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Arxiv: JWST's PEARLS: Improved Flux Calibration for NIRCam Published: 11/23/2023 3:02:17 AM Updated: 1/16/2024 5:46:53 PM

Paper abstract: The Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS),a JWST GTO program, obtained a set of unique NIRCam observations that haveenabled us to significantly improve the default photometric calibration acrossboth NIRCam modules. The observations consisted of three epochs of 4-band(F150W, F200W, F356W, and F444W) NIRCam imaging in the Spitzer IRAC Dark Field(IDF). The three epochs were six months apart and spanned the full duration ofCycle 1. As the IDF is in the JWST continuous viewing zone, we were able todesign the observations such that the two modules of NIRCam, modules A and B,were flipped by 180 degrees and completely overlapped each other's footprintsin alternate epochs. We were therefore able to directly compare the photometryof the same objects observed with different modules and detectors, and we foundsignificant photometric residuals up to ~ 0.05 mag in some detectors andfilters, for the default version of the calibration files that we used(jwst_1039.pmap). Moreover, there are multiplicative gradients present in thedata obtained in the two long-wavelength bands. The problem is less severe inthe data reduced using the latest pmap (jwst_1130.pmap as of September 2023),but it is still present, and is non-negligible. We provide a recipe to correctfor this systematic effect to bring the two modules onto a more consistentcalibration, to a photometric precision better than ~ 0.02 mag.