Reflective Engineered Diffusers for the Roman Space Telescope Simplified Relative Calibration System

The Wide Field Instrument (WFI) on NASA’s Roman Space Telescope (RST) has an on-board calibrator called the simplified Relative Calibration System (sRCS). The sRCS is designed to project flat-field calibration light onto the WFI focal plane in two modes of operation. One mode illuminates a quasi-Lambertian diffuser that blocks the full aperture of the telescope, and the other mode uses a small reflective engineered diffuser that sits behind each filter’s aperture mask in the WFI element wheel assembly (EWA). The second mode, called “Lamp On Lamp Off” (LOLO), uses these metallic-coated engineered diffusers to illuminate the focal plane concurrently with an exposure from the observatory and enables nonlinearity corrections. The relatively smaller size of the LOLO diffusers in comparison to the blocking diffuser motivates intentionally limiting the LOLO diffuser divergence angle to enhance flux. This presentation shows how diffuse divergence from metallic-coated engineered diffusers satisfies a nearly identical angle of incidence range at the WFI focal plane as a quasi-Lambertian diffuser. Further, it is shown that the LOLO diffusers are effective at producing a spatially uniform and smooth focal plane illumination when illuminated by a diffuse extended source.