The Coronagraph Instrument (CGI) for NASA's Wide Field Infrared Survey Telescope (WFIRST) will constitute a dramatic step forward for high-contrast imaging, integral field spectroscopy, and polarimetry of exoplanets and circumstellar disks, aiming to improve upon the sensitivity of current ground-based direct imaging facilities by 2-3 orders of magnitude. Furthermore, CGI will serve as a pathfinder for future exo-Earth imaging and characterization missions by demonstrating wavefront control, coronagraphy, and spectral retrieval in a new contrast regime, and by validating instrument and telescope models at unprecedented levels of precision. To achieve this jump in performance, it is critical to draw on the experience of ground-based high-contrast facilities. We discuss several areas of relevant commonalities, including: wavefront control, post-processing of integral field unit data, and calibration and observing strategies.
The SPHERE instrument, dedicated to high contrast imaging on VLT, has been routinely operated for more than 3 years, over a large range of conditions and producing observations from visible to NIR. A central part of the instrument is the high order adaptive optics system, named SAXO, designed to deliver high Strehl image quality with a balanced performance budget for bright stars up to magnitude R=9.
We take benefit now from the very large set of observations to revisit the assumptions and analysis made at the time of the design phase: we compare the actual AO behavior as a function of expectations. The data set consists of the science detector data, for both coronagraphic images and non-coronagraphic PSF calibrations, but also of AO internal data from the high frequency sensors and statistics computations from the real-time computer which are systematically archived, and finally of environmental data, monitored at VLT level. This work is supported and made possible by the SPHERE « Data Center » infrastructure hosted at Grenoble which provides an efficient access and the capability for the homogeneous analysis of this large and statistically-relevant data set.
We review in a statistical manner the actual AO performance as a function of external conditions for different regimes and we discuss the possible performance metrics, either derived from AO internal data or directly from the high contrast images. We quantify the dependency of the actual performance on the most relevant environmental parameters. By comparison to earlier expectations, we conclude on the reliability of the usual AO modeling. We propose some practical criteria to optimize the queue scheduling and the expression of observer requirements ; finally, we revisit what could be the most important AO specifications for future high contrast imagers as a function of the primary science goals, the targets and the turbulence properties.
We present the current results of the astrometric characterization of the VLT planet finder SPHERE over 2 years of on-sky operations. We first describe the criteria for the selection of the astrometric fields used for calibrating the science data: binaries, multiple systems, and stellar clusters. The analysis includes measurements of the pixel scale and the position angle with respect to the North for both near-infrared subsystems, the camera IRDIS and the integral field spectrometer IFS, as well as the distortion for the IRDIS camera. The IRDIS distortion is shown to be dominated by an anamorphism of 0.60±0.02% between the horizontal and vertical directions of the detector, i.e. 6 mas at 1 arcsec. The anamorphism is produced by the cylindrical mirrors in the common path structure hence common to all three SPHERE science subsystems (IRDIS, IFS, and ZIMPOL), except for the relative orientation of their field of view. The current estimates of the pixel scale and North angle for IRDIS are 12.255±0.009 milliarcseconds/pixel for H2 coronagraphic images and -1.70±0.08°. Analyses of the IFS data indicate a pixel scale of 7.46±0.02 milliarcseconds/pixel and a North angle of -102.18±0.13°. We finally discuss plans for providing astrometric calibration to the SPHERE users outside the instrument consortium.
The major source of noise in high-contrast imaging is the presence of slowly evolving speckles that do not average with time. The temporal stability of the point-spread-function (PSF) is therefore critical to reach a high contrast with extreme adaptive optics (XAO) instruments. Understanding on which timescales the PSF evolves and what are the critical parameters driving the speckle variability allow to design an optimal observing strategy and data reduction technique to calibrate instrumental aberrations and reveal faint astrophysical sources. We have obtained a series of 52 min, AO-corrected, coronagraphically occulted, high-cadence (1.6Hz), H-band images of the star HR 3484 with the SPHERE (Spectro-Polarimeter High-contrast Exoplanet REsearch1) instrument on the VLT. This is a unique data set from an XAO instrument to study its stability on timescales as short as one second and as long as several tens of minutes. We find different temporal regimes of decorrelation. We show that residuals from the atmospheric turbulence induce a fast, partial decorrelation of the PSF over a few seconds, before a transition to a regime with a linear decorrelation with time, at a rate of several tens parts per million per second (ppm/s). We analyze the spatial dependence of this decorrelation within the well-corrected radius of the adaptive optics system and show that the linear decorrelation is faster at short separations. Last, we investigate the influence of the distance to the meridian on the decorrelation.
