baselines induced errors, which is part of a larger error budget.
Differential chromatic dispersion in single-mode optical fibres leads to a loss of contrast of the white light fringe. For the GRAVITY instrument, this aspect is critical since it limits the fringe tracking performance. We present a real-time algorithm that compensates for differential dispersion due to varying fibre lengths using prior calibration of the optical fibres. This correction is limited by the accuracy to which the fibres stretch is known. We show how this affects the SNR on the white light fringe for different scenarios and we estimate how this phenomenon might eventually impact the astrometric accuracy of GRAVITY observations.
We use a numerical model of the birefringence in the VLT Interferometer (VLTI) and the Gravity instrument to study the astrometric phase errors that arise when two conditions are simultaneously present: differential birefringence between two VLTI arms, and different polarizations of the science and fringe tracker sources. We present measurements of the VLTI birefringence, that are used to validate our model. We show how a suitable alignment of the eigenvectors of the optical train eliminates the phase error.
The acquisition camera for the GRAVITY/VLTI instrument implements four functions: a) field imager: science field imaging, tip-tilt; b) pupil tracker: telescope pupil lateral and longitudinal positions; c) pupil imager: telescope pupil imaging and d) aberration sensor: The VLTI beam higher order aberrations measurement. We present the dedicated algorithms that simulate the GRAVITY acquisition camera detector measurements considering the realistic imaging conditions, complemented by the pipeline used to extract the data. The data reduction procedure was tested with real aberrations at the VLTI lab and reconstructed back accurately. The acquisition camera software undertakes the measurements simultaneously for all four AT/UTs in 1 s. The measured parameters are updated in the instrument online database. The data reduction software uses the ESO Common Library for Image Processing (CLIP), integrated in to the ESO VLT software environment.
GRAVITY1 is a 2nd generation Very Large Telescope Interferometer (VLTI) operated in the astronomical K-band. In the Beam Combiner Instrument2 (BCI) four Fiber Couplers3 (FC) will feed the light coming from each telescope into two fibers, a reference channel for the fringe tracking spectrometer4 (FT) and a science channel for the science spectrometer4 (SC). The differential Optical Path Difference (dOPD) between the two channels will be corrected using a novel metrology concept.5 The metrology laser will keep control of the dOPD of the two channels. It is injected into the spectrometers and detected at the telescope level. Piezo-actuated fiber stretchers correct the dOPD accordingly. Fiber-fed Integrated Optics6 (IO) combine coherently the light of all six baselines and feed both spectrometers. Assisted by Infrared Wavefront Sensors7 (IWS) at each Unit Telescope (UT) and correcting the path difference between the channels with an accuracy of up to 5 nm, GRAVITY will push the limits of astrometrical accuracy to the order of 10 μas and provide phase-referenced interferometric imaging with a resolution of 4 mas. The University of Cologne developed, constructed and tested both spectrometers of the camera system. Both units are designed for the near infrared (1.95 - 2.45 μm) and are operated in a cryogenic environment. The Fringe Tracker is optimized for highest transmission with fixed spectral resolution (R = 22) realized by a double-prism.8 The Science spectrometer is more diverse and allows to choose from three different spectral resolutions8 (R = [22, 500, 4000]), where the lowest resolution is achieved with a prism and the higher resolutions are realized with grisms. A Wollaston prism in each spectrometer allows for polarimetric splitting of the light. The goal for the spectrometers is to concentrate at least 90% of the ux in 2 × 2 pixel (36 × 36 μm2) for the Science channel and in 1 pixel (24 × 24 μm) in the Fringe Tracking channel. In Section 1, we present the arrangement, direction of spectral dispersion and shift of polarization channels for both spectrometers, and the curvature of the spectra in the science spectrometer. In Section 2 we determine the best focus position of the detectors. The overall contrast of images at different positions of the detector stage is computed with the standard deviation of pixel values in the spectra containing region. In Section 3 we analyze high dynamic range images for each spectrometer and resolution obtained at the afore determined best focus positions. We deduce the ensquared energy from the FWHM of Gaussian fits perpendicular to the spectra.
The GRAVITY acquisition camera has four 9x9 Shack-Hartmann sensors operating in the near-infrared. It measures the slow variations of a quasi-distorted wavefront of four telescope beams simultaneously, by imaging the Galactic Center field. The Shack-Hartmann lenslet images of the Galactic Center are generated. Since the lenslet array images are filled with the crowded Galactic Center stellar field, an extended object, the local shifts of the distorted wavefront have to be estimated with a correlation algorithm. In this paper we report on the accuracy of six existing centroid algorithms for the Galactic Center stellar field. We show the VLTI tunnel atmospheric turbulence phases are reconstructed back with a precision of 100 nm at 2 s integration.
LINC-NIRVANA (LN) is the near-infrared, Fizeau-type imaging interferometer for the large binocular telescope (LBT) on Mt. Graham, Arizona (elevation of 3267 m). The instrument is currently being built by a consortium of German and Italian institutes under the leadership of the Max Planck Institute for Astronomy in Heidelberg, Germany. It will combine the radiation from both 8.4 m primary mirrors of LBT in such a way that the sensitivity of a 11.9 m telescope and the spatial resolution of a 22.8 m telescope will be obtained within a 10.5×10.5 arcsec 2 scientific field of view. Interferometric fringes of the combined beams are tracked in an oval field with diameters of 1 and 1.5 arcmin. In addition, both incoming beams are individually corrected by LN’s multiconjugate adaptive optics system to reduce atmospheric image distortion over a circular field of up to 6 arcmin in diameter. A comprehensive technical overview of the instrument is presented, comprising the detailed design of LN’s four major systems for interferometric imaging and fringe tracking, both in the near infrared range of 1 to 2.4 μm, as well as atmospheric turbulence correction at two altitudes, both in the visible range of 0.6 to 0.9 μm. The resulting performance capabilities and a short outlook of some of the major science goals will be presented. In addition, the roadmap for the related assembly, integration, and verification process are discussed. To avoid late interface-related risks, strategies for early hardware as well as software interactions with the telescope have been elaborated. The goal is to ship LN to the LBT in 2014.
LINC-NIRVANA (LN) is the near-infrared, Fizeau-type imaging interferometer for the Large Binocular Telescope
(LBT) on Mt. Graham, Arizona, USA (3267m of elevation). The instrument is currently being built by a consortium of
German and Italian institutes under the leadership of the Max Planck Institute for Astronomy (MPIA) in Heidelberg,
Germany. It will combine the radiation from both 8.4m primary mirrors of LBT in such a way that the sensitivity of a
11.9m telescope and the spatial resolution of a 22.8m telescope will be obtained within a 10.5arcsec x 10.5arcsec
scientific field of view. Interferometric fringes of the combined beams are tracked in an oval field with diameters of 1
and 1.5arcmin. In addition, both incoming beams are individually corrected by LN’s multi-conjugate adaptive optics
(MCAO) system to reduce atmospheric image distortion over a circular field of up to 6arcmin in diameter.
This paper gives a comprehensive technical overview of the instrument comprising the detailed design of LN’s four
major systems for interferometric imaging and fringe tracking, both in the NIR range of 1 - 2.4μm, as well as
atmospheric turbulence correction at two altitudes, both in the visible range of 0.6 - 0.9μm. The resulting performance
capabilities and a short outlook of some of the major science goals will be presented. In addition, the roadmap for the
related assembly, integration and verification (AIV) process will be discussed. To avoid late interface-related risks,
strategies for early hardware as well as software interactions with the telescope have been elaborated. The goal is to ship
LN to the LBT in 2014.
GRAVITY is a second generation instrument for the VLT Interferometer, designed to enhance the near-infrared
astrometric and spectro-imaging capabilities of VLTI. Combining beams from four telescopes, GRAVITY will
provide an astrometric precision of order 10 micro-arcseconds, imaging resolution of 4 milli-arcseconds, and low
and medium resolution spectro-interferometry, pushing its performance far beyond current infrared interferometric
capabilities. To maximise the performance of GRAVITY, adaptive optics correction will be implemented
at each of the VLT Unit Telescopes to correct for the e_ects of atmospheric turbulence. To achieve this, the
GRAVITY project includes a development programme for four new wavefront sensors (WFS) and NIR-optimized
real time control system. These devices will enable closed-loop adaptive correction at the four Unit Telescopes
in the range 1.4-2.4 μm. This is crucially important for an e_cient adaptive optics implementation in regions
where optically bright references sources are scarce, such as the Galactic Centre. We present here the design of
the GRAVITY wavefront sensors and give an overview of the expected adaptive optics performance under typical
observing conditions. Bene_ting from newly developed SELEX/ESO SAPHIRA electron avalanche photodiode
(eAPD) detectors providing fast readout with low noise in the near-infrared, the AO systems are expected to
achieve residual wavefront errors of 400 nm at an operating frequency of 500 Hz.≤
Optical and opto-mechanical components in astronomical instruments are amongst the most expensive and
delicate single parts. Lenses made of special glasses or crystals are sometimes difficult to obtain (if at all),
especially with larger diameters and are figured and polished involving time-consuming and even risky procedures.
