Analysis and mitigation of pupil discontinuities on adaptive optics performance

Abstract

As already noticed in other telescopes, the presence of large telescope spiders and of a segmented deformable mirror in an Adaptive Optics system leads to pupil fragmentation and may create phase discontinuities. On the ELT telescope, a typical effect is the differential piston, where all disconnected areas of the pupil create their own piston, unseen locally but drastically degrading the final image quality. The poor sensitivity of the Pyramid WFS to differential piston will lead to these modes been badly seen and therefore badly controlled by the adaptive optics (AO) loop. In close loop operation, differential pistons between segments will start to appear and settle around integer values of the average sensing wavelength. These additional differential pistons are artificially injected by the adaptive optics control loop but do not have any real physical origin, contrary to the Low Wind Effect. In an attempt to reduce the impact of unwanted differential pistons that are injected by the AO loop, we propose a novel approach based on the pair-wise coupling of the actuators sitting on the edges of the deformable mirror segments. In this paper, we present the correction principle, its performance in nominal seeing condition, and its robustness relative to changing seeing conditions, wind speed and natural guide star magnitude. We show that the edge actuator coupling is a simple and robust solution and that the additional quadratic error relative to the reference case (i.e. no spiders) is of only 40 nm RMS, well within the requirements for HARMONI.

Real-time adaptive optics with pyramid wavefront sensors: part I. A theoretical analysis of the pyramid sensor model

“…These are used in the upcoming second part of the paper [39] which will be devoted to the application of iterative algorithms to the problem of wavefront reconstruction from pyramid wavefront sensor data. An extension of the analysis (linearization and calculation of adjoints) to the full pyramid sensor operator as well as the adaption of the aperture mask to segmented pupils on ELTs [17,40,55,66,67] is dedicated to future work.…”

A many-core CPU prototype of an MCAO and LTAO RTC for ELT-scale instruments

“…is being targeted for the ELT first light instrument HAR-MONI (Thatte et al 2014) SCAO mode (Schwartz et al 2018). Figures 8 and 7 show the latency scaling of both the SH-WFS RTC and Pyr-WFS RTC against sub-apertures/pixels across the pupil.…”

Review on methods for wavefront reconstruction from pyramid wavefront sensor data

“…8,27,[99][100][101][102][103] Furthermore, the modal basis is not as well-suited for pupils with spiders as the zonal approach, which allows significantly more degrees of freedom in the representation of wavefront. 49,69,[104][105][106][107] An alternative-decoupled-approach to AO loop control considers the two steps: wavefront reconstruction and DM fitting, separately and independently. In this situation, wavefront reconstruction can be based on a synthetic calibration (using a numerical implementation of the sensor's forward model) done independently from the shapes a DM can produce.…”