tokens: to 0.015 mag. Again, we used the Kron magnitudes for the subsequent analysis. An example of the excellent HEROES image quality, and also the various artefacts produced by bright objects in the field, is given in Fig. 3 showing the shell around the planetary nebula NGC 6543 (Cat’s Eye Nebula) that was ejected by its predecessor red giant star. The actual PN in the centre of the image is completely over-exposed. The high density of faint background objects shows the excellent sensitivity of the data. The NEP lies at relatively low Galactic latitudes, and therefore contains a rather large number of overexposed foreground stars showing up in this image (in cyan). In order to obtain reliable optical photometry for brighter objects, which are saturated in the HEROES HSC images, we also made use of the SDSS DR16 ( Ahumada et al. 2020 ), the Pan-STARRS DR1 DR2 ( Chambers et al. 2016 ), and Gaia DR2 ( Gaia Collaboration 2018 ) catalogues. We also cross-correlated our samples with the far-UV and near-UV photometry from the GALEX surveys ( Martin et al. 2005 ). Fig. 3. False colour HEROES image of the shell around the planetary nebula NGC 6543 (Cat’s Eye Nebula) close to the NEP. Blue represents the HSC g -band image and predominantly shows the green [OIII] line. Green represents the HSC r -band and predominantly shows H α . Red corresponds to the 4.5 μ m Spitzer image and mainly shows dust emission. 4.2. Mid-infrared observations of the HEROES field For photometric information in the 3−25 μ m band across the whole HEROES field we used the WISE all-sky survey catalogues. For the shorter wavelength W 1 (3.4 μ m) and W 2 (4.6 μ m) bands we used the CatWISE2020 all-sky catalogue ( Eisenhardt et al. 2020 ) (see also Marocco et al., in prep.), containing about 1.89 billion objects observed by the Wide-field Infrared Survey Explorer (WISE and NEOWISE), both of which have higher sensitivity and higher angular resolution than the original ALLWISE catalogue ( Cutri et al. 2013 ). The CatWISE2020 catalogue in the HEROES field contains 2.4 million objects and gives fluxes in the W 1 and W 2 band, which we converted to the AB magnitude system. For the longer wavelength W 3 (12 μ m) and W 4 (22 μ m) bands we used the original ALLWISE catalogue ( Cutri et al. 2013 ), again converting to AB magnitudes. At the faintest magnitudes the CatWISE2020 and ALLWISE catalogues are, however, severely confusion limited. In the centre of the HEROES field, we therefore also made use of the Spitzer Observations of the Euclid Deep Field North (Moneti et al., in prep.). The ESA Euclid mission is making great progress towards its launch, scheduled in 2022. Euclid ’s main goal is to survey a large fraction of the sky and image billions of galaxies to investigate dark energy and dark matter over the history of the Universe. Roughly 10% of the observing time will be dedicated to the Euclid Deep Fields, repeatedly observing three specific areas of the sky, covering a total of
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