The long term ideas for decomposing the photometric fits into the wcs jacobian term and the illumination correction are described in https://community.lsst.org/t/photometric-calibration-and-astrometric-jacobians/3232. Over the near term, fgcmcal has been ignoring the WCS jacobian and simply constraining the combined calibration term, as this is what can be handled in the PhotoCalib objects. However, in the case of the tract-based fgcmcal (
DM-16704), this might not work as well because there is not as many star observations (compared to a global fit) to constrain the relevant spatial variation terms. This ticket investigates, in conjunction with DM-20161, whether performance can be improved in tract-based fgcmcal by distinguishing these terms. If so, some functionality of combining a WCS jacobian BoundedField ( DM-15431) with a Chebyshev BoundedField will need to be added to afw. If this is not necessary, we can wait for the longer term solution when calexp images will be fluence images thus taking the wcs jacobian out of the equation when it comes to photometric calibration.