Euclid Data Release 1 (DR1) Euclid photometric masks

This paper describes the Euclid DR1 photometric masks, which trace spatial variations in the survey properties (e.g. satellite position angles, integration time, sky tiling, and survey footprint); in the instrumental performance of Euclid and the external data (EXT) it uses (e.g. survey coverage, depth, PSF width, quality flags, systematic effects and astrometric uncertainties); and in possible contamination from sky emission (e.g. galactic extinction and zodiacal light emission). Among this, the effective coverage mask is particularly important as it defines the effective survey area and can be used to exclude spurious objects when selecting Euclid photometric samples for scientific analyses. The Euclid photometric masks are built for each observed sky region (tile) and are provided as full-sky HEALPix maps (nested ordering and explicit indexing) at Nside = 16 384, equating to a resolution of 12"88, for both the Euclid Wide Survey (EWS) and Euclid Deep Survey (EDS). We estimate the total area of the DR1 EWS (EDS) to be 2008.8 (74.7) deg² and find that 82 (81)% of this area is preserved by the effective coverage mask. We observe that Euclid's instrumental properties are consistent with expectations, showing only minor relative variations across the sky. For DR1 EWS the standard deviation of the depth (computed for a 2" aperture and at 5 $σ$) is below 0.2 AB mag, with median values of 25.69, 24.14, 24.3, and 24.29 for the Euclid IE, YE, JE, and HE bands. Finally, we expect these photometric masks to be key to many science applications and in particular for cosmological analyses.