Kjetil Dohlen

Achromatic dual-zone phase mask stellar coronagraph

This paper presents a generalization of the Roddier & Roddier Phase Mask coronagraph for polychromatic obser- vations. It is shown that using a dual-zone phase mask, combined with complex apodization, both phase and size chromatism can be compensated simultaneously to produce high extinction of a point source over large bandwidths, for example the entire K band with a residual integrated starlight of 3.2 × 10 −4 and a star intensity level of 10 −6 at an angular separation of 3λ/D. Other advantages of the proposed technique include the compatibility with centrally obscured telescopes, absence of blind axes and no symmetrization of the images.

First light of the VLT planet finder SPHERE IV : Physical and chemical properties of the planets around HR8799

Context. The system of fourplanets discovered around the intermediate-mass star HR8799 offers a unique opportunity to test planet formation theories at large orbital radii and to probe the physics and chemistry at play in the atmospheres of self-luminous young (~30 Myr) planets. We recently obtained new photometry of the four planets and low-resolution (R ~ 30) spectra of HR8799 d and e with the SPHERE instrument (Paper III). n nAims. In this paper (Paper IV), we aim to use these spectra and available photometry to determine how they compare to known objects, what the planet physical properties are, and how their atmospheres work. n nMethods. We compare the available spectra, photometry, and spectral energy distribution (SED) of the planets to field dwarfs and young companions. In addition, we use the extinction from corundum, silicate (enstatite and forsterite), or iron grains likely to form in the atmosphere of the planets to try to better understand empirically the peculiarity of their spectrophotometric properties. To conclude, we use three sets of atmospheric models (BT-SETTL14, Cloud-AE60, Exo-REM) to determine which ingredients are critically needed in the models to represent the SED of the objects, and to constrain their atmospheric parameters (T_(eff), logu2009g, M/H). n nResults. We find that HR8799d and e properties are well reproduced by those of L6-L8 dusty dwarfs discovered in the field, among which some are candidate members of young nearby associations. No known object reproduces well the properties of planets b and c. Nevertheless, we find that the spectra and WISE photometry of peculiar and/or young early-T dwarfs reddened by submicron grains made of corundum, iron, enstatite, or forsterite successfully reproduce the SED of these planets. Our analysis confirms that only the Exo-REM models with thick clouds fit (within 2σ) the whole set of spectrophotometric datapoints available for HR8799 d and e for T_(eff) = 1200 K, logu2009g in the range 3.0−4.5, and M/H = +0.5. The models still fail to reproduce the SED of HR8799c and b. The determination of the metallicity, logu2009g, and cloud thickness are degenerate. n nConclusions. Our empirical analysis and atmospheric modelling show that an enhanced content in dust and decreased CIA of H_2 is certainly responsible for the deviation of the properties of the planet with respect to field dwarfs. The analysis suggests in addition that HR8799c and b have later spectral types than the two other planets, and therefore could both have lower masses.

Design, analysis, and testing of a microdot apodizer for the Apodized Pupil Lyot Coronagraph. II. The dot size impact. (Research Note)

Context. The Apodized Pupil Lyot Coronagraph (APLC) is a promising coronagraphic device for direct exoplanet detection with the European Extremely Large Telescope. This concept features amplitude apodization in the entrance aperture, and a small opaque Lyot mask in the focal plane. We present new near-IR laboratory results using binary apodizersxa0– the so-called microdot apodizerxa0– which represent a very attractive and advantageous solution for the APLC. Aims. Microdot apodizers introduce high-frequency noise whose characteristics depend on the pixel size. The aim of this work is to characterize the impact of the pixel size on the coronagraphic image. We aim to estimate both the noise intensity and its localization in the field of view. Methods. The microdot apodizer, consisting of an array of pixels with spatially variable density that are either opaque or transparent, was manufactured by lithography of a light-blocking metal layer deposited on a transparent substrate. A set of 5xa0masks has been designed with different pixel sizes, tested in the near-IR, and their behavior compared to theoretical models. Results. Stray light diffraction introduced by the finite pixel size was measured during experiments. The intensity decreases, and radial distance increases, when the pixel size gets smaller. Conclusions. The physical properties of these microdot apodizers have been demonstrated in the laboratory. The microdot apodizer is a suitable solution for any coronagraph using pupil amplitude apodization, if properly designed.

