15.3.3 Bad fits in SE++ due to missing data

Not all tiles have complete coverage in all bands (see Sec. 7 of the MER DR1 paper); however, the catalog contains information on objects located in any band. In principle, this would mean that, for bands with incomplete coverage, the photometry for objects located in that missing area should be set to NaN. Although the model fitting engine from SourceXtractor++ does not fit these sources, the output is set to (in most cases) the input guess for the object. This means that for incompletely covered tiles, the MER final catalogs contain numbers from the SE++ Sersic fitting that are not measured. We became aware of this only after the DR1 processing was already finished.

If users want to take this into account, we have also developed a program²² 2 Contact us at euclid-helpdesk@cosmos.esa.int with the subject “Bad MER catalog Sersic fitting results” that identifies Sersic-fitted sources with bad or missing photometry in the Euclid OU-MER DR1 catalogs and sets their flux values to NaN. It writes cleaned copies of the MER FINAL catalog, the MER morphology catalog, and – for DEEP tiles – the deep photometry catalog, leaving the originals untouched. The input catalogs for the given tile(s) are automatically located in the local DR1 data store.

For each requested tile, the program discovers all Sersic-fitted bands present in the catalog (from the FLUXERR_<band>_SERSIC columns), flags the bad objects per band, nulls their FLUX_<band>_SERSIC and FLUXERR_<band>_SERSIC values, and then propagates those nulls to the corresponding SERSIC* morphology columns and to the extended (EXT) Sersic fluxes.

An object is flagged as bad in a band if any of the following hold:

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  the fitted flux is NaN, or its error is zero or NaN

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  the flux error is outside the range ${\log }_{10}$(FLUXERR_<band>_SERSIC) in [-5, 5]

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  the fitted Sersic flux is indistinguishable from the detection flux (fractional difference $\le {10}^{-6}$), which is a sign of a failed fit.