Coarse alignment of the James Webb Space Telescope secondary and primary mirrors without a fine guidance sensor

Abstract

Launched December 25th 2021 to its final destination at the Sun-Earth Lagrange Point 2, JWST will revolutionize the way the universe is seen back to the early universe beginnings. During the six month commissioning phase of the mission, there were various activities performed which aligned the segmented telescope from eighteen unique Point Spread Functions (PSFs) to a single PSF. One of these steps is to globally align the Secondary Mirror (SM) Assembly (SMA) to the 18 Primary Mirror Segment Assemblies (PMSA) in the primary mirror. This alignment process happens in two phases, and they are named Global Alignment 1 (GA1) and Global Alignment 2 (GA2). A successful completion of GA1 entails coarsely aligning the SM and each PMSA to achieve an RMS Wavefront Error (WFE) less than 200nm. This is accomplished by generating PMSA wavefront maps which are used to determine a correction in the SM using Phase Retrieval and Decomposition. One of the requirements is that the Fine Guidance Sensor (FGS) can guide on a segment during the exposures, minimizing the impacts of motion blur on the images. This paper details a contingency method on how global alignment can be achieved using the observatories star trackers for guiding. The star trackers line of sight (LOS) stability and pointing stability performance are intended for coarse guiding whereas FGS provides fine guiding. It is expected that motion blur will increase should the observatory use coarse guiding during the GA1 exposures. Within the paper, we detail the maximum motion thresholds needed to achieve the desired placement of the SM, as well as exposure methods which can be used to handle motions that exceed the thresholds.