At infrared wavelengths (< 5μm), when the instrument is cooled to temperatures even below that of liquid
nitrogen, mechanical stress is induced between e.g. a glass lens and its metal mounting due to different heat
expansion coefficients of the materials involved. This can considerably degrade the performance of the whole
instrument. At infrared wavelengths the optical specifications considering surface roughness and form error
are less tight than in the optical due to the longer wavelengths involved. Hence metal mirrors with a surface
roughness and a form error of around 50 nm (RMS) may generally be favoured due to lower production costs then
lenses. Goal of the project described here is to manufacture plane, spherical or aspherical aluminum mirrors,
which are not hampered in the ways described above, in a cost effective procedure with optical specifications
(surface roughness and form error) of less than 100 nm (RMS) by means of direct diamond milling.
Operating on 6 interferometric baselines, i.e. using all 4 UTs, the 2nd generation VLTI instrument GRAVITY will deliver narrow angle astrometry with 10μas accuracy at K-band.
We present the system design of the science and fringe tracking spectrometers of GRAVITY: The fringe tracking spectrometer is optimised for highest sensitivity, providing a fixed spectral resolution. The science spectrometer provides 3 different low - medium spectral resolutions. Both spectrometers provide detector focus stages and deployable Wollaston prisms. The two spectrometers also feed the beams of the metrology laser system of GRAVITY backwards into the integrated optics beam-combiner, propagating back to the M2 mirrors of the 4 telescopes.
The Fringe and Flexure Tracking System (FFTS) is meant to monitor and correct atmospheric piston varia tion and instrumental vibrations and flexure during near-infrared interferometric image acquisition of LING NIRVANA. In close work with the adaptive optics system the FFTS enables homothetic imaging for the Large Binocular Telescope. One of the main problems we had to face is the connection between the cryogenic upper part of the instrument, e.g. detector head, and the lower ambient temperature part. In this ambient temperature part the moving stages are situated that move the detector head in the given field of view (FOV). We show how we solved this problem using the versatile material glass fiber reinforced plastics (GFRP's) and report in what way this material can be worked. We discuss in detail the exquisite characteristics of this material which we use to combine the cryogenic and ambient environments to a fully working system. The main characteristics that we focus on are the low temperature conduction and the tensile strength of the GFRP's. The low temperature conduction is needed to allow for a low heat-exchange between the cryogenic and ambient part whereas the tensile strength is needed to support heavy structures like the baffle motor and to allow for a minimum of flexure for the detector head. Additionally, we discuss the way we attached the GFRP to the remaining parts of the FFTS using a two component encapsulant.
The GRAVITY acquisition camera measurements are part of the overall beam stabilization by measuring each second
the tip-tilt and the telescope pupil lateral and longitudinal positions, while monitoring at longer intervals the full
telescope pupil, and the VLTI beam higher order aberrations.
The infrared acquisition camera implements a mosaic of field, pupil, and Shack Hartman type images for each telescope.
Star light is used to correct the tip-tilt while laser beacons placed at the telescope spiders are used to measure the pupil
lateral positions. Dedicated optimized algorithms are applied to each image, extracting the beam parameters and storing
them on the instrument database.
The final design is built into the GRAVITY beam combiner, around a structural plane where the 4 telescope folding
optics and field imaging lenses are attached. A fused silica prism assembly, kept around detector temperature, is placed
near to the detector implementing the different image modes.
Gravity aims at enhancing infrared imaging at VLTI to significantly improve our understanding of the physical processes
related to gravitation and accretion within compact objects. With its fiber-fed integrated optics, infrared wavefront
sensors, fringe tracker, beam stabilization and a novel metrology concept, GRAVITY will push the sensitivity and
accuracy of astrometry and interferometric imaging far beyond what is offered today. Four telescopes will be combined
in dual feed in the K band providing precision astrometry of order 10 micro-arcseconds, and imaging with 4-
milliarcsecond resolution. The fringe tracker and the scientific instrument host an identical integrated optics beam
combiner made by silica-on-silicon etching technology that is put inside a cryogenic vessel and cooled down to 200K to reduce thermal background and increase sensitivity.
This paper gives the design of the integrated beam combiner and of its fibered array that allows feeding the combiner
with stellar light. Lab measurement of spectral throughput and interferometric performance for beam combiners made by Flame Hydrolysis Deposition and by Plasma-Enhanced Chemical Vapor Deposition (PECVD) are given. The procedure
to glue together the beam combiner and its fibered array is described as well as the tests to validate the performance and the ageing effects at low temperature. Finally the thermal analysis and the eigen-frequency study of the whole device are presented.
The super-massive 4 million solar mass black hole (SMBH) SgrA* shows variable emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flux density excursions are dominated by a single state power law, with the low states of SgrA* are limited by confusion through the unresolved stellar background. We show that for 8-10m class telescopes blending effects along the line of sight will result in artificial compact star-like objects of 0.5-1 mJy that last for about 3-4 years. We discuss how the imaging capabilities of GRAVITY at the VLTI, LINC-NIRVANA at the LBT and METIS at the E-ELT will contribute to the investigation of the low variability states of SgrA*.
LINC-NIRVANA (LN) is a German /Italian interferometric beam combiner camera for the Large Binocular Telescope. Due to homothetic imaging, LN will make use of an exceptionally large field-of-view. As part of LN, the Fringe-and-Flexure-Tracker system (FFTS) will provide real-time, closed-loop measurement and correction of pistonic and flexure signals induced by the atmosphere and inside the telescope-instrument system. Such
compensation is essential for achieving coherent light combination over substantial time intervals (~10min.).
The FFTS is composed of a dedicated near-infrared detector, which can be positioned by three linear stages within the curved focal plane of LN. The system is divided into a cryogenic (detector) and ambient (linear stages) temperature environment, which are isolated from each other by a moving baffie. We give an overview of the current design and implementation stage of the FFTS opto-mechanical components. The optical components represent an update of the original design to assess slow image motion induced by the LN instrument separately.
A two stage blocking system is implemented in the GRAVITY science and the fringe tracking spectrometer optical
design. The blocking system consists of a dichroic beam splitter and two long wave band-pass filters with the top
level requirements of high transmission of the science light in the K-Band (1.95 - 2.45 μm) region and high blocking power optical density (OD) ≥ 8 for each filter at the metrology laser wavelength of 1.908 μm. The laser metrology blocking filters were identified as one critical optical component in the GRAVITY science and fringe tracker
spectrometer design. During the Phase-C study of GRAVITY all the filters were procured and individually tested in terms of spectral response at K-band, transmission, blocking (OD) and reflection at the metrology laser wavelength. We present the measurements results of the full metrology blocking system in its final configuration as to be implemented in the GRAVITY spectrometers.
We present the latest status of the control system of the LN (LINC-NIRVANA) FFTS (Fringe and Flexure Tracker
System) for the LBT. The software concept integrates the sensor data and control of the various subsystems
and provides the interaction with the whole LN instrument. Varying conditions and multiple configurations for
observations imply a flexible interconnection of the control loops for the hardware manipulators with respect
to the time-critical data analysis of the fringe detection. In this contribution details of the implementation of
the algorithms on a real-time Linux PC are given. By considering the results from simulations of the system
dynamics, lab experiments, atmospheric simulations, and telescope characterization the optimal parameter setup
for an observation can be chosen and basic techniques for adaption to changing conditions can be derived.
Operating on 6 interferometric baselines, i.e. using all 4 unit telescopes (UTs) of the Very Large Telescope
Interferometer (VLTI) simultaneously, the 2nd generation VLTI instrument GRAVITY will deliver narrow-angle
astrometry with 10μas accuracy at the infrared K-band. At this angular resolution, GRAVITY will be able to
detect the positional shift of the photo-center of a flare at the Galactic Center within its orbital timescale of
about 20 minutes, using the observed motion of the flares as dynamical probes of the gravitational field around
the supermassive black hole Sgr A*.
