Global sky background images for JWST/NIRISS Wide-Field Slitless Spectroscopy

Abstract: We present updated empirical background images for JWST/NIRISS Wide-Field Slitless Spectroscopy (WFSS) for both orthogonal grisms (GR150C, GR150R) crossed with all NIRISS WFSS wide-band filters (F090W, F115W, F150W, F200W). The background images are created using carefully vetted science and calibration exposures and improve the quality of previous background reference files. We present our methodology to create the background images and assess their quality using background subtracted science data. Overall, background residuals reach below 1% of the sky brightness for all filter and grism combinations, corresponding to a decrease in the background RMS of a factor of up to 7 from previous background reference files. The new background images are available on the Calibration Reference Data System (CRDS) as of context 1365 and we recommend using them for improved quality of NIRISS WFSS data.