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, installed in a slot in WFI’s element wheel. With a passband of 0.75-1.8µm, the Prism Assembly enables survey of Supernova Type 1a redshifts in the range of 0.2-1.7. It achieves a spectral resolving power of 170 > R > 70 across the field.

A late add-on to the Roman mission, the Prism Assembly had restrictions on size, mass and geometry, and a challenging schedule. Despite this, the Prism Assembly is a compact high-performance spectrographic element, implemented as a refractive, all-spherical surfaces, 2-element optical design using S-TIH-1 glass and CaF2. In this paper we describe the Prism Assembly from design and implementation through alignment, optical performance test and calibration, to delivery for installation in the WFI instrument.