Johanan Codona, Air Force Research Laboratory; Michael Hart, HartSCI LLC and University of Arizona; Lauren Schatz, University of Arizona; Mala Mateen, Air Force Research Laboratory
Keywords: Adaptive Optics, Pyramid Wavefront Sensors, Performance, Noise Sensitivity.
Abstract:
We investigated the relative performance of three-sided and four-sided pyramid wavefront sensors (PWFS3 and PWFS4). Three-sided pyramids are easier to manufacture and are reasonably expected to be less sensitive to sensor shot and read noise, simply because there are 3/4 as many pixels contributing noise for the same plate scale. For this study, we assumed high Strehl ratios, allowing us to analyze the performance without modulation. We assumed a linear reconstructor method for processing the WFS images. The background-subtracted and flat-fielded images were normalized by the total image count to make the response independent of guide star flux. We then subtracted an ideal flat-wavefront image, leaving us with a delta-image that goes to zero when the wavefront is flat. The pupil image plate scale is selected to allow adequate sampling of the irradiance compared with the DM resolution. This minimizes the number of pixels for the highest-resolution desired, minimizing the amount of of read and shot noise entering the reconstructor. We simulated an AO-equipped 1.5 m telescope with a 12×12 actuator DM and 36 sensor pixels across a pupil image. For this study we did not use pyramid modulation. We also considered guide star brightnesses of between 4 and 14th magnitude. The delta-image (image minus ideal image) pixels were unfolded into a vector that is multiplied by the reconstructor matrix to give the DM actuator updates. We used 2500 weak Kolmogorov phase screens to estimate both the PWFS3 and PWFS4 reconstructors. Each reconstructor was estimated using SVD and tested on an independent set of wavefronts. The optimum number of SVD modes was selected by minimizing the residuals for both the PWFS3 and PWFS4. Possibly as a result of the differential in nonlinear response, the PWFS3 rms aberration suppression after one iteration was 8% while the PWFS4 suppression was slightly better at 7.5%. However, as expected, the PWFS3 sensitivity to read and shot noise was less due to the the smaller number of image pixels required for a given spatial resolution. The measured rms WFE residual was consistent with the square root of 3/4, or 87%. We present a comparative error budget analysis between PWFS3, PWFS4, along with a similar-resolution Shack-Hartmann WFS.
Date of Conference: September 11-14, 2018
Track: Poster