Compact prism assembly for slit-less spectroscopy capability in Roman Space Telescope Wide-Field Instrument

The Roman Space Telescope (Roman) is a three-mirror anastigmat design with a 2.4-m primary mirror. It will be based in the L2 orbit optimized for observations of cosmic expansion using the Wide-Field Instrument (WFI) and exoplanet discovery using the coronagraph instrument (CGI). The WFI features a 300-megapixel near-infrared detector array providing a field of view >100 times larger than that of the Hubble Space Telescope. To enable a low-resolution spectroscopy functionality for Roman, a compact Prism Assembly was added and installed in a slot in WFI’s element wheel. The Prism Assembly has a 0.76 to 1.8 μm passband enabling a survey of Supernova Type 1a redshifts in the range of 0.2 to 1.7. It achieves a spectral resolving power of 170>R>70 across the field. The Prism Assembly had restrictions on size, mass, and geometry, and a challenging schedule due to being a late addition to Roman. Despite this, the Prism Assembly is a compact high-performance spectrographic element, implemented as a refractive, all-spherical surfaces, two-element optical design using S-TIH1 glass and CaF2. We describe the Prism Assembly from design and implementation through alignment, optical performance test, and calibration to delivery for installation in the WFI instrument.