Marcel Carbillet

The infra-red dual imaging and spectrograph for SPHERE: design and performance

The SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) planet finder instrument for ESOs VLT telescope, scheduled for first light in 2011, aims to detect giant extra-solar planets in the vicinity of bright stars by the aid of an extreme-AO turbulence compensation system and to characterize the objects found through spectroscopic and polarimetric observations. Dual imaging observations within the Y, J, H and Ks atmospheric windows (~0.95 - 2.32μm) will be done by the aid of the IRDIS cryogenic camera. We describe briefly the science goals of IRDIS and present its system architecture. Current status of the instrument design is presented, and expected performance is described in terms of end-to-end simulations.

Shadows cast on the transition disk of HD 135344B - Multiwavelength VLT/SPHERE polarimetric differential imaging

Context. The protoplanetary disk around the F-type star HD 135344B (SAO 206462) is in a transition stage and shows many intriguing structures both in scattered light and thermal (sub-) millimeter emission which are possibly related to planet formation processes. Aims. We aim to study the morphology and surface brightness of the disk in scattered light to gain insight into the innermost disk regions, the formation of protoplanets, planet-disk interactions traced in the surface and midplane layers, and the dust grain properties of the disk surface. Methods. We have carried out high-contrast polarimetric differential imaging (PDI) observations with VLT/SPHERE and obtained polarized scattered light images with ZIMPOL in the R and I-bands and with IRDIS in the Y and J-bands. The scattered light images and surface brightness profiles are used to study in detail structures in the disk surface and brightness variations. We have constructed a 3D radiative transfer model to support the interpretation of several detected shadow features. Results. The scattered light images reveal with unprecedented angular resolution and sensitivity the spiral arms as well as the 25 au cavity of the disk. Multiple shadow features are discovered on the outer disk with one shadow only being present during the second observation epoch. A positive surface brightness gradient is observed in the stellar irradiation corrected (r(2)-scaled) images in southwest direction possibly due to an azimuthally asymmetric perturbation of the temperature and/or surface density by the passing spiral arms. The disk integrated polarized flux, normalized to the stellar flux, shows a positive trend towards longer wavelengths which we attribute to large (2 pi alpha \textgreater= lambda) aggregate dust grains in the disk surface. Part of the non-azimuthal polarization signal in the U-phi image of the J-band observation can be attributed to multiple scattering in the disk. Conclusions. The detected shadow features and their possible variability have the potential to provide insight into the structure of and processes occurring in the innermost disk regions. Possible explanations for the presence of the shadows include a 22 degrees misaligned inner disk, a warped disk region that connects the inner disk with the outer disk, and variable or transient phenomena such as a perturbation of the inner disk or an asymmetric accretion flow. The spiral arms are best explained by one or multiple protoplanets in the exterior of the disk although no gap is detected beyond the spiral arms up to 1.0.

SPHERE ZIMPOL: overview and performance simulation

The ESO planet finder instrument SPHERE will search for the polarimetric signature of the reflected light from extrasolar planets, using a VLT telescope, an extreme AO system (SAXO), a stellar coronagraph, and an imaging polarimeter (ZIMPOL). We present the design concept of the ZIMPOL instrument, a single-beam polarimeter that achieves very high polarimetric accuracy using fast polarization modulation and demodulating CCD detectors. Furthermore, we describe comprehensive performance simulations made with the CAOS problem-solving environment. We conclude that direct detection of Jupiter-sized planets in close orbit around the brightest nearby stars is achievable with imaging polarimetry, signal-switching calibration, and angular differential imaging.

Diamonds in HD 97048: A Closer Look

We present adaptive optics high angular resolution (~01) spectroscopic observations in the 3 μm region of the Herbig Ae/Be star HD 97048. For the first time, we spatially resolve the emission in the diamond features at 3.43 and 3.53 μm and in the adjacent continuum. Using both the intensity profiles along the slit and reconstructed two-dimensional images of the object, we derive FWHM sizes consistent with the predictions for a circumstellar disk seen pole-on. The diamond emission originates in the inner region (R 15 AU) of the disk.

Detection of a Moving Source in Speckle Noise. Application to Exoplanet Detection

Astronomical instruments able to detect the direct light of extra solar planets are currently under development. This paper focuses on instruments that will acquire a set of successive images where the planet (the source in more general purposes) moves in a known manner on a speckled background. Performant signal processing tools are required to account for the very low signal-to-noise ratio of the data. In the astrophysical context, the background arises mainly from the light scattered by the parent star. An accurate-but general-data model has been proposed in previous works to statistically describe optical images taking into account the spatial correlation of the wavefront complex amplitude. First, an estimator of the position and the intensity of the potential source is proposed. Because of several kinds of numerical constraints, it is derived from a simplified Gaussian data model. Under reasonable constraints on the motion of the source, the estimators are proved to be consistent, even under the accurate data model. For the detection test, we propose to threshold a linear statistics that arises from the intensity estimation. The threshold needs to be precisely related to the probability of false alarm (PFA) and the probability of detection (PD). Under the detailed model, the distribution of the data is only reachable through its moment generating function. We propose therefore to estimate analytically PFA and PD using the saddlepoint approximation. To evaluate the quality of these estimations, a Monte Carlo analysis is applied to monodimensional simulated data. The saddlepoint approximation proves to be very accurate, unlike the Gaussian approximation or even a low-order Gram-Charlier series approximation.