Within the international GRAVITY consortium, the 1. Physikalische Institut of the University of Cologne
is responsible for the development and construction of the two spectrometers of the camera system: one for
the science object, and one for the fringe tracking object, both being operated at cryo-vacuum. In this paper
we present the phase-C final optical design of the two spectrometers as it got derived from the scientific and
technical requirements and as it was presented and reviewed successfully at the Final Design Review (FDR) at
the European Southern Observatory (ESO) in October 2011.
We present design results of the 2nd generation VLTI instrument GRAVITY beam stabilization and light injection
subsystems. Designed to deliver micro-arcsecond astrometry, GRAVITY requires an unprecedented stability of the
VLTI optical train. To meet the astrometric requirements, we have developed a dedicated 'laser guiding system',
correcting the longitudinal and lateral pupil position as well as the image jitter. The actuators for the correction are
provided by four 'fiber coupler' units located in the GRAVITY cryostat. Each fiber coupler picks the light of one
telescope and stabilizes the beam. Furthermore each unit provides field de-rotation, polarization analysis as well as
atmospheric piston correction. Using a novel roof-prism design offers the possibility of on-axis as well as off-axis fringe
tracking without changing the optical train. Finally the stabilized beam is injected with minimized losses into singlemode
fibers via parabolic mirrors. We present lab results of the first guiding- as well as the first fiber coupler prototype
regarding the closed loop performance and the optical quality. Based on the lab results we discuss the on-sky
performance of the system and the implications concerning the sensitivity of GRAVITY.
LINC-NIRVANA is a near-Infrared homothetic, beam combining camera for the Large Binocular Telescope that offers Multi-Conjugate Adaptive Optics wavefront correction and fringe tracking to achieve a time-stable fringe pattern. Therefore, the trajectory of the reference source has to be followed as accurate as possible for a precise point spread function acquisition. The presented measurement campaign shows detector positioning errors exceeding the requirements significantly and indicates that these huge errors arise from the software, while the installed hardware matches the requirements.
GRAVITY belongs to the 2nd generation of the Very Large Telescope Interferometer (VLTI) and will operate inK-band on 6 baselines using all 4 Unit Telescopes of the VLT. With an unprecedented astrometrical accuracy of l0μas it will be, amongst others, capable of detecting the highly relativistic motion of the photocenter of a flare surrounding the supermassive black hole at the Galactic Centre, and thus probe General Relativity. The contribution of the University of Cologne within the international GRAVITY-Consortium is the design, manufacturing, qualification and assembly of the Fringe Tracking Spectrometer and the Science Spectrometer in the Beam Combiner Instrument (BCI). The BCI will be located in the interferometric lab of the VLTI. The spectrometers will be operated at 85K in a 200K environment in the BCI. We present the design and qualification of a linear displacement mechanism, which will be used at the focus stages of the detectors in both spectrometers and at the zoom stage in the Fringe Tracking Spectrometer. The mechanism consists of 4 double-hinged compliant joints which support the stage and provide a linear motion along the optical axis. The stage characterization at room and cryogenic conditions are presented.
We present the adaptive optics simulations we have performed to dimension the Gravity adaptive optics wavefront
sensor. We first computed the optimal WFS bandpass, depending on the sampling frequency, detector readout
noise and reference source colour/temperature. We then performed adaptive optics simulations with the YAO
simulation tool for different WFS parameters (number of subpupils, number of pixels per subpupil, loop frequency,
reference source magnitude, etc). Results demonstrate that the Gravity adaptive optics top-level requirements
can be fulfilled with a 9×9 subaperture Shack-Hartmann with 4 pixels per subaperture using an H+K filter, a
larger filter being recommended for sources bluer than 770 K reference source of the Galactic Centre.
Operating on 6 interferometric baselines, i.e. using all 4 UTs, the 2nd generation VLTI instrument GRAVITY will
deliver narrow angle astrometry with 10μas accuracy at the infrared K-band.
Within the international GRAVITY consortium, the Cologne institute is responsible for the development and
construction of the two spectrometers: one for the science object, and one for the fringe tracking object.
Optically two individual components, both spectrometers are two separate units with their own housing and interfaces
inside the vacuum vessel of GRAVITY. The general design of the spectrometers, however, is similar. The optical layout
is separated into beam collimator (with integrated optics and metrology laser injection) and camera system (with
detector, dispersive element, & Wollaston filter wheel). Mechanically, this transfers to two regions which are separated
by a solid baffle wall incorporating the blocking filter for the metrology Laser wavelength. The optical subunits are
mounted in individual rigid tubes which pay respect to the individual shape, size and thermal expansion of the lenses.
For a minimized thermal background, the spectrometers are actively cooled down to an operating temperature of 80K in
the ambient temperature environment of the GRAVITY vacuum dewar. The integrated optics beam combiner and the
metrology laser injection, which are operated at 200/240K, are mounted thermally isolated to the cold housing of the
spectrometers.
The optical design has shown that the alignment of the detector is crucial to the performance of the spectrometers.
Therefore, in addition to four wheel mechanisms, six cryogenic positioning mechanisms are included in the mechanical
design of the detector mount.
GRAVITY is a VLTI second generation instrument designed to deliver astrometry at the level of 10 μas. The
beam transport to the beam combiner is stabilized by means of a dedicated guiding system whose specifications
are mainly driven by the GRAVITY astrometric error budget. In the present design, the beam is monitored using
an infrared acquisition camera that implements a mosaic of field, pupil and Shack-Hartmann images for each of the telescopes. Star and background H-band light from the sky can be used to correct the tip-tilt and pupil lateral position, within the GRAVITY specifications, each 10 s. To correct the beam at higher frequencies laser guiding beams are launched in the beam path, on field and pupil planes, and are monitored using position sensor detectors. The detection, in the acquisition camera, of metrology laser light back reflected from the telescopes, is also being investigated as an alternative for the pupil motion control.
We present the Fiber Coupler subsystem of the future VLTI instrument GRAVITY. GRAVITY is specifically designed
to deliver micro-arcsecond astrometry and deep interferometric imaging. The Fiber Coupler is designed to feed the light
from a science and a reference object into single-mode fibers. The Fiber Coupler consists of four independent units. The
units de-rotate the FoV. A motorized half-wave plate allows rotating the liner polarization axis. Each unit provides
actuators for fast piston actuation, tip-tilt correction and pupil stabilization for one of the beams from four VLT
telescopes. The actuators are operated in closed-loop. Together with a dedicated Laser Guiding System, this allows to
stabilize the beams and maximize the coherently coupled light. The fast piston actuator provides the crucial fringe
tracking capability at a bandwidth of >220Hz. A special roof prism design allows to either split the FoV or to serve as a
50/50 beam splitter without changing the optical path. This offers the possibility of on-axis as well as off-axis fringe
tracking. The optical train consists solely of mirrors, which ensures an achromatic behavior and maximum throughput.
The sophisticated optical design compensates for aberrations which are introduced by off-axis parabolic mirrors. This
allows to achieve Strehl ratios of >95% across the FoV.
LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). In close
cooperation with the Adaptive Optics systems of LINC-NIRVANA the Fringe and Flexure Tracking System
(FFTS) is a fundamental component to ensure a complete and time-stable wavefront correction at the position
of the science detector in order to allow for long integration times at interferometric angular resolutions. In this
contribution, we present the design and the realization of the ongoing FFTS laboratory tests, taking into account
the system requirements. We have to sample the large Field of View and to follow the reference source during
science observations to an accuracy of less than 2 microns. In particular, important tests such as cooling tests
of cryogenic components and tip - tilt test (the repeatability and the precision under the different inclinations)
are presented. The system parameters such as internal flexure and precision are discussed.
GRAVITY is an adaptive optics assisted Beam Combiner for the second generation VLTI instrumentation. The
instrument will provide high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the
astronomical K-band for faint objects. We describe the wide range of science that will be tackled with this instrument,
highlighting the unique capabilities of the VLTI in combination with GRAVITY. The most prominent goal is to observe
highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky
Way. We present the preliminary design that fulfils the requirements that follow from the key science drivers: It includes
an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near-infrared wavefrontsensing
adaptive optics; fringe-tracking on secondary sources within the field of view of the VLTI and a novel metrology
concept. Simulations show that 10 μas astrometry within few minutes is feasible for a source with a magnitude of
mK = 15 like Sgr A*, given the availability of suitable phase reference sources (mK = 10). Using the same setup, imaging of mK = 18 stellar sources in the interferometric field of view is possible, assuming a full night of observations and the corresponding UV coverage of the VLTI.