The surveys dedicated to the search for extrasolar planets with the recently installed extreme-AO, high contrast Planet Imagers generally include hundreds of targets, to be observed sometimes repeatedly, generally in Angular Differential Imaging Mode. Each observation has to fulfill several time-dependent constraints, which makes a manual elaboration of an optimized planning impossible. We have developed a software (SPOT), an easy to use tool with graphical interface that allows both long term (months, years) and dynamic (nights) optimized scheduling of such surveys, taking into account all relevant constraints.
Tests show that excellent schedules and high filling efficiencies can be obtained with execution times compatible with real-time scheduling, making possible to take in account complex constraints and to dynamically adapt planning to unexpected circumstances even during their execution. Moreover, such a tool is very valuable during survey preparations to build target lists and calendars.
SPOT could be easily adapted for scheduling observations other instruments or telescopes.
Direct imaging of exoplanet is one of the most exciting field of planetology today. The light coming from exoplanet orbiting their host star witnesses for the chemical composition of the atmosphere, and the potential biomarkers for life. However, the faint flux to be imaged, very close to the huge flux of the parent star, makes this kind of observation extremely difficult to perform from the ground. The direct imaging instruments (SPHERE , GPI ) are nowaday reaching lab maturity. Such instrument imply the coordination of XAO for atmospherical turbulence real-time correction, coronagraphy for star light extinction, IR Dual band camera, IFS, and visible polarimetry. The imaging modes include single and double difference (spectral and angular). The SPHERE project is now at the end of AIT phase. This paper presents the very last results obtained in laboratory, with realistic working conditions. These AIT results allows one to predict on-sky performance, that should come within the next weeks after re-installation at Very Large Telescope at Paranal.
In November 2012, we installed an L-band annular groove phase mask (AGPM) vector vortex coronagraph (VVC) inside NACO, the adaptive optics camera of ESO’s Very Large Telescope. The mask, made out of diamond subwavelength gratings has been commissioned, science qualified, and is now offered to the community. Here we report ground-breaking on-sky performance levels in terms of contrast, inner working angle, and discovery space. This new practical demonstration of the VVC, coming a few years after Palomar’s and recent record-breaking lab experiments in the visible (E. Serabyn et al. 2013, these proceedings), shows once again that this new-generation coronagraph has reached a high level of maturity.
In the framework of the understanding of extrasolar systems, the study of host stars is a fundamental point. We
need to understand the link between them and the presence of companions, i.e. what makes a star becoming
a host star. In this perspective, we used the instrument called VEGA, situated at Mount Wilson (California)
on the CHARA array to perform optical interferometric measurements. Interferometry at visible wavelengths
allows reaching very high spatial frequencies well adapted for very small (less than 1 millisecond of arc) angular
diameters. Therefore, we can access limb darkening measurements which is one of the very few directly measurable
constraints on the structure of the atmosphere of a star. From this we can derive stars fundamental
parameters. A precise measurement within spectral lines is also a very powerful tool to study the temperature
and density structure of the atmosphere of distant stars. Besides, the detection of exoplanets is also related to
this method. Combined with the radial velocity method and the transit method, one can study the atmosphere
of exoplanets and learn more about their internal structure. We started a large program of observations made of
40 stars hosting exoplanets and observable by VEGA/CHARA. We will measure their limb darkened diameters
and derive their parameters. We also aim at better understanding stellar noise sources like spots, and study
surface brightness relationships.
SPHERE, the ESO extra-solar planet imager for the VLT is aimed at the direct detection and spectral characterization of
extra-solar planets. Its whole design is optimized towards reaching the highest contrast in a limited field of view and at
short distances from the central star. SPHERE has passed its Final Design Review (FDR) in December 2008 and it is in
the manufacturing and integration phase. We review the most challenging specifications and expected performance of
this instrument; then we present the latest stage of the design chosen to meet the specifications, the progress in the
manufacturing as well as the integration and test strategy to insure gradual verification of performances at all levels.
High angular resolution imaging with adaptive optics (AO) has allowed significant progress in the study of disks and
companions around stars over the past decades. This technique is also expected to lead to major breakthroughs in the
next 10 years. We review the results obtained so far with AO and their impact on the understanding of how planetary
systems form and evolve.