On-sky multiwavelength phasing of segmented telescopes with the Zernike phase contrast sensor

Future extremely large telescopes will adopt segmented primary mirrors with several hundreds of segments. Cophasing of the segments together is essential to reach high wavefront quality. The phasing sensor must be able to maintain very high phasing accuracy during the observations, while being able to phase segments dephased by several micrometers. The Zernike phase contrast sensor has been demonstrated on-sky at the Very Large Telescope. We present the multiwavelength scheme that has been implemented to extend the capture range from ±λ/2 on the wavefront to many micrometers, demonstrating that it is successful at phasing mirrors with piston errors up to ±4.0u2009 μm on the wavefront. We discuss the results at different levels and conclude with a phasing strategy for a future extremely large telescope.

Active Optics: stress polishing of toric mirrors for the VLT SPHERE adaptive optics system

The manufacturing of toric mirrors for the Very Large Telescope-Spectro-Polarimetric High-Contrast Exoplanet Research instrument (SPHERE) is based on Active Optics and stress polishing. This figuring technique allows minimizing mid and high spatial frequency errors on an aspherical surface by using spherical polishing with full size tools. In order to reach the tight precision required, the manufacturing error budget is described to optimize each parameter. Analytical calculations based on elasticity theory and finite element analysis lead to the mechanical design of the Zerodur blank to be warped during the stress polishing phase. Results on the larger (366 mm diameter) toric mirror are evaluated by interferometry. We obtain, as expected, a toric surface within specification at low, middle, and high spatial frequencies ranges.

EPICS: the exoplanet imager for the E-ELT

Presently, dedicated instrument developments at large telescopes (SPHERE for the VLT, GPI for Gemini) are about to discover and explore self-luminous giant planets by direct imaging and spectroscopy. The next generation of 30m-40m ground-based telescopes, the Extremely Large Telescopes (ELTs), have the potential to dramatically enlarge the discovery space towards older giant planets seen in reflected light and ultimately even a small number of rocky planets. EPICS is a proposed instrument for the European ELT, dedicated to the detection and characterization of expolanets by direct imaging and spectroscopy. ESO recently launched a phase-A study for EPICS with a large European consortium which - by simulations and demonstration experiments - will investigate state-of-the-art diffraction and speckle suppression techniques to deliver highest contrasts. The final result of the study in 2010 will be a conceptual design and a development plan for the instrument. Here we present first results from the phase-A study and discuss the main challenges and science capabilities of EPICS.

Simulation of planet detection with the SPHERE integral field spectrograph

Aims. We present simulations of the perfomances of the future SPHERE IFS instrument designed for imaging extrasolar planets in the near infrared (Y, J, and H bands). Methods. We used the IDL package code for adaptive optics simulation (CAOS) to prepare a series of input point spread functions (PSF). These feed an IDL tool (CSP) that we designed to simulate the datacube resulting from the SPHERE IFS. We performed simulations under different conditions to evaluate the contrast that IFS will be able to reach and to verify the impact of physical propagation within the limits of the near field of the aperture approximation (i.e. Fresnel propagation). We then performed a series of simulations containing planet images to test the capability of our instrument to correctly classify the found objects. To this purpose we developed a separated IDL tool. Results. We found that using the SPHERE IFS instrument and appropriate analysis techniques, such as multiple spectral differential imaging (MDI), spectral deconvolution (SD), and angular differential imaging (ADI), we should be able to image companion objects down to a luminosity contrast of similar to 10(-7) with respect to the central star in favorable cases. Spectral deconvolution resulted in the most effective method for reducing the speckle noise. We were then able to find most of the simulated planets (more than 90% with the Y-J-mode and more than the 95% with the Y-H-mode) for contrasts down to 3 x 10(-7) and separations between 0.3 and 1.0 arcsec. The spectral classification is accurate but seems to be more precise for late T-type spectra than for earlier spectral types. A possible degeneracy between early L-type companion objects and field objects (flat spectra) is highlighted. The spectral classification seems to work better using the Y-H-mode than with the Y-J-mode.Aims. We presentsimulations of the perfomances of the future SPHERE IFS instrument desig n d for imaging extrasolar planets in the near infrared (Y, J, and H bands). Methods. We used the IDL package code for adaptive optics simulation ( CAOS) to prepare a series of input point spread functions (PSF). These feed an IDL tool (CSP) that we designed to simula te the datacube resulting from the SPHERE IFS. We performed simulations under di fferent conditions to evaluate the contrast that IFS will be ab l to reach and to verify the impact of physical propagation within the limits of the near field of the apertur e approximation (i.e. Fresnel propagation). We then perfor med a series of simulations containing planet images to test the capabilit y of our instrument to correctly classify the found objects. To this purpose we developed a separated IDL tool. Results. We found that using the SPHERE IFS instrument and appropriat e analysis techniques, such as multiple spectral di fferential imaging (MDI), spectral deconvolution (SD), and angular di fferential imaging (ADI), we should be able to image companion objects down to a luminosity contrast of ∼ 10−7 with respect to the central star in favorable cases. Spectra l deconvolution resulted in the most effective method for reducing the speckle noise. We were then ab le to find most of the simulated planets (more than 90% with the Y-J-mode and more than the 95% with the Y-H-mode) for contras ts down to 3× 10−7 and separations between 0.3 and 1.0 arcsec. The spectral classification is accurate but seems to be more p recise for late T-type spectra than for earlier spectral typ es. A possible degeneracy between early L-type companion objects and field objects (flat spectra) is highlighted. The spectral classifi cation seems to work better using the Y-H-mode than with the Y-J-mode.

