Post-facto photon-fixing

Abstract

Single photon avalanche detectors (SPADs) have emerged as a potential successor of current wavefront sensor (WFS) and science camera detectors. Due to their lack of readout noise and ability to operate at very high frame-rates they are particularly well suited for low flux imaging – as is often the case when observing with an adaptive optics (AO) system. However, while the underlying detector technologies are radically different, current applications of SPADs in AO settings have treated them simply as standard CMOS sensors with desirable noise properties. In this work we present a new method which takes advantage of the SPADs high-frame rates when used as a science camera to compensate for servo-lag created by delays inherent in the AO control loop. Typically, science cameras have exposures many orders of magnitude longer than the WFS. When a SPAD is used, we instead have potentially thousands of science camera frames between each wavefront measurement. We show that by deconvolving these “between” frames with the residual slopes from the next WFS measurement we can undo the effects of servo lag caused by the delay of the WFS sensing, reading, and sending the updated commands to the deformable mirror. We show that our method is capable of improving both Strehl ratio and contrast in simulation, showing it to be a promising direction for further research.