GRAVITY is a second generation instrument for the VLTI. It will combine four telescopes in the K band and perform fringe tracking on stars as faint as 10 magnitude with a lambda/8 accuracy, thus counterbalancing atmospheric piston and UTs longitudinal vibrations, despite flux drop-outs due to residual tip-tilt jitter. To achieve such a performance, new developments have to be tested. We have developed a complete simulator so as to improve algorithms and establish an efficient fringe tracking strategy. In addition, a prototype of the fringe tracker for GRAVITY is being built up in order to demonstrate the results of this simulator. We present here the current status of these developments, achieved by simulating realistic tracking at VLTI.
Operating on 6 interferometric baselines, i.e. using all 4 unit telescopes (UTs) of the Very Large Telescope
Interferometer (VLTI) simultaneously, the 2nd generation VLTI instrument GRAVITY will deliver narrow-angle
astrometry with 10μas accuracy at the infrared K-band. At this angular resolution, GRAVITY will be able to
detect the positional shift of the photo-center of a flare at the Galactic Center within its orbital timescale of
about 20 minutes, using the observed motion of the flares as dynamical probes of the gravitational field around
the supermassive black hole Sgr A*.
Within the international GRAVITY consortium, the 1. Physikalische Institut of the University of Cologne is
responsible for the development and construction of the two spectrometers of the camera system: one for the
science object, and one for the fringe tracking object. In this paper we present the phase-B optical design of the
two spectrometers as it got derived from the scientific and technical requirements and as it passed the preliminary
design review (PDR) at the European Southern Observatory (ESO) successfully in late 2009.
LINC-NIRVANA is the near-infrared Fizeau interferometric imaging camera for the Large Binocular Telescope (LBT).
For an efficient interferometric operation of LINC-NIRVANA the Fringe and Flexure Tracking System (FFTS) is
mandatory: It is a real-time servo system that allows to compensate atmospheric and instrumental optical pathlength
differences (OPD). The thereby produced time-stable interference pattern at the position of the science detector enables
long integration times at interferometric angular resolutions.
As the development of the FFTS includes tests of control software and robustness of the fringe tracking concept in a
realistic physical system a testbed interferometer is set up as laboratory experiment.
This setup allows us to generate point-spread functions (PSF) similar to the interferometric PSF of the LBT via a
monochromatic (He-Ne laser) or a polychromatic light source (halogen lamp) and to introduce well defined, fast varying
phase offsets to simulate different atmospheric conditions and sources of instrumental OPD variations via dedicated
actuators.
Furthermore it comprises a piston mirror as actuator to counteract the measured OPD and a CCD camera in the focal
plane as sensor for fringe acquisition which both are substantial devices for a fringe tracking servo loop. The goal of the
setup is to test the performance and stability of different control loop algorithms and to design and optimize the control
approaches.
We present the design and the realization of the testbed interferometer and comment on the fringe-contrast behavior.
Interferometric measurements of optical path length differences of stars over large baselines can deliver extremely
accurate astrometric data. The interferometer GRAVITY will simultaneously measure two objects in the field
of view of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) and
determine their angular separation to a precision of 10 μas in only 5 minutes. To perform the astrometric
measurement with such a high accuracy, the differential path length through the VLTI and the instrument has
to be measured (and tracked since Earth's rotation will permanently change it) by a laser metrology to an even
higher level of accuracy (corresponding to 1 nm in 3 minutes). Usually, heterodyne differential path techniques
are used for nanometer precision measurements, but with these methods it is difficult to track the full beam size
and to follow the light path up to the primary mirror of the telescope. Here, we present the preliminary design of a differential path metrology system, developed within the GRAVITY project. It measures the instrumental differential path over the full pupil size and up to the entrance pupil location. The differential phase is measured by detecting the laser fringe pattern both on the telescopes' secondary mirrors as well as after reflection at the primary mirror. Based on our proposed design we evaluate the phase measurement accuracy based on a full budget of possible statistical and systematic errors. We show that this metrology design fulfills the high precision requirement of GRAVITY.
The dynamics of stars and gas undoubtedly shows the existence of a 4 million solar mass black hole at the
center of the Milky Way: Sagittarius A* (SgrA*). Violent flare emission allows us to probe the immediate
environment of the central mass. Near-infrared polarimetry now shows signatures of strong gravity that are
statistically significant against randomly polarized red noise. Using these signatures we can derive spin and
inclination information of SgrA*. A combined synchrotron self Compton (SSC) and adiabatic expansion model
with source components peaking in the sub-mm domain can fully account for the observed flare flux densities
and the time delays towards the (sub-)mm flares that have been reported in some cases. We discuss the expected
centroid paths of the NIR images and summarize how the geometrical structure of the emitting region (i.e.
spot shape, presence of a torus or spiral-arm pattern etc.) affects this centroid tracks. While most of the
mentioned geometries are able to fit the observed fluxes, future NIR interferometry with GRAVITY at the
VLT will break some of the degeneracies between different emission models. In this contribution we summarize
several GRAVITY science cases for SgrA*. Our simulations propose that focusing GRAVITY observations on
the polarimetry mode could reveal a clear centroid track of the spot(s). A non-detection of centroid shifts cannot
rule out the multi-component model or spiral arms scenarios. However, a clear wander between alternating
centroid positions during the flares will prove the idea of bright long-lived spots occasionally orbiting the central
black hole.
LINC-NIRVANA (LN) is a German/Italian interferometric beam combiner camera for the Large Binocular
Telescope. Due to homothetic imaging, LN will make use of an exceptionally large field-of-view. As part of LN,
the Fringe-and-Flexure-Tracker system (FFTS) will provide real-time, closed-loop measurement and correction
of pistonic and flexure signals induced by the atmosphere and inside the telescope-instrument system. Such
compensation is essential for achieving coherent light combination over substantial time intervals (~ 10min.).
The FFTS is composed of a dedicated near-infrared detector, which can be positioned by three linear stages
within the curved focal plane of LN. The system is divided into a cryogenic (detector) and ambient (linear
stages) temperature environment, which are isolated from each other by a moving baffle. We give an overview
of the current design and implementation stage of the FFTS opto-mechanical and electronic components. We
present recent important updates of the system, including the development of separated channels for the tracking
of piston and flexure. Furthermore, the inclusion of dispersive elements will allow for the correction of atmospheric
differential refraction, as well as the induction of artificial dispersion to better exploit the observational-conditions
parameter space (air mass, brightness).
A two stage blocking system is implemented in the GRAVITY science and the fringe tracking spectrometer optical
design. The blocking system consists of a dichroic mirror and a long wave band-pass filter with the top level
requirements of high transmission of the science light in the K-Band (1.95 - 2.5 μm) region and high blocking power
optical density (OD) ≥ 8 for the metrology laser wavelength at 1.908 μm. The laser metrology blocking filters have been
identified as one critical optical component in the GRAVITY science and fringe tracker spectrometer design.
During the Phase-B study of GRAVITY we procured 3 blocking filter test samples for demonstration and qualification
tests. We present the measurements results of an effective blocking of the metrology laser wavelength with a long wave
band-pass filter at OD=12.
We present the latest status of the fringe detecting algorithms for the LINC-NIRVANA FFTS (Fringe and Flexure
Tracker System). The piston and PSF effects of the system from the top of the atmosphere through the telescopes and
multi-conjugate AO systems to the detector are discussed and the resulting requirements for the FFTS outlined.
The Fringe and Flexure Tracker System (FFTS) of the LINC-NIRVANA instrument is designed to monitor and
correct the atmospheric piston variations and the instrumental vibrations and flexure at the LBT during the
NIR interferometric image acquisition. In this contribution, we give an overview of the current FFTS control
design, the various subsystems, and their interaction details. The control algorithms are implemented on a realtime
computer system with interfaces to the fringe and flexure detector read-out electronics, the OPD vibration
monitoring system (OVMS) based on accelerometric sensors at the telescope structure, the piezo-electric actuator
for piston compensation, and the AO systems for offloading purposes. The FFTS computer combines data from
different sensors with varying sampling rate, noise and delay. This done on the basis of the vibration data and the
expected power spectrum of atmospheric conditions. Flexure effects are then separated from OPD signals and
the optimal correcting variables are computed and distributed to the actuators. The goal is a 120 nm precision
of the correction at a bandwidth of about 50 Hz. An end-to-end simulation including models of atmospheric
effects, actuator dynamics, sensor effects, and on-site vibration measurements is used to optimize controllers and
filters and to pre-estimate the performance under different observation conditions.
