Simulating image coaddition with the Nancy Grace Roman Space Telescope: I. Simulation methodology and general results

TL;DR

This work develops and tests a linear coaddition framework (Imcom) to reconstruct a fully sampled, uniform PSF mosaic from undersampled Roman Space Telescope data. By optimizing a coaddition matrix ${\mathbfss T}$ to minimize PSF leakage $U_\alpha/C$ while controlling output noise $\Sigma_{\alpha\alpha}$, the method yields output images with a user-defined target PSF $\Gamma$, demonstrated on a $0.64$ deg$^2$ DC2-based Roman region with realistic masks, dithers, and detector effects. The paper provides a detailed description of input data, postage-stamp and block-mosaic coaddition, and PSF/noise diagnostics, including observed Moiré patterns and the impact of charge diffusion on fidelity. It also documents known issues and practical considerations for implementing large-area, ultra-precise coadds, with companion Paper II delivering the statistical weak-lensing implications and ellipticity biases.

Abstract

The upcoming Nancy Grace Roman Space Telescope will carry out a wide-area survey in the near infrared. A key science objective is the measurement of cosmic structure via weak gravitational lensing. Roman data will be undersampled, which introduces new challenges in the measurement of source galaxy shapes; a potential solution is to use linear algebra-based coaddition techniques such as Imcom that combine multiple undersampled images to produce a single oversampled output mosaic with a desired "target" point spread function (PSF). We present here an initial application of Imcom to 0.64 square degrees of simulated Roman data, based on the Roman branch of the Legacy Survey of Space and Time (LSST) Dark Energy Science Collaboration (DESC) Data Challenge 2 (DC2) simulation. We show that Imcom runs successfully on simulated data that includes features such as plate scale distortions, chip gaps, detector defects, and cosmic ray masks. We simultaneously propagate grids of injected sources and simulated noise fields as well as the full simulation. We quantify the residual deviations of the PSF from the target (the "leakage"), as well as noise properties of the output images; we discuss how the overall tiling pattern as well as Moiré patterns appear in the final leakage and noise maps. We include appendices on interpolation algorithms and the interaction of undersampling with image processing operations that may be of broader applicability. The companion paper ("Paper II") explores the implications for weak lensing analyses.

Figures (19)

• Figure 1: The coverage (number of exposures) in each of the 4 bands in the $48\times 48$ arcmin region considered in this paper. Each sub-panel shows one of the filters. The 18-chip "pawprint" feature of the Roman focal plane is easily visible, as is the presence of two roll angles from the two passes in each filter.
• Figure 2: An example of a simulated mask. This is the $512\times 512$ lower-right corner of SCA 11 in a H158-band observation (ID 8836). Reference pixels are shown in dark yellow; permanently masked pixels are shown in black; and pixels rejected in this observation only due to cosmic rays are shown in red-orange.
• Figure 3: A $384\times 384$ cutout of a $1/f$ noise field generated in Sec. \ref{['ss:noise']}. The grayscale is a linear stretch from $-16$ to $+16$.
• Figure 4: The hierarchical structure of mosaic coadds in this paper. The mosaic (panel [a]) is defined by a center, a map projection, and a number of blocks ( BLOCK$\times$ BLOCK). Each block (panel [b]) is itself composed of postage stamps; we make an $n_1\times n_1$ array, with padding of PAD postage stampps around the rim so that the blocks overlap. The postage stamps (panel [c]) are composed of an $n_2\times n_2$ grid of output pixels, with a transition region around the edge that is merged at the block processing level before writing to a FITS file. The postage stamp is built from all un-masked input pixels in all input images within a given acceptance radius of the stamp.
• Figure 5: The workflow for coaddition of a block in this paper. There are two repositories: the postage stamp coaddition ( furry-parakeet) and the mosaic driver with interfaces to the simulations ( fluffy-garbanzo).