Phase retrieval algorithm for JWST Flight and Testbed Telescope

Abstract

An image-based wavefront sensing and control algorithm for the James Webb Space Telescope (JWST) is presented. The algorithm heritage is discussed in addition to implications for algorithm performance dictated by NASA's Technology Readiness Level (TRL) 6. The algorithm uses feedback through an adaptive diversity function to avoid the need for phase-unwrapping post-processing steps. Algorithm results are demonstrated using JWST Testbed Telescope (TBT) commissioning data and the accuracy is assessed by comparison with interferometer results on a multi-wave phase aberration. Strategies for minimizing aliasing artifacts in the recovered phase are presented and orthogonal basis functions are implemented for representing wavefronts in irregular hexagonal apertures. Algorithm implementation on a parallel cluster of high-speed digital signal processors (DSPs) is also discussed.

Focal-plane wavefront sensing for segmented telescope

“…This shows that pupil-Plane (PP) Wavefront Sensors (WFS) are the most considered devices to cophase segmented mirrors. On the other side, the James Web Space Telescope (JWST) plans to use a Focal-Plane (FP) WFS for the fine cophasing of its segments [5]. The advantage of using a focal-plane wavefront sensor is that the recorded images are the results of the interference of all the segments.…”

Wave-front sensing for space active optics: Rascasse project

“…One of the main advantages of this method is its hardware simplicity. That is the reason why phase diversity was retained for measuring the spherical aberration of Hubble Space Telescope [9] or for the fine cophasing of JWST segments [10]. In these applications the object was known because it was a non-resolved star.…”

Qualification of a null lens using image-based phase retrieval

“…There are two general categories of phaseretrieval algorithms: In iterative-transform algorithms (based on the Misell-Gerchberg-Saxton algorithm)4, light is simulated traveling back and forth through the optical system -from the exit pupil of the optical system to the light detector, and then backwards from the detector to the exit pupil -repeatedly until the algorithm converges. At each destination (first at the detector, and then at the exit pupil), tbe amplitude of the light field is replaced with data from optical models or measurements while leaving the phase (the wavefront) untouched 5 .…”