GRAVITY is a 2nd generation VLTI Instrument which operates on 6 interferometric baselines by using all 4
UTs. It will offer narrow angle astrometry in the infrared K-band with an accuracy of 10 ìas.
The University of Cologne is part of the international GRAVITY consortium and responsible for the design
and manufacturing of the two spectrometers. One is optimized for observing the science object, providing three
different spectral resolutions and optional polarimetry, the other is optimized for a fast fringe tracking at a spectral
resolution of R=22 with optional polarimetry. In order to achieve the necessary image quality, the current
mechanical design foresees 5 motorized functions, 2 linear motions and 3 filter wheels. Additionally the latest
optical design proposal includes 20 degrees of freedom for manual adjustments distributed over the different
optical elements.
Both spectrometers require precise linear and rotational movements on micrometer or arcsecond scales. These
movements will be realized using custom linear stages based on compliant joints. These stages will be driven
by actuators based on a Phytron/Harmonic Drive combination. For dimensioning and in order to qualify the
reliability of these mechanisms, it is necessary to evaluate the mechanisms on the base of several prototypes. Due
to the cryogenic environment the wheel mechanisms will be driven by Phytron stepper motors, too. A ratchet
mechanism, which is currently in the beginning of his design phase, will deliver the required precision to the
filter wheels.
This contribution will give a first impression how the next mechanical prototypes will look like. Besides, advantages
of purchasing and integrating a distance sensor and a resolver are reported. Both are supposed to work
under cryogenic conditions and should achieve high resolutions for the measuring of movements inside the test
cryostat.
LINC-NIRVANA is the near-infrared homothetic imaging camera for the Large Binocular Telescope. Once
operational, it will provide an unprecedented combination of angular resolution, sensitivity and field of view. Its
Fringe and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA.
It is tailored to compensate low-order phase perturbations in real-time to allow for a time-stable
interference pattern in the focal plane of the science camera during the integration. Two independent control
loops are realized within FFTS: A cophasing loop continuously monitors and corrects for atmospheric and
instrumental differential piston between the two arms of the interferometer. A second loop controls common
and differential image motion resulting from changing orientations of the two optical axes of the interferometer.
Such changes are caused by flexure but also by atmospheric dispersion.
Both loops obtain their input signals from different quadrants of a NIR focal plane array. A piezo-driven
piston mirror in front of the beam combining optics serves as actuator in the cophasing loop. Differential piston
is determined by fitting a parameterized analytical model to the observed point spread function of a reference
target. Tip-tilt corrections in the flexure loop are applied via the secondary mirrors. Image motion is sensed for
each optical axis individually in out-of-focus images of the same reference target.
In this contribution we present the principles of operation, the latest changes in the opto-mechanical design,
the current status of the hardware development.
Differential measurements with dual feed stellar interferometers using large baselines can deliver extremely accurate
astrometric data. Separating the phase difference measured on the stars from the path length differences occurring within
the interferometric instrument itself requires the use of laser interferometers. Usually heterodyne differential path
techniques are used for nanometer precision measurements. With these methods it is usually difficult to track the full
beam size and follow the light path up to the secondary mirror. We will report on the concept and first tests of a
differential path metrology system, developed within the GRAVITY project, that allows one to measure the instrumental
differential path over the full pupil size and up to the entrance pupil location. The differential phase is measured by
detecting the laser fringe pattern created on the telescopes' secondaries. This novel method is almost free from systematic
errors since the stellar and laser light are traveling along a common optical path.
LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). Its Fringe
and Flexure Tracking System (FFTS) is mandatory for an effcient interferometric operation of LINC-NIRVANA:
the task of this cophasing system is to assure a time-stable interference pattern in the focal plane of the camera.
A testbed interferometer, set up as laboratory experiment, is used to develop the FFTS control loop and
to test the robustness of the fringe tracking concept. The geometry of the resulting interferometric intensity
distribution in the focal plane of the implemented CCD corresponds to that of the LBT PSF. The setup allows to
produce monochromatic (He-Ne laser) and polychromatic (halogen lamp) PSFs and allows to actively introduce
well defined low-order phase perturbations, namely OPD and differential tip/tilt. Furthermore, all components
that are required in a fringe tracking servo loop are included: a sensor for fringe acquisition and an actuator
to counteract measured OPD. With this setup it is intended to determine the performance with which a fringe
tracking control loop is able to compensate defined OPD sequences, to test different control algorithms, and to
optimize the control parameters of an existing servo system.
In this contribution we present the design and the realization of the testbed interferometer. Key parameters
describing the white light testbed interferometer, such as fringe contrast and thermal sensitivity are discussed.
The effects of all controllable phase perturbations are demonstrated.
We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase.
GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*,
the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this
goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner
operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources
within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches
the scientific needs; in particular that 10µas astrometry is feasible for a source with a magnitude of K=15 like Sgr A*,
given the availability of suitable phase reference sources.
LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). Its Fringe
and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA:
the task of this cophasing system is to assure a time-stable interference pattern in the focal plane of the camera.
Differential piston effects will be detected and corrected in a real-time closed loop by analyzing the PSF of
a guide star at a frequency of 100Hz-200Hz. A dedicated piston mirror will then be moved in a corresponding
manner by a piezo actuator. The long-term flexure tip/tilt variations will be compensated by the AO deformable
mirrors.
A testbed interferometer has been designed to simulate the control process of the movement of a scaled
piston mirror under disturbances. Telescope vibration and atmospheric variations with arbitrary power spectra
are induced into the optical path by a dedicated piezo actuator. Limiting factors of the control bandwith are
the sampling frequency and delay of the detector and the resonance frequency of the piston mirror. In our setup
we can test the control performance under realistic conditions by considering the real piston mirrors dynamics
with an appropriate software filter and inducing a artificial delay of the PSF detector signal. Together with
the expected atmospheric OPD variations and a realistic vibration spectrum we are able to quantify the piston
control performance for typical observation conditions. A robust control approach is presented as result from
in-system control design as provided by the testbed interferometer with simulated dynamics.
GRAVITY is a 2nd generation VLTI instrument that operates in the K-band and uses up to 4 telescopes simultaneously.
GRAVITY will provide interferometric astrometry of two objects in a 2 arcsecond field of view at
an astrometric precision of 10 μas. Using all four UTs and six interferometric baselines, it will allow for phase-referenced
imaging at mas resolution in combination with spectroscopic and polarimetric observing capabilities.
The large field of view of the VLTI delay lines is worldwide unique on a 140 m baseline, and no other VLTI
instrument is taking advantage of that outstanding capability so far.
In this paper we present the optical and mechanical design of the two spectrometers of the instrument.
The presented design resulted from the successful Phase A study of the system and provides low-resolution
spectroscopy using grisms and Wollaston prisms for polarimetry.
GRAVITY, a VLTI second generation instrument, requires a fringe tracker combining four beams. Its design is
driven by the observation of the Galactic Center and implies stringent fringe sensor specifications. We present
the simulations of the fringe tracking closed-loop performance with an optical path difference (OPD) turbulence
spectrum using a Kolmogorov model of the atmosphere for typical seeing conditions at VLTI (r0 = 0.95 m,
t0 = 47 ms at 2.2 μm). We show that the total residual OPD standard deviation can be as low as λ/12 at
a sampling frequency of 350 Hz on a guide star with a magnitude of mK = 10. To obtain this performance,
we compared several 4-beam pairwise co-axial combination conceptual architectures and show that the optimal
4-beam combination is the one measuring the OPD on the six baselines.
