Quantifying telescope phase discontinuities external to adaptive optics 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%.

Low wind effect on VLT/SPHERE: impact, mitigation strategy, and results

“…Nonetheless, it did not completely remove the effect: from December 2017 to May 2018, medium-strength LWE events occurred when the wind speed was below 1 m/s. Several solutions based on modifications of the AO control loop and wavefront sensing strategy have already been proposed for SPHERE [16][17][18] or other high-contrast instruments, and might be implemented in a future upgrade of the instrument 22 .…”

“…A solution addressing directly the root cause of the problem at the telescope level was preferred over a solution at the instrument level, which would imply changes in the WFS strategy, whether it implies only software changes, such use of phase diversity algorithms with the DTTS 16,17 or also hardware changes, such as the asymmetric Fourier pupil wavefront sensor. 18 The reason behind is that SPHERE is not the only VLT instrument sensitive to the LWE and the solution had therefore to be instrument-independent.…”

Review on methods for wavefront reconstruction from pyramid wavefront sensor data

“…105 A typical approach toward the elimination of the LWE consists of its measurement and a consequent correction with a corresponding offset being applied on the DM. It has been demonstrated that focal plane techniques such as focal plane phase diversity [138][139][140][141]145,146 or focal plane sharpening 146 can continuously provide information about the LWE in the loop.…”

“…Other instruments will rely on dedicated focal-plane wavefront sensing employed for an interferometric restoration of the noncommon path phase maps. Such speckle-analyzing techniques, such as phase-sorting interferometry 149 or phase diversity, 146,150,151 can provide continuously in the loop information about the NCPAs, which is used to iteratively correct for them with the main DM. Another approach is to use focal plane sharpening methods for a blind control of the DM.…”

“…Another approach is to use focal plane sharpening methods for a blind control of the DM. 146,151,152 When the NCPA measurements are available, the usual way to correct for them in a closed loop is to inject the measured phase shape offset onto the DM 27,153 or to introduce an offset in the WFS measurements, making the DM converge in closed loop to the shape, which cancels out the NCPAs in the science path. 142,154 Both of these schemes of compensation make the WFS see the NCPA and involve a reconstruction algorithm assuming linearity of both the sensor and the reconstructor.…”