Design of the extreme AO system for SPHERE, the planet-finder instrument of the VLT

The SPHERE system aims at the detection of extremely faint sources (giant extra-solar planet) in the vinicity of bright stars. Such a challenging goal automatically requires the use of a coronagraphic device to cancel out the flux coming from the star and smart imaging technics which have to be added to reach the required contrast for exo-planet detection (typically 10-6 - 10-7 in contrast). In this frame of the SPHERE project a global system study has demonstrated the feasibility of an AO system for the direct exoplanets detection. A detailed description of this system is proposed in this paper. The main trade-offs are discussed and justified and all the subsystems briefly presented. The realization phase has begun in 2006 and we foresee to obtain a first light at the VLT in 2010.

The SPHERE XAO system SAXO: integration, test, and laboratory performance

Direct detection and spectral characterization of extra-solar planets is one of the most exciting and challenging areas in modern astronomy due to the very large contrast between the host star and the planet at very small angular separations. SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research in Europe) is a second-generation instrument for the ESO VLT dedicated to this scientific objective. It combines an extreme adaptive optics system, various coronagraphic devices and a suite of focal instruments providing imaging, integral field spectroscopy and polarimetry capabilities in the visible and near-infrared spectral ranges. The extreme Adaptive Optics (AO) system, SAXO, is the heart of the SPHERE system, providing to the scientific instruments a flat wavefront corrected from all the atmospheric turbulence and internal defects. We present an updated analysis of SAXO assembly, integration and performance. This integration has been defined in a two step process. While first step is now over and second one is ongoing, we propose a global overview of integration results. The main requirements and system characteristics are briefly recalled, then each sub system is presented and characterized. Finally the full AO loop first performance is assessed. First results demonstrate that SAXO shall meet its challenging requirements.

Experimental results with a second-generation Roddier & Roddier phase mask coronagraph

Context. Coronagraphic techniques are required to observe substellar mass companions close to nearby bright stars by direct imagery. Phase mask coronagraphs are particularly interesting because they give access to the innermost regions. While the principle of the first such concept was validated experimentally a decade ago, the achieved brightness attenuation was too low to be conclusive, probably due to the imperfect thickness profile of the mask. Aims. We have manufactured and tested a second-generation Roddier & Roddier coronagraph in preparation for the development of more elaborate phase mask designs, planned to be used in the future European Extremely Large Telescope. Methods. A monolithic phase mask was made by ion beam machining. Experimentally obtained coronagraphic images were compared with simulated images. Results. Good agreement with theory was obtained. A peak attenuation of 216 was achieved, and a contrast of ∼10 −5 was measured at 5.7 λ/D. The results exploring contrasts obtained at different distances from the star for different mask dimensions are particularly interesting, confirming predictions made in the literature.