Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userOptimizing the subwavelength grating of L-band Annular Groove Phase Masks for high coronagraphic performance
68
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Context. The Annular Groove Phase Mask (AGPM) is one possible implementation of the vector vortex coronagraph, where the helical phase ramp is produced by a concentric subwavelength grating. For several years, we have been manufacturing AGPMs by etching gratings into synthetic diamond substrates using inductively coupled plasma etching. Aims. We aim to design, fabricate, optimize, and evaluate new L-band AGPMs that reach the highest possible coronagraphic performance, for applications in current and forthcoming infrared high-contrast imagers. Methods. Rigorous coupled wave analysis (RCWA) is used for designing the subwavelength grating of the phase mask. Coronagraphic performance evaluation is performed on a dedicated optical test bench. The experimental results of the performance evaluation are then used to accurately determine the actual profile of the fabricated gratings, based on RCWA modeling. Results. The AGPM coronagraphic performance is very sensitive to small errors in etch depth and grating profile. Most of the fabricated components therefore show moderate performance in terms of starlight rejection (a few 100:1 in the best cases). Here we present new processes for re-etching the fabricated components in order to optimize the parameters of the grating and hence significantly increase their coronagraphic performance. Starlight rejection up to 1000:1 is demonstrated in a broadband L filter on the coronagraphic test bench, which corresponds to a raw contrast of about 1e-5 at two resolution elements from the star for a perfect input wave front on a circular, unobstructed aperture. Conclusions. Thanks to their exquisite performance, our latest L-band AGPMs are good candidates for installation in state-of-the-art and future high-contrast thermal infrared imagers, such as METIS for the E-ELT.
rate research
Read More
Phase-mask coronagraphs are known to provide high contrast imaging capabilities while preserving a small inner working angle, which allows searching for exoplanets or circumstellar disks with smaller telescopes or at longer wavelengths. The AGPM (Annular Groove Phase Mask, Mawet et al. 2005) is an optical vectorial vortex coronagraph (or vector vortex) induced by a rotationally symmetric subwavelength grating (i.e. with a period smaller than {lambda}/n, {lambda} being the observed wavelength and n the refractive index of the grating substrate). In this paper, we present our first mid- infrared AGPM prototypes imprinted on a diamond substrate. We firstly give an extrapolation of the expected coronagraph performances in the N-band (~10 {mu}m), and prospects for down-scaling the technology to the most wanted L- band (~3.5 {mu}m). We then present the manufacturing and measurement results, using diamond-optimized microfabrication techniques such as nano-imprint lithography (NIL) and reactive ion etching (RIE). Finally, the subwavelength grating profile metrology combines surface metrology (scanning electron microscopy, atomic force microscopy, white light interferometry) with diffractometry on an optical polarimetric bench and cross correlation with theoretical simulations using rigorous coupled wave analysis (RCWA).
Quasi-static aberrations in coronagraphic systems are the ultimate limitation to the capabilities of exoplanet imagers both ground-based and space-based. These aberrations - which can be due to various causes such as optics alignment or moving optical parts during the observing sequence - create light residuals called speckles in the focal plane that might be mistaken for a planets. For ground-based instruments, the presence of residual turbulent wavefront errors due to partial adaptive optics correction causes an additional difficulty to the challenge of measuring aberrations in the presence of a coronagraph. In this paper, we present an extension of COFFEE, the coronagraphic phase diversity, to the estimation of quasi-static aberrations in the presence of adaptive optics-corrected residual turbulence. We perform realistic numerical simulations to assess the performance that can be expected on an instrument of the current generation. We perform the first experimental validation in the laboratory which demonstrates that quasistatic aberrations can be corrected during the observations by means of coronagraphic phase diversity.
The vortex coronagraph is an optical instrument that precisely removes on-axis starlight allowing for high contrast imaging at small angular separation from the star, thereby providing a crucial capability for direct detection and characterization of exoplanets and circumstellar disks. Telescopes with aperture obstructions, such as secondary mirrors and spider support structures, require advanced coronagraph designs to provide adequate starlight suppression. We introduce a phase-only Lyot-plane optic to the vortex coronagraph that offers improved contrast performance on telescopes with complicated apertures. Potential solutions for the European Extremely Large Telescope (E-ELT) are described and compared. Adding a Lyot-plane phase mask relocates residual starlight away from a region of the image plane thereby reducing stellar noise and improving sensitivity to off-axis companions. The phase mask is calculated using an iterative phase retrieval algorithm. Numerically, we achieve a contrast on the order of $10^{-6}$ for a companion with angular displacement as small as $4~lambda/D$ with an E-ELT type aperture. Even in the presence of aberrations, improved performance is expected compared to either a conventional vortex coronagraph or optimized pupil plane phase element alone.
Context. The next generation of space-borne instruments dedicated to the direct detection of exoplanets requires unprecedented levels of wavefront control precision. Coronagraphic wavefront sensing techniques for these instruments must measure both the phase and amplitude of the optical aberrations using the scientific camera as a wavefront sensor. Aims. In this paper, we develop an extension of coronagraphic phase diversity to the estimation of the complex electric field, that is, the joint estimation of phase and amplitude. Methods. We introduced the formalism for complex coronagraphic phase diversity. We have demonstrated experimentally on the Tr`es Haute Dynamique testbed at the Observatoire de Paris that it is possible to reconstruct phase and amplitude aberrations with a subnanometric precision using coronagraphic phase diversity. Finally, we have performed the first comparison between the complex wavefront estimated using coronagraphic phase diversity (which relies on time-modulation of the speckle pattern) and the one reconstructed by the self-coherent camera (which relies on the spatial modulation of the speckle pattern). Results. We demonstrate that coronagraphic phase diversity retrieves complex wavefront with subnanometric precision with a good agreement with the reconstruction performed using the self-coherent camera. Conclusions. This result paves the way to coronagraphic phase diversity as a coronagraphic wave-front sensor candidate for very high contrast space missions.
Despite promising astrometric signals, to date there has been no success in direct imaging of a hypothesized third member of the Sirius system. Using the Clio instrument and MagAO adaptive optics system on the Magellan Clay 6.5 m telescope, we have obtained extensive imagery of Sirius through a vector apodizing phase plate (vAPP) coronagraph in a narrowband filter at 3.9 microns. The vAPP coronagraph and MagAO allow us to be sensitive to planets much less massive than the limits set by previous non-detections. However, analysis of these data presents challenges due to the targets brightness and unique characteristics of the instrument. We present a comparison of dimensionality reduction techniques to construct background illumination maps for the whole detector using the areas of the detector that are not dominated by starlight. Additionally, we describe a procedure for sub-pixel alignment of vAPP data using a physical-optics-based model of the coronagraphic PSF.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