The SPHERE is an exo-solar planet imager, which goal is to detect giant exo-solar planets in the vicinity of bright stars
and to characterize them through spectroscopic and polarimetric observations. It is a complete system with a core made
of an extreme-Adaptive Optics (AO) turbulence correction, pupil tracker and interferential coronagraphs. At its back
end, an Infra-Red Dual-beam Imaging and Spectroscopy science module and an integral field spectrograph work in
the Near Infrared (NIR) Y, J, H and Ks bands (0.95 - 2.32μm) and a high resolution polarization camera covers the
visible (0.6 - 0.9 μm) region. We describe briefly the science goals of the instrument and deduce the top-level
requirements. This paper presents the system architecture, and reviews each of the main sub-systems. The results of the
latest end-to-end simulations are shown and an update of the expected performance is given. The project has been
officially kicked-off in March 2006, it is presently undergoing Preliminary Design Review and is scheduled for 1st
light in early 2011. This paper reviews the present design of SPHERE but focuses on the changes implemented since
this project was presented the last time to this audience.
Amplitude apodization of a telescope's pupil can be used to reduce the diffraction rings (Airy rings) in the PSF to allow high contrast imaging. Rather than achieving this apodization by selectively removing light at the edges of the pupil, we propose to produce the desired apodized pupil by redistributing the pupil's light. This lossless apodization concept can yield a high contrast PSF which allows the efficient detection of Earth-sized planets around stars at ~10pc with a 2m visible telescope in space. We review the current status of a JPL-funded study of this concept for the Terrestrial Planet Finder (TPF) mission, including a lab experiment and extensive computer simulations.
We will present a design of a high contrast 3D spectrometer with the attractive characteristics to provide very low differential aberrations. This concept, very well suited for an instrument as planet finder, is based on our expertise in slicer unit. Thanks to a control of diffraction and roughness effect, this new concept will provide spectrograph entrance sub-slit with a high purity. Very low level of cross-talk signal is expected. Associated with the slicer unit, we will present a spectrograph concept allowing to guaranty a negligible level of differential aberration between wavelength in the spectrum.
In November 2001, the VLT has been equipped for the first time with an adaptive optics system, NAOS. NAOS has been designed to provide good image quality over a wide range of conditions, allowing thus a large variety of astrophysical programs, from Solar System to extragalactic studies. NAOS feeds a camera CONICA which provides imaging, coronagraphic, spectroscopic and polarimetric capabilities between 1 and 5 microns. NAOS and CONICA (hereafter NACO) have been commissioned over the past months. We present in this paper the first images recorded by NACO during the commissioning period, illustrating the capabilities of this new instrument.
The Adaptive Optics NIR Instrument NAOS-CONICA has been commissioned at the VLT (UT4) between November 2001 and March 2002. After summarizing the observational capabilities of this multimode instrument in combination with the powerful AO-system, we will present first on sky results of the instrumental performance for several non-direct imaging modes: High spatial resolution slit-spectroscopy in the optical and thermal NIR region has been tested. For compact sources below 2 arcsec extension, Wollaston prism polarimetry is used. For larger objects the linear polarization pattern can be analyzed by wire grids down to the diffraction limit. Coronographic masks are applied to optimize imaging and polarimetric capabilities. The cryogenic Fabry-Perot Interferometer in combination with an 8m-telescope AO-system is shown to be a powerful tool for imaging spectroscopy (3D-scans).
The 8-m class telescopes are now in full operation, while 100-m
baseline interferometers (VLTI, KeckI) are starting routine
operation too. A working group from the French high angular
resolution community tried to identify what could be our
post-VLT/VLTI instruments after 2010. Possible future instruments,
ground or space-based, can be split into three main categories:
Extremely large filled aperture telescopes, diluted
interferometric arrays for direct imaging, and diluted
interferometric arrays for aperture synthesis imaging. These
concepts are compared in terms of observing capabilities and
performances (spatial resolution, field of view, imaging
capability, sensitivity, photometric dynamical range, etc.),
technological issues (adaptive optics, phasing, instrument mount,
etc.) and R&D priorities.
We present and discuss the capabilities of the infrared polarimetric modes of the ESO-VLT adaptive optics system NAOS-CONICA. Commissioning results obtained both with wire-grids and Wollaston prisms are shown. In particular, NACO observations of the Calabash
reflection nebula are compared with earlier, seeing limited, results
obtained at ESO to illustrate the new potential offered by adaptive
optics assisted polarimetry on an 8m class telescope.
