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The Nancy Grace Roman Space Telescope Coronagraph Instrument is ahigh-contrast imager, polarimeter, and spectrometer that will enable the studyof exoplanets and circumstellar disks at visible wavelengths($\sim$550--850~nm) at contrasts 2--3 orders of magnitude better than cancurrently be achieved by ground or space-based direct imaging facilities. Tocapitalize on this sensitivity, precise flux calibration will be required. TheRoman Coronagraph, like other space-based missions, will use on-orbitflatfields to measure and correct for phenomena that impact the measured totaleffective throughput. However, the Coronagraph does not have internal lampsources, therefore we have developed a method to perform flatfield calibrationsusing observations of extended sources, such as Uranus and Neptune, using acombination of rastering the Coronagraph's Fast Steering Mirror, tiling theplanet across the field of view, and matched-filter image processing. Here weoutline the process and present the results of simulations using images ofUranus and Neptune from the Hubble Space Telescopes Wide Field Camera 3, infilters approximate to the Coronagraph's Band 1 and Band 4. The simulations areperformed over the Coronagraph's direct imaging and polarimetric modes. Wemodel throughput effects in 3 different spatial frequency regimes including 1)high spatial frequency detector pixel-to-pixel quantum efficiency variations,2) medium spatial frequency "measles" caused by particle deposition on thedetector or other focal-plane optics post-launch, and 3) low spatial frequencydetector fringing caused by self-interference due to internal reflections inthe detector substrate as well as low spatial frequency vignetting at the edgesof the Coronagraph's field of view. We show that Uranus and Neptune can be usedas astrophysical flat sources with high precision ($\sim$0.5% relative error)

Flatfield Calibrations with Astrophysical Sources for the Nancy Grace Roman Space Telescope's Coronagraph Instrument