Roman coronagraph simulations of exozodi observations in the presence of wavefront errors

Abstract. The Coronagraph Instrument on board of the Nancy Grace Roman Space Telescope will demonstrate key technologies that will prepare the ground for the Habitable Worlds Observatory. The current predictions for the Roman Coronagraph’s detection limit range from ∼10−8 to a few 10−9, which would allow for groundbreaking science, such as potentially imaging Jupiter-like exoplanets around mature stars or imaging exozodi disks in scattered light. However, the performance of the instrument depends on many factors. Simulating images with varying optical error sources can help us connect instrument and observatory performance to potential science yield. We present a tool, corosims, to simulate observations of arbitrary astrophysical scenes with the Roman Coronagraph in the presence of evolving wavefront errors. This tool wraps around the Roman Coronagraph PROPER diffraction model and detector simulator. We use corosims to investigate the potential degeneracy between jitter-induced speckles and both hot and warm exozodi disk structures. First, we simulate observations of warm exozodi around τ Ceti, with varying jitter levels. We predict that with nominal post-correction pointing jitter performance (∼0.3 mas RMS), the Roman Coronagraph should be sensitive to 12× zodis worth of dust, assuming a face-on (worst case scenario) inclination. We further predict that its sensitivity degrades to 35× zodis if jitter on-target is 3× worse than the nominal value. This estimate assumes the best-modeled wavefront control and stability values from the Roman project, including additional “model uncertainty factors” on performance. We find that, although jitter hinders warm exozodi detection by increasing the noise, jitter residuals are unlikely to result in a false positive, particularly if two-color imaging is conducted. However, if a faint, hot exozodi disk falls <λ/D projected separation, it may not be distinguishable from jitter-induced speckle residuals of comparable brightness. Finally, we discuss the degeneracies induced between flux and separation retrieved near the inner working angle due to the sharp edge of the Roman Coronagraph’s focal plane mask.