Distribution of galaxies in the PAUS W3 field, with points coloured according to the log of each galaxy’s Qz value – this quantity describes the quality of photometric redshift estimation (low Qz = high photo-z quality) through a combination of the goodness-of-fit of SED templates to narrow-band photometry, and the shape of the resulting redshift probability density function; see Sects. 2.2 and 2.6. Circular holes in the footprint correspond to masks around foreground stars, with the largest gap (top-right) covering the Pinwheel Galaxy (Messier 101).