I am trying to understand how the Monster reference catalogue was used for DP2 LSSTCam calibration.
On the DP2 RSP Butler, I find the reference catalogue the_monster_20250219, from the collection:
refcats/DM-49042/the_monster_20250219
When I inspect this catalogue, I see synthetic photometry for systems including ComCam, DES, LATISS, and SDSS, but I do not see any monster_LSSTCam_* or SynthLSST flux columns.
I also noticed that the current obs_lsst/config/lsstCam/filterMap.py maps LSSTCam filters to monster_ComCam_* fields.
Could someone clarify which Monster photometric columns and filter mapping were actually used for DP2 LSSTCam calibration? I understand that ComCam uses the same nominal ugrizy filters as LSSTCam, so I wonder whether this similarity is the reason for the current mapping. In particular, I would like to understand why there are no LSSTCam/SynthLSST columns in the_monster_20250219, even though the the_monster repository contains DES-to-SynthLSST colour transformations.
Thanks for your question! It seems like your digging uncovered the right answer – for DP2 we didn’t derive new synthetic LSSTCam photometry for the reference catalog, but just pointed to the LSSTComCam synthetic photometry in the Monster. We plan to update the Monster refcat to include LSSTCam, but have not gotten to it yet (and may as well wait until after Gaia DR4 comes out in December?).
The Monster is used for the “first guess” at calibration. Thus, as you correctly surmised, minor differences between LSSTComCam and LSSTCam are not that important, since we refine the photometric calibration after the “initial guess” using FGCM (Forward Global Calibration Method), which incorporates information from the full dataset.
DMTN-277 describes how the Monster refcat is created, but not how we apply it to data. Section 5.7 of PSTN-019 has a brief note about its application, and there is some information about DP2 calibration in RTN-115.
Hope this is helpful! As with most of our documents, DMTN-277 is a living document that will be updated when we update the reference catalog.