Demonstration of low-order wavefront sensing and control system of Roman coronagraph instrument in thermal vacuum testing environment

Prior to its delivery to the Goddard Space Flight Center (GSFC) on May 17, 2024, the integrated coronagraph instrument (CGI) underwent testing and verification at the Jet Propulsion Laboratory. We detail the test outcomes of the focus control loop (FCL) and Zernike control loop (ZCL) in the CGI’s thermal vacuum (TVAC) test environment. To obtain and maintain the required wavefront stability, the CGI features a dedicated subsystem, the low-order wavefront sensing and control (LOWFSC), which detects wavefront alterations using starlight reflected off the focal plane mask, converts these changes into Zernike modes Z2-Z11, and independently controls Z2/Z3, Z4, and Z5-Z11 through three distinct control loops: line of sight control loop (LCL), FCL, and ZCL. Unlike the LCL, the testing of FCL and ZCL has been unfeasible without an appropriate testbed equipped with the necessary optical stimuli. Thus, the CGI TVAC tests mark the first system-level evaluation of FCL and ZCL prior to the CGI’s handover to GSFC. The tests and subsequent analysis affirm that both loops are operating as intended, successfully meeting the key requirements of CGI. This document delineates various test configurations, presents the collected data, and scrutinizes the resulting outcomes. In addition, it includes the dark hole contrast data acquired through the exoplanetary systems camera (EXCAM). By concurrently measuring the EXCAM dark hole images alongside the LOWFSC observation, we were able to assess the CGI’s contrast sensitivity to the Zernike wavefront errors, though the accuracy of these measurements suffered from unintentional partial saturation of the dark hole images.