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

TL;DR

This work introduces corosims, a dedicated end-to-end simulator for the Roman Coronagraph that propagates evolving wavefront errors into realistic exozodi observations. By wrapping around the PROPER-based cgisim model, it enables systematic studies of jitter, drift, and speckle evolution on exozodi detectability, using Tau Ceti as a case study. The results quantify how disk inclination and jitter impact detection limits (e.g., ≈12 zodis for face-on and ≈1 zodi for edge-on under nominal jitter) and identify degeneracies between jitter-induced speckles and compact hot exozodi signals, with mitigation strategies discussed (e.g., multi-band imaging). The work underscores the instrument's potential for precursor exozodi science for Habitable Worlds Observatory and provides a public toolset for planning and interpreting future observations.

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 1e-8 to a few 1e-9, which would allow for groundbreaking science, such as potentially imaging Jupiter-like planets. 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 science yield. Here we present corosims, a tool to simulate observations of astrophysical scenes with the Coronagraph with evolving errors. This tool wraps around the Coronagraph PROPER diffraction model and detector simulator. We use it to investigate the potential degeneracy between jitter-induced speckles and both hot and warm exozodi disk structures. First, we simulate observations of exozodi around Tau Ceti, with varying jitter. We predict that with nominal post-correction pointing jitter performance (~0.3 mas RMS), the Roman Coronagraph should be sensitive to 12x zodis worth of dust, assuming a face-on (worst case scenario) inclination. We further predict that its sensitivity degrades to 35x zodis if jitter on-target is 3x worse than the nominal value. This estimate assumes the best-modeled wavefront control and stability values from the project, including additional model uncertainty factors. We find that, while jitter hinders warm exozodi detection, jitter residuals are unlikely to result in a false positive. However, if a faint hot exozodi falls at small 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 the sharp edge of the Roman Coronagraph's focal plane mask.

Figures (12)

• Figure 1: The crosses indicate the positions where the $\delta$EF are computed to simulate jitter and drift as explained in Sec. \ref{['sec:jitter']}. Finer resolution is required near the center of focal pane mask, where the variability of the PSF structure is more important and where the object is expected to spend most of the acquisition time. This representation is for the case of the HLC mode. The cloud centered at [0,0] indicates a representative pre-LOWFSC jitter for the first 32 hours of the Observing Scenario 11 (OS11)Krist2023. The green line represents the pre-LOWFS drift of the instrument for the first 10 hours of OS11. The inset shows a zoomed in view of the inner region where a finer resolution of $\delta$EF is computed.
• Figure 2: Dark hole images from the OS11 data package batch 0 (left), and from corosims (right), generated with the same inputs, except for the jitter implementation and deformable mirror shapes. The differences between speckles structures is due to the use of different deformable mirror shapes. White lines indicate inner and outer working angles (IWA and OWA), at 3 and 9 $\lambda/D$, respectively.
• Figure 3: Normalized intensity comparison between the simulation package OS11 and the results from simulations with corosims for the same inputs. Two different batches are shown: batch 0 (left), from images of the first reference acquisition, batch 100 (right), from images of the first roll science acquisition. The small difference in flux is due to the different DM solutions used, which result in a different normalized intensity. The uptick in flux in the dark hole at the end of Batch 0 (left) is caused by the shear at the instrument carrier interface.
• Figure 4: Flowchart illustrating the RDI observation strategy and post-processing steps that are taken to compute a final, post-processed image and an SNR map.
• Figure 5: Final images of simulated observations of $\tau$ Ceti's disk for different levels of zodi and inclinatons of the disk. The disk model, described in Sec. \ref{['sec:disk_model']} is convolved with the point response function (PRF) and added to a speckle series, generated as described in Sec. \ref{['sec:simus_description']}. The simulations include a reference observation for PSF subtraction and two rolls, these images show the PSF subtracted product with the two rolls combined. These simulations include detector noise. As expected, at lower inclinations the disk is harder to detect and the signal gets mixed with the residual speckles of comparable brightness. Note that the top row provides a visualization of the images obtained with no exozodi present, i.e. post-calibration speckle residuals only.