As a near-infrared (NIR) wide field interferometric imager offering an angular resolution of about 10 milliarcseconds
LINC-NIRVANA at the Large Binocular Telescope will be an ideal instrument for imaging the center of the
Milky Way especially in conjunction with mm/sub-mm interferometers like CARMA, ATCA or, in the near
future, ALMA. Sagittarius A* (Sgr A*) is the electromagnetc manifestation of the ~4×106M super-massive
black hole (SMBH) at the Galactic Center. First results from a mult-wavelength campaign focused on Sgr A*,
based on the VLT
and on CARMA, ATCA, and the IRAM 30m-telescope, in May 2007 show that the NIR
data are consistent with partially depolarized non-thermal emission from confined hot spots in relativistic orbits
around SgrA*. A 3mm flare following a May 2007 NIR flare is consistent with SSC emission from adiabatically
expanding plasma in a wind or jet. With the LBT and ALMA we will be able to study the spectral evolution
of NIR/sub-mm/mm flare emission in order to constrain the emission mechanism, the jet/wind physics, and
possibly determine the angular momentum of the SMBH. LINC/NIRVANA will also serve to investigate the
stellar population and dynamics in the cluster surrounding Sgr A*. A particular emphasis will lie on examining
dust embedded and young stars and to unravel the star formation history in the cluster.
For the 0.3 parsec core radius central star cluster the investigation of will be investigated.
The JWST Mid-Infrared Instrument (MIRI) is designed to meet the JWST science requirements for mid-IR capabilities
and includes an Imager MIRIM provided by CEA (France). A double-prism assembly (DPA) allows MIRIM to perform
low-resolution spectroscopy. The MIRIM DPA shall meet a number of challenging requirements in terms of optical and
mechanical constraints, especially severe optical tolerances, limited envelope and very high vibration loads.
The University of Cologne (Germany) and the Centre Spatial de Liege (Belgium) are responsible for design,
manufacturing, integration, and testing of the prism assembly. A companion paper (Fischer et al. 2008) is presenting the
science drivers and mechanical design of the DPA, while this paper is focusing on optical manufacturing and overall
verification processes.
The first part of this paper describes the manufacturing of Zinc-sulphide and Germanium prisms and techniques to ensure
an accurate positioning of the prisms in their holder. (1) The delicate manufacturing of Ge and ZnS materials and (2) the
severe specifications on the bearing and optical surfaces flatness and the tolerance on the prism optical angles make this
process innovating. The specifications verification is carried out using mechanical and optical measurements; the
implemented techniques are described in this paper.
The second part concerns the qualification program of the double-prism assembly, including the prisms, the holder and
the prisms anti-reflective coatings qualification. Both predictions and actual test results are shown.
We present how it is achieved to mount a double prism in the filter wheel of MIRIM - the imager of JWST's Mid
Infrared Instrument. In order to cope with the extreme conditions of the prisms' surroundings, the low resolution
double prism assembly (LRSDPA) design makes high demands on manufacturing accuracy. The design and the
manufacturing of the mechanical parts are presented here, while 'Manufacturing and verification of ZnS and Ge
prisms for the JWST MIRI imager' are described in a second paper [1]. We also give insights on the astronomical
possibilities of a sensitive MIR spectrometer. Low resolution prism spectroscopy in the wavelength range from
5-10 microns will allow to spectroscopically determine redshifts of objects close to/at the re-ionization phase of
the universe.
The correction of atmospheric differential piston and instrumental flexure effects is mandatory for interferometric operation of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in real-time. In the fringe tracking concept that we present, differential piston information is gathered in the image plane by analyzing the PSF of a reference star anywhere in the large field of view of the LBT. We have developed and tested a fast PSF analysis algorithm that allows to clearly identify differential piston even in the case of low S/N. We present performance estimates of the algorithm. Since the performance of the FFTS algorithm has a strong impact on the overall sky coverage of LINC-NIRVANA, we studied the required limiting magnitudes of the fringe tracking reference star for different scenarios. As the FFTS may not necessarily operate on the science target, but rather uses a suitable reference star at a certain angular distance to the science target, differences between piston values at the two positions add to the residual piston of the FFTS. We have dealt with the question of differential piston angular anisoplanatism and studied a possible improvement of the isopistonic patch size by the use of multi-conjugate adaptive optics (MCAO). In its final stage, LINC-NIRVANA will be equipped with such a system.
Current and future opportunities for interferometric observations of the Galactic Center in the near- and mid-infrared (NIR/MIR) wavelength domain are highlighted. Main emphasis is being put on the Large Binocular Telescope (LBT) and the Very Large Telescope Interferometer (VLTI). The Galactic Center measurements of stellar orbits and strongly variable NIR and X-ray emission from Sagittarius A* (SgrA*) at the center of the Milky Way have provided the strongest evidence so far that the dark mass concentration at this position is associated with a super massive black hole. Similar dark mass concentrations seen in many galactic nuclei are most likely super massive black holes as well. High angular resolution interferometric observations in the NIR/MIR will provide key information on the central massive black hole and the stellar cluster it is embedded in. These observations have already started: Recent results on the luminous dust enshrowded star IRS3 using MIDI at the VLTI are presented and future scientific possibilities in the GC using MIDI at the VLTI in the MIR and GRAVITY in the NIR are highlighted. As a NIR wide field interferometric imager offering an angular resolution of about 10 milliarcseconds LINC/NIRVANA at the Large Binocular Telescope will be an ideal instrument for imaging galactic nuclei including the center of the Milky Way.
The correction of atmospheric differential piston and instrumental flexure effects is mandatory for interferometric operation of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in a real-time closed loop. Being a Fizeau-Interferometer, the LBT provides a large field of view (FoV). The FFTS can make use of the large FoV and increase the sky coverage of the overall instrument if it is able to acquire the light of a suitable fringe tracking reference star within the FoV. For this purpose, the FFTS detector needs to be moved to the position of the reference star PSF in the curved focal plane and needs to precisely follow its trajectory as the field rotates. Sub-pixel (1 pixel = 18.5 micron) positioning accuracy is required over a travel range of 200mm x 300mm x 70mm. Strong are the constraints imposed by the need of a cryogenic environment for the moving detector. We present a mechanical design, in which the Detector Positioning Unit (DPU) is realized with off-the-shelf micro-positioning stages, which can be kept at ambient temperature. A moving baffle will prevent the intrusion of radiation from the ambient temperature environment into the cryogenic interior of the camera. This baffle consists of two nested disks, which synchronously follow any derotation - or repositioning trajectory of the DPU. The detector, its fanout board and a filter wheel are integrated into a housing that is mounted on top of the DPU and that protects the FFTS detector from stray light. Long and flexible copper bands allow heat transfer from the housing to the LINC-NIRVANA heat exchanger.
We present the adaptive optics assisted, near-infrared VLTI instrument - GRAVITY - for precision narrow-angle astrometry and interferometric phase referenced imaging of faint objects. Precision astrometry and phase-referenced interferometric imaging will realize the most advanced vision of optical/infrared interferometry with the VLT. Our most ambitious science goal is to study motions within a few times the event horizon size of the Galactic Center massive black hole and to test General Relativity in its strong field limit. We define the science reference cases for GRAVITY and derive the top level requirements for GRAVITY. The installation of the instrument at the VLTI is planned for 2012.
LINC-NIRVANA is the interferometric near-infrared imaging camera for the Large Binocular Telescope (LBT). Being able to observe at wavelength bands from J to K (suppported by an adaptive optics system operating at visible light) LINC-NIRVANA will provide an unique and unprecedented combination of high angular resolution (~ 9 milliarcseconds at 1.25μm), wide field of view (~ 100 arcseconds2 at 1.25μm), and large collecting area (~ 100m2).
One of the major contributions of the 1. Physikalische Institut of the University of Cologne to this project is the development and provision of the Fringe and Flexure Tracking System (FFTS). In addition to the single-eye adaptive optics systems the FFTS is a crucial component to ensure a time-stable wavefront correction over the full aperture of the double-eye telescope, a mandatory pre-requisite for interferometric observations.
Using a independent HAWAII 1 detector array at a combined focus close to the science detector, the Fringe and Flexure Tracking System analyses the complex two-dimensional interferometric point spread function (PSF) of a suitably bright reference source at frame rates of up to several hundred Hertz. By fitting a parameterised theoretical model PSF to the preprocessed image-data the FFTS determines the amount of pistonic phase difference and angular misalignment between the wavefronts of the two optical paths of LINC-NIRVANA. For every exposure the corrective parameters are derived in real-time and transmitted to a dedicated piezo-electric fast linear mirror for simple path lengths adjustments, and/or to the adaptive optics systems of the single-eye telescopes for more complicated corrections.
