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

TL;DR

This paper introduces two methods using Phase Diversity and image sharpening to estimate and correct phase discontinuities in telescope pupils, demonstrated on VLT and Keck, improving image quality and contrast.

Contribution

It presents novel application of Phase Diversity and focal plane sharpening to estimate pupil phase errors in realistic telescope scenarios, including the low wind effect and segment piston errors.

Findings

Phase Diversity estimates LWE within 30 nm RMS WFE.

Single image Phase Diversity can estimate LWE using natural defocus.

Correction of piston errors improves Strehl ratio by 12%.

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\%