Quantifying telescope phase discontinuities external to AO-systems by use of Phase Diversity and Focal Plane Sharpening

Abstract

We propose and apply two methods to estimate pupil plane phase discontinuities for two realistic scenarios on VLT and Keck. The methods use both Phase Diversity and a form of image sharpening. For the case of VLT, we simulate the `low wind effect' (LWE) which is responsible for focal plane errors in the SPHERE system in low wind and good seeing conditions. We successfully estimate the simulated LWE using both methods, and show that they are complimentary to one another. We also demonstrate that single image Phase Diversity (also known as Phase Retrieval with diversity) is also capable of estimating the simulated LWE when using the natural de-focus on the SPHERE/DTTS imager. We demonstrate that Phase Diversity can estimate the LWE to within 30 nm RMS WFE, which is within the allowable tolerances to achieve a target SPHERE contrast of 10${}^{-6}$. Finally, we simulate 153 nm RMS of piston errors on the mirror segments of Keck and produce NIRC2 images subject to these effects. We show that a single, diverse image with 1.5 waves (PV) of focus can be used to estimate this error to within 29 nm RMS WFE, and a perfect correction of our estimation would increase the Strehl ratio of a NIRC2 image by 12\%

arXiv comment16 pages, 8 figures, Accepted for publication in JATIS

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Cited by in corpus (7)

• Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor. III. On-sky validation in VLT/SPHERE
• Calibration of the island effect: Experimental validation of closed-loop focal plane wavefront control on Subaru/SCExAO
• Low Wind Effect on VLT/SPHERE : impact, mitigation strategy, and results
• Phasing segmented telescopes via deep learning methods: application to a deployable CubeSat
• Redundant apodization for direct imaging of exoplanets 2: Application to island effects
• Simultaneous estimation of segmented telescope phasing errors and non-common path aberrations from adaptive optics corrected images
• High Speed Focal Plane Wavefront Sensing with an Optical Chopper