Paper detail

Simulating Image Coaddition with the Nancy Grace Roman Space Telescope. IV. Hyperparameter Optimization and Experimental Features

For weak gravitational lensing cosmology with the forthcoming Nancy Grace Roman Space Telescope, image coaddition, or construction of oversampled images from undersampled ones, is a critical step in the image processing pipeline. In the previous papers in this series, we have re-implemented the {\sc Imcom} algorithm, which offers control over point spread functions in coadded images, and applied it to state-of-the-art image simulations for Roman. In this work, we systematically investigate the impact of {\sc Imcom} hyperparameters on the quality of measurement results. We re-coadd the same $16$ blocks ($1.75 \times 1.75 \,{\rm arcmin}^2$, $2688 \times 2688$ pixels each) from OpenUniverse2024 simulations with $26$ different configurations in each of $5$ bands. We then compare the results in terms of $12$ objective evaluation criteria, including internal diagnostics of {\sc Imcom}, properties of coadded noise frames, measurements of injected point sources, and time consumption. We demonstrate that: i) the Cholesky kernel is the best known linear algebra strategy for {\sc Imcom}, ii) for our measurements, a wide Gaussian target output PSF outperforms a smoothed Airy disk or a narrow Gaussian, iii) kernel-specific settings are worth considering for future coaddition, and iv) {\sc Imcom} experimental features studied in this work are either inconsequential or detrimental. We end this paper by discussing current and next steps of {\sc Imcom}-related studies in the context of Roman shear and clustering measurements.