Expected gain in the pyramid wavefront sensor with limited Strehl ratio

Abstract

Context. One of the main properties of the pyramid wavefront sensor is that, once the loop is closed, and as the reference star image shrinks on the pyramid pin, the wavefront estimation signal-to-noise ratio can considerably improve. This has been shown to translate into a gain in limiting magnitude when compared with the Shack-Hartmann wavefront sensor, in which the sampling on the wavefront is performed before the light is split into four quadrants, which does not allow the quality of the focused spot to increase. Since this property is strictly related to the size of the re-imaged spot on the pyramid pin, the better the wavefront correction, the higher the gain. Aims. The goal of this paper is to extend the descriptive and analytical computation of this gain that was given in a previous paper, to partial wavefront correction conditions, which are representative for most of the wide field correction adaptive optics systems. Methods. After focusing on the low Strehl ratio regime, we analyze the minimum spatial sampling required for the wavefront sensor correction to still experience a considerable gain in sensitivity between the pyramid and the Shack-Hartmann wavefront sensors. Results. We find that the gain can be described as a function of the sampling in terms of the Fried parameter.

Operation of a layer-oriented multiconjugate adaptive optics system in the partial illumination regime

“…Binning this way reduces the equivalent readout noise on the individual subapertures. This improvement, coupled with the well-known sensitivity gain, 29,30 is a direct benefit of pyramid wavefront sensing. Comparing the local fluxes in the same subaperture of the four pupil images gives the local tilt.…”

Review on methods for wavefront reconstruction from pyramid wavefront sensor data

“…The mentioned advantages of the pyramid sensor lead to significant improvements in the closed-loop performance of AO systems compared to systems equipped with the SH-WFS. For instance, in astronomical AO systems, the PWFS has been reported to provide higher Strehl ratios as well as higher guide star limiting magnitudes, 26 which results in an increased sky coverage 19 and lower residual speckle levels for high contrast imaging. Recently, it has been shown that all the advantages of the pyramid sensor are kept even when significant levels of NCPA have to be sensed and corrected by an AO system.…”

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

“…It is gradually gaining more and more attention from the scientific community, especially in astronomical AO, due to its increased sensitivity, improved signal-to-noise ratio, robustness to spatial aliasing and adjustable spatial sampling compared to the other popular wavefront sensor choice -the Shack-Hartmann (SH) sensor. Several theoretical studies [8,21,22,48,61,62,70,80,82,81,85] including numerical simulations and laboratory investigations with optical test benches [4,33,52,59,78,84] have Figure 1: Principle of wavefront correction [2]. A deformable mirror reflects a perturbed wavefront and propagates the corrected, planar wavefront to the science camera yielding improved image quality.…”