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We investigate the focal plane wavefront sensing technique, known as Phase Diversity, at the scientific focal plane of a segmented mirror telescope with an adaptive optics (AO) system. We specifically consider an optical system imaging a point source in the context of (i) an artificial source within the telescope structure and (ii) from AO-corrected images of a bright star. From our simulations, we reliably disentangle segmented telescope phasing errors from non-common path aberrations (NCPA) for both a theoretical source and on-sky, AO-corrected images where we have simulated the Keck/NIRC2 system. This quantification from on-sky images is appealing, as its sensitive to the cumulative wavefront perturbations of the entire optical train; disentanglement of phasing errors and NCPA is therefore critical, where any potential correction to the primary mirror from an estimate must contain minimal NCPA contributions. Our estimates require a one-minute sequence of short-exposure, AO-corrected images; by exploiting a slight modification to the AO-loop, we find that 75 defocused images produces reliable estimates. We demonstrate a correction from our estimates to the primary and deformable mirror results in a wavefront error reduction of up to 67% and 65% for phasing errors and NCPA, respectively. If the segment phasing errors on the Keck primary are on the order of ~130 nm RMS, we show we can improve the H-band Strehl ratio by up to 10% by using our algorithm. We conclude our technique works well to estimate NCPA alone from on-sky images, suggesting it is a promising method for any AO-system.
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Non Common Path Aberrations (NCPA) are often considered as a critical issue in Adaptive Optics (AO) systems, since they introduce bias errors between real wavefronts propagating to the science detectors and those measured by the Wavefront Sensor (WFS). This is especially true when the AO system is coupled to a coronagraph instrument intended for the discovery and characterization of extra-solar planets, because useful planet signals could be mistaken with residual speckles generated by NCPA. Therefore, compensating for those errors is of prime importance and is already the scope of a few theoretical studies and experimental validations on-sky. This communication presents the conceptual optical design of a pseudo-interferometer arrangement suitable to accurate NCPA calibration, based on two WFS cooperating in real-time. The concept is applicable to both classical imaging and spectroscopy assisted by AO, and to high-contrast coronagraphs searching for habitable extra-solar planets. Practical aspects are discussed, such as the choice of WFS and coronagraph types, or specific requirements on additional hardware components, e.g. dichroic beamsplitters
The two main advantages of exoplanet imaging are the discovery of objects in the outer part of stellar systems -- constraining models of planet formation --, and its ability to spectrally characterize the planets -- information on their atmosphere. It is however challenging because exoplanets are up to 1e10 times fainter than their star and separated by a fraction of arcsecond. Current instruments like SPHERE/VLT or GPI/Gemini detect young and massive planets because they are limited by non-common path aberrations (NCPA) that are not corrected by the adaptive optics system. To probe fainter exoplanets, new instruments capable of minimizing the NCPA is needed. One solution is the self-coherent camera (SCC) focal plane wavefront sensor, whose performance was demonstrated in laboratory attenuating the starlight by factors up to several 1e8 in space-like conditions at angular separations down to 2L/D. In this paper, we demonstrate the SCC on the sky for the first time. We installed an SCC on the stellar double coronagraph (SDC) instrument at the Hale telescope. We used an internal source to minimize the NCPA that limited the vortex coronagraph performance. We then compared to the standard procedure used at Palomar. On internal source, we demonstrated that the SCC improves the coronagraphic detection limit by a factor between 4 and 20 between 1.5 and 5L/D. Using this SCC calibration, the on-sky contrast is improved by a factor of 5 between 2 and 4L/D. These results prove the ability of the SCC to be implemented in an existing instrument. This paper highlights two interests of the self-coherent camera. First, the SCC can minimize the speckle intensity in the field of view especially the ones that are very close to the star where many exoplanets are to be discovered. Then, the SCC has a 100% efficiency with science time as each image can be used for both science and NCPA minimization.
This paper introduces an analytical method to calculate segment-level wavefront error tolerances in order to enable the detection of faint extra-solar planets using segmented telescopes in space. This study provides a full treatment of spatially uncorrelated segment phasing errors for segmented telescope coronagraphy, which has so far only been approached using ad hoc Monte-Carlo simulations. Instead of describing the wavefront tolerance globally for all segments, our method produces spatially dependent requirements. We relate the statistical mean contrast in the coronagraph dark hole to the standard deviation of the wavefront error of each individual segment on the primary mirror. This statistical framework for segment-level tolerancing extends the Pair-based Analytical model for Segmented Telescope Imaging from Space (PASTIS), which is based uniquely on a matrix multiplication for the optical propagation. We confirm our analytical results with Monte-Carlo simulations of E2E optical propagations through a coronagraph. Comparing our results for the Apodized Pupil Lyot Coronagraph designs for the Large UltraViolet Optical InfraRed (LUVOIR) telescope to previous studies, we show general agreement but provide a relaxation of the requirements for a significant subset of segments. These requirement maps are unique to any given telescope geometry and coronagraph design. The spatially uncorrelated segment tolerances we calculate are a key element of a complete error budget that will also need to include allocations for correlated segment contributions. We discuss how the PASTIS formalism can be extended to the spatially correlated case by deriving the statistical mean contrast and its variance for a non-diagonal aberration covariance matrix. The PASTIS tolerancing framework therefore brings a new capability that is necessary for the global tolerancing of future segmented space observatories.
Circumstellar environments are now routinely observed by dedicated high-contrast imagers on large, ground-based observatories. These facilities combine extreme adaptive optics and coronagraphy to achieve unprecedented sensitivities for exoplanet detection and spectral characterization. However, non-common path aberrations (NCPA) in these coronagraphic systems represent a critical limitation for the detection of giant planets with a contrast lower than a few $10^{-6}$ at very small separations ($<$0.3$^{primeprime}$) from their host star. In 2013 we proposed ZELDA, a Zernike wavefront sensor to measure these residual quasi-static phase aberrations and a prototype was installed in SPHERE, the exoplanet imager for the VLT. In 2016, we demonstrated the ability of our sensor to provide a nanometric calibration and compensation for these aberrations on an internal source in the instrument, resulting in a contrast gain of 10 at 0.2$^{primeprime}$ in coronagraphic images. However, initial on-sky tests in 2017 did not show a tangible gain in contrast when calibrating the NCPA internally and then applying the correction on sky. In this communication, we present recent on-sky measurements to demonstrate the potential of our sensor for the NCPA compensation during observations and quantify the contrast gain in coronagraphic data.
The major noise source limiting high-contrast imaging is due to the presence of quasi-static speckles. Speckle noise originates from wavefront errors caused by various independent sources, and it evolves on different timescales pending to their nature. An understanding of quasi-static speckles originating from instrumental errors is paramount for the search of faint stellar companions. Instrumental speckles average to a fixed pattern, which can be calibrated to a certain extent, but their temporal evolution ultimately limit this possibility. This study focuses on the laboratory evidence and characterization of the quasi-static pinned speckle phenomenon. Specifically, we examine the coherent amplification of the static speckle contribution to the noise variance in the scientific image, through its interaction with quasi-static speckles. The analysis of a time series of adaptively corrected, coronagraphic images recorded in the laboratory enables the characterization of the temporal stability of the residual speckle pattern in both direct and differential coronagraphic images. We estimate that spoiled and fast-evolving quasi-static speckles present in the system at the angstrom/nanometer level are affecting the stability of the static speckle noise in the final image after the coronagraph. The temporal evolution of the quasi-static wavefront error exhibits linear power law, which can be used in first order to model quasi-static speckle evolution in high-contrast imaging instruments.
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