Laboratory characterization of widefield filter transmission for the Nancy Grace Roman Space Telescope's Wide Field...

We present optical test and analysis methods developed to characterize filter transmission across the 0.4×0.8-deg field of view of the Nancy Grace Roman Space Telescope's Wide Field Instrument (WFI). Cosmological surveys for supernovae and photometric redshifts drive stringent requirements on photometric calibration with sub-percent accuracy, necessitating precise characterization of wavelength-dependent transmission across the field. WFI employs eight filters spanning 0.48-2.3 µm, and its wide field of view yields angles of incidence up to 12 deg on filter elements, causing field-dependent blueshifts of the interference filter edges. We develop several approaches to model and measure these spatially varying optical properties through three complementary techniques: filter witness measurements, pupil imaging, and focus-diverse amplitude retrieval. A semi-empirical model accounts for both angle-of-incidence effects and variations in filter coating thickness, which we validate using laboratory measurements. The methods provide a characterization of the intrinsic filter wavelength response to better than 0.01% precision across the field. This precision supports the overall 0.06% wavelength error budget requirement that includes system-level characterization uncertainties, ultimately enabling sub-percent photometric calibration for Roman's cosmological surveys.