Optical alignment of the Coronagraph Instrument (CGI) was completed in time to begin its full-functional and environmental testing in late 2023 and its integration into the Roman Space Telescope (RST) in summer 2024. The optics of the CGI relay the optical pupil of the RST five times so that science operations, such as coronagraphy and wavefront control, can be conducted in the different internal pupil and image planes. Within the pupil relays, the CGI has multiple active optical assemblies, including a fast-steering mirror, a focus-control mirror, two deformable mirrors, and six precision alignment mechanisms that articulate different masks and apertures into the beam.
Initial alignment of the CGI optics was completed in the reverse direction, using a commercial dynamic Twyman-Green interferometer to measure the wavefront error through each relay as optics were added sequentially from back to front. The end-to-end wavefront error was initially verified using surrogate optics in place of the active optical assemblies, to allow their simultaneous development and test. Throughout alignment, pupil and image planes were referenced and coaligned optically using fiducials, including spherically mounted retroreflectors (SMRs) that were positioned by a laser tracker and measured by the interferometer camera.
Upon end-to-end alignment of the pupil-relay optics, the active optical assemblies were integrated and aligned individually, and the entire CGI alignment was then optimized. The CGI optical subsystem was also mapped to SMR fiducials, which will later be used to integrate CGI into the RST observatory and verify its alignment to the Telescope’s line of sight. This paper details the many alignment steps required to successfully achieve the performance criteria of the CGI.
The Roman Space Telescope will have the first advanced coronagraph in space, with deformable mirrors (DMs) for wavefront control (WFC), low-order wavefront sensing and maintenance, and a photon-counting detector. It is expected to be able to detect and characterize mature, giant exoplanets in reflected visible light. Over the past decade, the performance of the coronagraph in its flight environment has been simulated with increasingly detailed diffraction and structural/thermal finite-element modeling. With the instrument now being integrated in preparation for launch within the next few years, the present state of the end-to-end modeling, including the measured flight components such as DMs, is described. The coronagraphic modes, including characteristics most readily derived from modeling, are thoroughly described. The methods for diffraction propagation, WFC, and structural and thermal finite-element modeling are detailed. The techniques and procedures developed for the instrument will serve as a foundation for future coronagraphic missions, such as the Habitable Worlds Observatory.
The Nancy Grace Roman Space Telescope Coronagraph Instrument is a critical technology demonstrator for NASA’s Habitable Worlds Observatory. With a predicted visible-light flux ratio detection limit of 10−8 or better, it will be capable of reaching new areas of parameter space for both gas giant exoplanets and circumstellar disks. It is in the final stages of integration and test at the Jet Propulsion Laboratory, with an anticipated delivery to payload integration in the coming year. This paper will review the instrument systems, observing modes, potential observing applications, and overall progress toward instrument integration and test.
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