15.3.4 Inconsistent worst resolution PSF assignment

During the aperture photometry process, the MER pipeline homogenizes the source stamps in each band to the worst resolution PSF (PSF with the larger FWHM value). This is done on a source by source basis, and the worst PSF normally comes from one of the EXT bands.

After DR1 validation it was found that in few MER tiles that were only partially covered is one of the EXT bands, the assignment of the worst PSF was not correct. Sources falling in regions not covered by an EXT band could be assigned a low resolution PSF from that band. Their aperture photometry measurements used larger apertures than the should have used, which added some extra noise to the measurements. Fig. 15.3 shows the problem for tile 10183462. The HSC r band data covered a small fraction of the tile (upper panel). Euclid sources falling in empty HCS r regions were assigned a PSF from that band as the worst resolution PSF (middle panel). The MER pipeline has been updated to make sure that sources falling in empty regions do not get assigned a PSF from that band (bottom panel).