MICADO will enable the ELT to perform diffraction limited near-infrared observations at first light. The instrument’s capabilities focus on imaging (including astrometric and high contrast) as well as single object spectroscopy. This contribution looks at how requirements from the observing modes have driven the instrument design and functionality. Using examples from specific science cases, and making use of the data simulation tool, an outline is presented of what we can expect the instrument to achieve.
ERIS is an instrument that will both extend and enhance the fundamental diffraction limited imaging and spectroscopy capability for the VLT. It will replace two instruments that are now being maintained beyond their operational lifetimes, combine their functionality on a single focus, provide a new wavefront sensing module that makes use of the facility Adaptive Optics System, and considerably improve their performance. The instrument will be competitive with respect to JWST in several regimes, and has outstanding potential for studies of the Galactic Center, exoplanets, and high redshift galaxies. ERIS had its final design review in 2017, and is expected to be on sky in 2020. This contribution describes the instrument concept, outlines its expected performance, and highlights where it will most excel.
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.
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 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 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.
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.
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.
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 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.
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.
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.
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.
GRAVITY is a second generation VLTI instrument, combining the light of four telescopes and two objects
simultaneously. The main goal is to obtain astrometrically accurate information. Besides correctly measured stellar
phases this requires the knowledge of the instrumental differential phase, which has to be measured optically during the
astronomical observations. This is the purpose of a dedicated metrology system. The GRAVITY metrology covers the
full optical path, from the beam combiners up to the reference points in the beam of the primary telescope mirror,
minimizing the systematic uncertainties and providing a proper baseline in astrometric terms. Two laser beams with a
fixed phase relation travel backward the whole optical chain, creating a fringe pattern in any plane close to a pupil. By
temporal encoding the phase information can be extracted at any point by means of flux measurements with photo
diodes. The reference points chosen sample the pupil at typical radii, eliminating potential systematics due differential
focus. We present the final design and the performance estimate, which is in accordance with the overall requirements
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.
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.
This paper aims at giving an update on the most versatile Adaptive Optics fed instrument to date, the well
known and successful NACO*. Although NACO is only scheduled for about two more years† at the Very Large
Telescope (VLT), it keeps on evolving with additional operation modes bringing original astronomical results.
The high contrast imaging community uses it creatively as a test-bench for SPHERE‡ and other second generation
planet imagers. A new visible wavefront sensor (WFS) optimized for Laser Guide Star (LGS) operations has
been installed and tested, the cube mode is more and more required for frame selection on bright sources, a
seeing enhancer mode (no tip/tilt correction) is now offered to provide full sky coverage and welcome all kind
of extragalactic applications, etc. The Instrument Operations Team (IOT) and Paranal engineers are currently
working hard at maintaining the instrument overall performances but also at improving them and offering new
capabilities, providing the community with a well tuned and original instrument for the remaining time it is
being used. The present contribution delivers a non-exhaustive overview of the new modes and experiments that
have been carried out in the past months.
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.
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.
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.
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.
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.
The Phase-Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) facility is scheduled for installation
in the Very Large Telescope Interferometer observatory in Paranal, Chile, in the second half of 2008. Its goal
is to provide an astrometric accuracy in the micro-arcsecond range. High precision astrometry can be applied
to explore the dynamics of the dense stellar cluster. Especially models for the formation of stars near super
massive black holes or the fast transfer of short-lived massive stars into the innermost parsec of our galaxy can
be tested. By measuring the orbits of stars close the the massive black hole one can probe deviations from a
Keplerian motion. Such deviations could be due to a swarm of dark, stellar mass objects that perturb the point
mass solution. At the same time the orbits are affected by relativistic corrections which thus can be tested. The
ultimate goal is to test the effects of general relativity in the strong gravitational field. The latter can be probed
with the near infrared flares of SgrA* which are most likely due to accretion phenomena onto the black hole.
We study the expected performance of PRIMA for astrometric measurements in the Galactic Center based on
laboratory measurements and discuss possible observing strategies.
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.
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.
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.
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.