Deconvolution is a necessary tool for the exploitation of adaptive optics corrected images, because the correction is partial. The Maximum A Posteriori (MAP) framework is used to derive a deconvolution method (MISTRAL) that combines the data with our knowledge of the noise statistics as well as our prior information about the object and the variability of the Point Spread Function. The deconvolution of experimental and scientific data illustrates the capabilities of this method.
NAOS is the first adaptive optics system installed at the VLT 8m telescopes. It was designed, manufactured and tested by a french Consortium under an ESO contract, to provide compensated images to the high angular resolution IR spectro-imaging camera (CONICA) in the 1 to 5 μm spectral range. It is equipped with a 185 actuator deformable mirror, a tip/tilt mirror and two wavefront sensors, one in the visible and one in the near IR spectral range. It has been installed in November at the Nasmyth focus B of the VLT UT4. During the first light run in December 2001, NAOS has delivered a Strehl ratio of 50 under average seeing conditions for bright guide stars. The diffraction limit of the telescope has been achieved at 2.2 μm. The closed loop operation has been very robust under bad seeing conditions. It was also possible to obtain a substantial correction with mV=17.6 and mK=13.1 reference stars. The on-sky acceptance tests of NAOS-CONICA were completed in May 2002 and the instrument will be made available to the European astronomical community in October by ESO. This paper describes the system and present the on-sky performance in terms of Strehl ratio, seeing conditions and guide star magnitude.
We have designed, realized and tested a dedicated software tool defined so as to enable a wide non-specialist community to perform optimal adaptive optics observations with VLT instrument NAOS-CONICA. We first precise the requirements derived from NAOS complexity due to a large number of configurations, ESO/VLT operational policy including service mode observations and limited human interaction during observations, and astronomical observation requirements. We then present the developed software tool, so-called "Preparation Software", that couples a user-friendly interface that accepts observation conditions (including seeing, star magnitude etc...) and an elaborate simulator of adaptive optics based on the NAOS characteristics.
Observing at high angular resolution from the ground is not made possible with Adaptive Optics alone, and besides the turbulence residuals, atmospheric refraction, thermal background or instrument's mechanical flexures may also severely limit the gain of optical quality that AO techniques are supposed to provide. We describe here how NAOS, the newly installed AO system on the VLT, has been designed to accommodate for these unavoidable effects. In particular, beam chopping, flexures compensation and AO tracking on reference objects with a significant relative motion will be addressed. It will thus be shown how long term astronomical observations at the diffraction limit can be carried out with an AO system under regular ground level conditions, thanks to the implementation of original technical solutions.
In the following years, the VLTI will offer new observing capabilities at high angular resolution: a large number of observing nights with a better spatial frequencies coverage, improved sensibilities and accuracy, and various observing modes covering near and thermal infrared. However, both the choice between all the involved observing parameters and the still partial information of intensity interferometry (compared to classical imaging) require specific study, dedicated to each astronomical interest. We discuss here the interest of long baseline interferometry to investigate the circumstellar environment of stars, as they evolve from the pre-main sequence stage with massive disks to the main sequence tenuous `debris' disks. These transient environments give us very interesting information on the physics of planetary formation and the dynamics of young planetary systems. We will identify the specific information the long baseline interferometry can provide, and how it would impact this issue. We derive then the required observing capabilities and discuss how VLTI will fulfill such requirements: what will be the impact of VLTI on (proto)planetary disks studies, with which instrument and which observing mode?
NAOS is the adaptive optics system to be installed at one of the Nasmyth foci of the very large telescope (VLT). It will provide compensated image to the high angular resolution IR spectro-imaging camera which covers the 1-5 micrometers spectral bands. our French consortium is the sub-contractor of ESO for the design, manufacturing, integration and test of NAOS. For bright sources, the specification is to reach 70 percent Strehl ratio under average seeing conditions. Two wavefront sensors, one in the visible spectral range and one in the near IR spectral range, will equip the adaptive optics system. We foresee to obtained the first light at the VLT unit telescope 1 in mid-2000.
The combination of adaptive optics and chronographic techniques providing at the same time high angular resolution and high dynamic range is a powerful tool to study the close environment of bright objects in different fields of astrophysics. A stellar coronography dedicated to the ESO adaptive optics system ADONIS has been designed and manufactured by Observatoire de Grenoble and is now offered to the astronomical community. We present here the design and performance analysis of this new coronograph and illustrate its impact for a few astrophysical programs.
The design of adaptive optics system requires astrophysically driven requirements and specifications. We present here the study performed to help specifying and designing the ESO Nasmyth Adaptive Optics System of the VLT.