In this paper we present the basic concept and currect status of the opto-mechanical design of the Fringe and Flexure Tracker, the operating principle of the fringe and flexure tracking loops, and the encouraging result of a laboratory test of the piston control loop.
The Fringe and Flexure Tracking System (FFTS) is designed to correct
the atmospheric piston variations and the instrumental flexure during the NIR interferometric image acquisition of the LINC-NIRVANA camera at the LBT. The interferometric image quality depends on the performance of these corrections.
Differential piston and flexure effects will be detected and corrected in a real-time closed loop by analyzing the PSF of a guide star at a frequency of up to several hundred Hz. A dedicated piston mirror will then be moved in a corresponding manner by a piezo actuator.
The FFTS is expected to provide a residual piston of better then 0.1 λ at the central wavelength of the science band. Thus, the required correction bandwidth is 10-20 Hz as differential piston simulations of different seeing conditions indicate. Therefore, a sampling frequency of 100-200 Hz is required to correct OPD variations. The upper limit for the loop frequency is the resonance frequency of the mirror and the response function respectively.
The piston mirror as the actuator and the FFTS detector as the sensor
feedback are embedded in a very complex system. Many control loop aspects like sampling frequencies, delays, controller algorithm and control bandwidth have to be identified. With accurate simulations of the system the limits of atmospheric and instrumental conditions for reliable closed loops can be determined against the respective control parameters. We present strategies for the closed-loop control of the piston correction which are suitable to achieve the 0.1 λ requirement and the optimal overall imaging performance with a sufficient "all-purpose" control stability.
KEYWORDS: Computing systems, Image processing, Interferometry, Near infrared, Signal processing, Atmospheric corrections, Sensors, Control systems, Telescopes, Point spread functions
The correction of atmospheric differential piston and instrumental flexure effects is mandatory for optimum interferometric performance of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in a real-time closed loop. On a timescale of milliseconds, image data of the order of 4K bytes has to be retrieved from the FFTS detector, analyzed, and the results have to be sent to the control system. The need for a reliable communication between several processes within a confined period of time calls for solutions with good real-time performance. We investigated two soft real-time options for the Linux platform. The design we present takes advantage of several features that follow the POSIX standard with improved real-time performance, which were implemented in the new Linux kernel (2.6.12). Several concepts, such as synchronization, shared memory, and preemptive scheduling are considered and the performance of the most time-critical parts of the FFTS software is tested.
MIRI, the Mid-InfraRed Instrument, is one of the 4 instruments currently under development for the NASA/ESA
James Webb Space Telescope. Together with the US, MIRI is built by a consortium of 28 European institutes
under the lead-management of ESA. The instrument consists of two main modules, a spectroscopic and an
imaging part. The imager will allow imaging, coronography and low resolution spectroscopy. The latter mode
will use a ZnS-Ge-double-prism assembly as dispersive element.
In this contribution, we present the design concept for the mounting of this double prism assembly which places
the prisms into the optical path of the imager via an interface to the imager's filterwheel. Despite the very
limited available space in the filterwheel and the high weight of the prisms (in comparison to the other filters
in the filterwheel), the kinematic mounting of the individual prisms guarantees exact placement with smallest
possible induced forces into the prisms. The here presented design of the development model of the double prism
assembly is based upon GEM calculation. Experimental thermal and vibrational tests will be performed by the
time of this conference.
On the way to the Extremely Large Telescopes (ELT) the Large Binocular
Telescope (LBT) is an intermediate step. The two 8.4m mirrors create a masked aperture of 23m. LINC-NIRVANA is an instrument taking advantage of this opportunity. It will get, by means of Multi-Conjugated Adaptive Optics (MCAO), a moderate Strehl Ratio over a 2 arcmin field of view, which is used for Fizeau (imaging) interferometry in J,H and K. Several MCAO concepts, which are
proposed for ELTs, will be proven with this instrument. Studies of sub-systems are done in the laboratory and the option to test them on sky are kept open. We will show the implementation of the MCAO concepts and control aspects of the instrument and present the road map to the final installation at LBT. Major milestones of LINC-NIRVANA, like preliminary design review or final design review are already done or in preparation. LINC-NIRVANA is one of the
few MCAO instruments in the world which will see first light and go into operation within the next years.
LINC-NIRVANA is an imaging interferometer for the Large Binocular Telescope (LBT) and will make use of multi-conjugated adaptive optics (MCAO) with two 349 actuators deformable mirrors (DM), two 672 actuator deformable secondary mirrors and a total of 4 wavefront sensors (WFS) by using 8 or 12 natural guide stars each. The goal of the MCAO is to increase sky coverage and achieve a medium Strehl-ratio over the 2 arcmin field of view. To test the concepts and prototypes, a laboratory setup of one MCAO arm is being built. We present the layout of the MCAO prototype, planned and accomplished tests, especially for the used Xinetics DMs, and a possible setup for a test on sky with an existing 8m class telescope.
The correction of atmospherical differential piston and instrumental flexure effects is mandatory for full interferometric performance of the LBT NIR interferometric imaging camera LINC-NIRVANA. This is the task of the Fringe and Flexure Tracking System (FFTS), which is part of the contribution of the I. Physikalische Institut of the University of Cologne to the project. Differential piston and flexure effects will be detected and corrected in a real-time closed loop by analyzing the PSF of a guide star at a frequency of up to several hundred Hz.
Numerous critical design parameters for both FFTS hardware and control loop have to be derived from simulations. Detailed knowledge of the special shape of the LBT interferometric PSF as a function of a variety of parameters is required to design the fringe tracking control loop. In this paper we will show the results of our software that allows us to generate polychromatic interferometric PSFs for a number of different scenarios.
Our fringe detection algorithm is based on an analytic model which is fitted to the acquired PSF. We present the results of the evaluation of the algorithm in terms of speed and residual piston, as well as the first successful implementation of the algorithm in a closed loop system.
Simulations of the time evolution of differential piston have been performed in order to investigate necessary correction frequencies and the variation of differential piston across the usable field of view. These simulations are based on the Layer Oriented Adaptive Optics performance simulator "LOST" of the Osservatorio Astriofisico di Arcetri.
LINC-NIRVANA is a near-infrared (1-2.4 micron) beam-combiner instrument for the Large Binocular Telescope (LBT). LINC-NIRVANA is being built by a consortium of groups at the Max-Planck-Institut fur Astronomie in Heidelberg, the Osservatorio Astrofisico di Arcetri in Florence, the Universitat zu Koln, and the Max-Planck-Institut fur Radioastronomie in Bonn. The MPI fur Radioastronomie is responsible for the near-infrared detector for the fringe and flexure tracking system (FFTS).
We describe the design and construction of the detector control electronics as well as the first laboratory measurements of performance parameters of the NIR detector for the fringe and flexure tracking system of the LBT LINC-NIRVANA instrument. This detector has to record LBT interferograms of suitable reference stars in the FOV at a frame rate of the order of 200 frames per second using, for example, 32 x 32-pixel subframes. Moreover, special noise reduction techniques have to be applied. The fringe-tracker interferograms are required for monitoring and closed-loop correction of the atmospheric optical path difference of the two LBT wavefronts (see C. Straubmeier et al., "A fringe and flexure tracking system for LINC-NIRVANA: basic design and principle of operation"). We will describe our laboratory measurements of maximum frame rate, readout noise, photometric stability, and other important parameters together with first measurements of laboratory simulations of LBT interferograms.
As a near-infrared (NIR) wide field interferometric imager offering an
angular resolution of about 10 milliarcseconds LINC/NIRVANA at the
Large Binocular Telescope will be an ideal instrument for
imaging of galactic nuclei including the center of the Milky Way.
Recent optical/IR imaging surveys can now quite successfully be used
to search for star-galaxy pairs that are suitable for interferometric
observation with LINC NIRVANA. These objects can then be used to efficiently investigate galaxy interaction, nuclear activity, and star formation in distant galaxies. In the NIR these investigations will be carried out at scales below 100~pc for z<0.05 and at scales
below 500~pc at z<2.
The Galactic Center measurements of stellar orbits and strongly
variable NIR and X-ray emission from Sagittarius A* at the center of the Milky Way have provided the strongest evidence so far that the dark mass concentrations seen in many galactic nuclei are most likely super massive black holes. Observations with LINC NIRVANA will allow to simultaneously investigate the stellar dynamics of the entire central cluster, the determination of the amount of extended mass within the cusp region, and to monitor the activity of the 3 million solar mass black hole at the position of Sagittarius A* at separations of only about 10 light hours or 15 Schwarzschild radii.