End to End Simulation of AO-assisted coronagraphic differential imaging: estimation of performance for SPHERE

SPHERE (Spectro Polarimetric High contrast Exoplanet REsearch), the planet finder instrument for the VLT is designed to study relatively bright extrasolar giant planets around young or nearby stars. SPHERE is a set of three instruments fed by the same AO-system, two of them share the same coronagraph. This complex system has been modeled with Fourier Optics to investigate the performance of the whole instrument. In turns, this end-to-end model was useful to analyze the sensitivity to various parameters (WFE, alignment of the coronagraph, differential aberrations) and to put some specifications on the sub-systems. This paper presents some example of sensitivity analysis and some contrast performance of the instruments as a function of the flux for the main observing mode of SPHERE: the Dual Band Imaging (DBI), equivalent to the Spectral Differential Imaging technique.

The CAOS problem-solving environment: recent developments

We present recent developments of the CAOS problem-solving environment (PSE), an IDL-based software tool whose original aim was to define and simulate as realistically as possible the behavior of a generic adaptive optics (AO) system -from the atmospheric propagation of light, to the sensing of the wave-front aberrations and the correction through a deformable mirror- but which results in a widely more general tool now. In fact, the different developments made through the last years result in a very versatile numerical tool complete of a global graphical interface (the CAOS Application Builder), a general utilities library (the CAOS Library), and different packages dedicated to a wide range of astronomical-optics-related scientific topics: the original package designed for end-to-end AO system simulations (the Software Package CAOS), an image simulation/ reconstruction package with interferometric capabilities (the Software Package AIRY), an extension of the latter specialized for the LBT instrument LINC-NIRVANA (the Software Package AIRY-LN), an ad hoc package dedicated to the VLT instrument SPHERE (the Software Package SPHERE), and an embedment of the analytical AO simulation code PAOLA (the Software Package PAOLAC).We present the status of the whole CAOS PSE, together with the most recent developments, and plans for the future of the overall tool.

Layer-oriented wavefront sensor for MAD: status and progress report

A demonstrator of the multi-conjugate adaptive optics concept is under construction at ESO and will be installed on the Nasmyth focus of the VLT. This demonstrator called MAD will have two different wavefront sensor channels: Shack-Hartmann and Layer-Oriented; in this article we only describe the Layer-Oriented one. The Layer-Oriented wavefront sensor can select eight reference stars in the two arc-minutes corrected field of view in order to have a maximum of two references in each quarter of the field. XY stages will remotely adjust the position of each reference star selector. The starlight will be fed onto two detectors and two completely independent loops will drive the deformable mirrors, one conjugated to the ground and the other to an altitude of approximately 8 km. The Layer-Oriented wavefront sensor will use the same CCDs than the Shack-Hartmann channel and the pupil will be divided into 9×9 subapertures both for the high and for the ground layer. The spatial sampling of the subapertures will be different for the two CCDs and their integration time will be tuned to typical values of the conjugated altitudes characteristic wind speed. The overall status of the instrument with respect to optics, mechanics, electronics and software is given hereafter. We also summarize the progress on the procurement phase and give the time schedule for the assembling, integration and testing phases.

Fine cophasing of segmented aperture telescopes with ZELDA, a Zernike wavefront sensor in the diffraction-limited regime

Segmented aperture telescopes require an alignment procedure with successive steps from coarse alignment to monitoring process in order to provide very high optical quality images for stringent science operations such as exoplanet imaging. The final step, referred to as fine phasing, calls for a high sensitivity wavefront sensing and control system in a diffraction-limited regime to achieve segment alignment with nanometric accuracy. In this context, Zernike wavefront sensors represent promising options for such a calibration. A concept called the Zernike unit for segment phasing (ZEUS) was previously developed for ground-based applications to operate under seeing-limited images. Such a concept is, however, not suitable for fine cophasing with diffraction-limited images. We revisit ZELDA, a Zernike sensor that was developed for the measurement of residual aberrations in exoplanet direct imagers, to measure segment piston, tip, and tilt in the diffraction-limited regime. We introduce a novel analysis scheme of the sensor signal that relies on piston, tip, and tilt estimators for each segment, and provide probabilistic insights to predict the success of a closed-loop correction as a function of the initial wavefront error. The sensor unambiguously and simultaneously retrieves segment piston and tip-tilt misalignment. Our scheme allows for correction of these errors in closed-loop operation down to nearly zero residuals in a few iterations. This sensor also shows low sensitivity to misalignment of its parts and high ability for operation with a relatively bright natural guide star. Our cophasing sensor relies on existing mask technologies that make the concept already available for segmented apertures in future space missions.

Deconvolution methods for LINC/NIRVANA data reduction

LINC/NIRVANA (LN) is the German-Italian beam combiner for the Large Binocular Telescope (LBT). It is a Fizeau interferometer and it will provide multiple images of the same target corresponding to different orientations of the baseline. For each one of these images the resolution is not uniform over the field since it is the resolution of a 22.8m mirror in the direction of the baseline and that of a 8.4m mirror in the orthogonal one. Therefore a unique high-resolution image can only be obtained by means of deconvolution methods. Four-years ongoing work of our group on this problem has already clarified the effects of partial adaptive optics corrections and partial coverage of the u,v plane and has produced the Software Package AIRY, a set of modules IDL-based and CAOS-compatible, which can be used for simulation and/or deconvolution of multiple images from the LBT instrument LN. In this paper we present a general approach to the design of methods for the simultaneous deconvolution of multiple images of the same object. These can include both quick-look methods, to be used for routinely process LN images, and ad-hoc methods for specific classes of astronomical objects. We describe several examples of these methods whose implementation and validation is in progress. Finally we present the last version of the Software Package AIRY.