Spectroscopy and polarimetry design and flight instrument calibration for the Roman Coronagraph

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Abstract
The Nancy Grace Roman Space Telescope Coronagraph Instrument (CGI) will be the first high-contrast imager in space with active wavefront control, advancing several key technologies for future missions such as the Habitable Worlds Observatory (HWO).1,2 Future mission concepts have baselined a lenslet-based integral field spectrograph (IFS) for their spectroscopy modes,3,4 which provides data that simultaneously span two spatial dimensions and one spectral dimension. There are many operational, calibration, and post-processing advantages to such a design, but for cost, mass, and power reasons, a simplified Amici prism and slit are implemented in the primary CGI imaging path. CGI did advance the IFS maturity to a preliminary design review (PDR)-level design; the details of which and comparison to the Amici prism can be found in Groff et al.5 Although imaging spectroscopy will continue to be a priority for the HWO coronagraph instrument, we have focused on enabling spectroscopic capability in CGI so that Roman can support both technology and science-related demonstrations related to spectroscopy. Here, we describe the design, assembly, testing, and calibration of the spectroscopic and polarization imaging modes up through instrument integration and test (I&T). Both modes are included in the primary imaging path to the Exoplanet Imaging Camera (ExCam) because they are designed to have a zero angular deviation optical axis. We refer to the polarization and spectroscopy modes as the Wollaston and Zero Optical Deviation (ZOD) spectroscopy modes, respectively. The effort was quite expansive, with the ZOD concept first presented in August 2019, approved to proceed at CGI PDR in September 2019, combined ZOD+Wollaston System Requirements Review (SRR) and PDR in March 2020, tabletop critical design review (CDR) (with preliminary instrument environments) in January 2021, bond qualification in April 2021, combined final CDR and manufacturing readiness review (MRR) in August 2021, pre-ship review (PSR) and delivery to CGI in January 2022. The calibration test program and supporting algorithms for ground and on-orbit spectral calibration followed using the flight spare units. Final ground calibration of the flight units took place during the CGI thermal vacuum (TVAC) test campaign in April 2024.
Funding Information
  • National Aeronautics and Space Administration (80NM0018D0004)

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