LINC-NIRVANA is the interferometric near-infrared imaging camera for the Large Binocular Telescope (LBT). Operating at JHK bands LINC-NIRVANA will provide an unique and unprecedented combination of high angular resolution (~9 milliarcseconds at 1.25 µm), wide field of view (~100 arcseconds2 at 1.25 µm), and large collecting area (~100 m2).
One of the major contributions of the I. Physikalische Institut of the University of Cologne to this project is the development of the Fringe and Flexure Tracking System (FFTS). In close cooperation with the Adaptive Optics systems of LINC-NIRVANA the FFTS is a fundamental component to ensure a complete and time-stable wavefront correction at the position of the science detector in order to allow for long integration times at interferometric angular resolutions.
Using a dedicated near-infrared detector array at a combined focus close to the science detector, the Fringe and Flexure Tracking System analyses the interferometric point spread function (PSF) of a suitably bright reference source at frame rates of several hundred Hertz up to 1 kHz. By fitting a parameterized theoretical model PSF to the preprocessed image-data the FFTS determines the amount of pistonic phase difference and the amount of an angular misalignment between the wavefronts of the two optical paths of LINC-NIRVANA. For every exposure the correcting parameters are derived in real-time and transmitted to the respective control electronics, or the Adaptive Optics systems of the single-eye telescopes, which will adjust their optical elements accordingly.
In this paper we present the opto-mechanical hardware design, the principle of operation of the software control algorithms, and the results of first numerical simulations and laboratory experiments of the performance of this Fringe and Flexure Tracking System.
We present a 1:3 scale model of the LINC-NIRVANA interferometer. This
laboratory Fizeau, or image plane, interferometer allows us to test many aspects of LINC-NIRVANA before the final instrument is integrated. We have used this testbed interferometer to practice alignment procedures, verify the optical design, show that point spread functions with low (10\%) Strehl ratio can maintain high fringe contrast, and test the fringe tracking algorithm by running the interferometer in a closed piston loop.
The LINC-NIRVANA instrument is a 1-2.5 micron Fizeau interferometric imager, which combines the light of the two 8.4 m mirrors of the Large Binocular Telescope on Mt. Graham in Arizona. The cryogenic camera forms the heart of the science channel of this instrument, delivering a 1 arcmin diameter field of view with 5 mas spatial resolution. The center 10x10 arcseconds, initially limited by the size of the 2048x2048 Hawaii-2 detector, are used for science observations. For simplicity, the camera has a fixed, F/32 optical path of the combined beams, leading to wavelength-dependent sampling. We describe the main components of the camera, as well as present the calculations of interferometric performance and the required opto-mechanical tolerances. We demonstrate that specially designed components can reach these specifications.
KEYWORDS: James Webb Space Telescope, Spectroscopy, Mirrors, Sensors, Electronics, Imaging systems, Optical components, Mid-IR, Optical filters, Picture Archiving and Communication System
MIRI is one of three focal plane instruments for the JWST covering the wavelengths region 5...28 μm. It is jointly developed by US and European institutes with the latter ones being responsible for the complete optical bench assembly, cryomechanisms, calibration source and the related electronics. MIRI is the combination of an imager with coronographic and low-resolution spectroscopic capabilities and a high-resolution integral-field spectrometer. These diverse options require several mechanisms to select a specific observing mode: (1) a filter wheel with bandpass filters, coronographic masks and a prism, (2) two grating/dichroic wheels with dispersing and order-sorting elements and (3) a flip mirror to direct the beam of an internal black body source into the spectrometer section. All mechanisms are required to operate under laboratory conditions (warm launch) as well as in the cryovacuum in space. The heat dissipation has to be small and the reliability and precision very high. Our low risk approach is the application of successfully qualified and flown components of the ISOPHOT (ISO) and PACS (HERSCHEL) instruments. We will report on the concept developed in phase B.
LINC-NIRVANA is a Fizeau interferometer which will be built for the Large Binocular Telescope (LBT). The LBT exists of two 8.4m mirrors on one mounting with a distance of 22.8m between the outer edges of the two mirrors. The interferometric technique used in LINC-NIRVANA provides direct imaging with the resolution of a 23m telescope in one direction and 8.4m in the other. The instrument uses multi-conjugated adaptive optics (MCAO) to increase the sky coverage and achieve the diffraction limit in J, H, K over a moderate Field of View (2 arcmin in diameter). During the preliminary design phase the team faced several problems similar to those for an instrument at a 23m telescope. We will give an overview of the current design, explain problems related to 20m class telescopes and present solutions.
A high-speed photometer, "OPTIMA" short for Optical Pulsar Timing Analyzer, has been designed as a sensitive, portable detector to observe optical pulsars and other highly variable sources. The detector contains eight fiber fed avalanche photodiode single photon counters, a GPS timing receiver, a CCD camera for target acquisition and a computerized control unit. The central fibers are configured as a hexagonal bundle around the target fiber, while one fiber is located at a distance of ~1' as a monitor for the night sky background. Recently a rotating polarization filter and a 4-color prism spectrograph have been added to the system as optional equipment. Since January 1999 OPTIMA has been used on different telescopes to measure detailed lightcurves and polarization of the Crab Pulsar, in a search for optical emission from the Geminga pulsar, and for the timing of cataclysmic variables and X-ray transients.
Fizeau interferometry at the Large Binocular Telescope (LBT) offers significant advantages over other facilities in terms of spatial resolution, field of view, and sensitivity. We provide an update of the LINC-NIRVANA project, which aims to bring a near-infrared and visible wavelength Fizeau beam combiner to the LBT by late 2005. As with any complex instrument, a number of detailed requirements drive the final design adopted.
The I. Physikalische Institut of the University of Cologne is participating in an international collaboration with the Max-Planck-Institut fur Astronomie in Heidelberg and the Osservatorio Astrofisico di Arcetri for the development of LINC/NIRVANA, the Near-Infrared/Visible Interferometric Camera for the Large Binocular Telescope (LBT). LINC/NIRVANA will be one of the two interferometric camera systems of the LBT and will operate at wavelengths from 0.6 μm to 2.4 μm, with the long wavelength regime between 1.0 μm and 2.5 μm being covered by LINC (LBT INterferometric Camera} and the shorter wavelengths part from 0.6 μm to 1.0 μm being processed by NIRVANA (Near-InfraRed/Visible Adaptive iNterferometer for Astronomy}.
The main contributions of the Cologne institute to this camera will be the 77K dewar and the Fringe and Flexure Tracker (FFT) for the near-infrared part on the system. Detecting and correcting the fast pistonic aberrations of the atmosphere and the slow flexure of the instrument in a closed-loop operation, the presence and proper function of the FFT is mandatory for a time-stable image quality at highest interferometic resolutions. In order to get the best possible image correction for LINC, the FFT will be located inside the camera dewar at an interferometric focus close the one of the near-infrared science detector. Using simple optical elements it will continuously monitor the time-variable phase difference and pupil locations of the incoming wavefronts from the two arms of the twin-telescope.
In this article we give a short overview of the camera concept of LINC and present the current status of the design and development of the FFT going on at our institute at the University of Cologne.
Stellar proper motions, radial velocities and accelarations obtained with high angular resolution techniques over the past decade have convincingly proven the presence of a central compact dark mass of 3x106 M. This mass is most likely associated with the compact radio source Sagittarius A* and represents one of the best candidates for a super massive Black Hole.
This contribution summarizes some important observational facts and outlines the future possibilities for interferometric observations of the Galactic Center. In the near future interferometric observations of that region with the LBT, VLTI and the Keck Interferometer will be possible. Detailed measurements of the stellar orbits close to the center will allow us to precisely determine the compactness, extent and shape of any extended mass contribution e.g. due to a central stellar cusp. Emphasis will be put on the potential of the NIR LBT interferometric camera LINC. Given the combination of large telescope apertures, adaptive optics, and interferometry it is likely that stars with orbital time scales of the order of one year will be detected. Theses sources, however, will most likely not be on simple Keplerian orbits. The effects of measurable prograde relativistic and retrograde Newtonian periastron shifts will result in rosetta shaped orbits. An increased interferometric point source sensitivity will also allow for an effective search and monitoring of an IR counterpart of SgrA*.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.