Fig. 4

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ERO zodiacal light flat-field. This is a combination of large scales (>10ʺ) derived solely from the zodiacal light observed in any long VIS exposures and small scales (<10ʺ) coming from a weighted average of five high-S/N LED internal calibration images. These have wavelengths of 573 nm, 610 nm, 660 nm, 720 nm, and 890 nm, matching spectral coverage of VIS. The colour scale is arbitrary, set to explore the full range of intensity within each image. The top five images show the wavelength-dependent evolution of {gain × quantum efficiency}. This large-scale pattern is removed in the flat-field, replaced by the one from the pure zodiacal light image (bottom left). The overall left-to-right gradient is caused by the telescopic off-axis illumination and the VIS shutter effect (see also Euclid Collaboration: Cropper et al. 2025). The final ERO VIS zodiacal flat-field (bottom centre) incorporates all scales present in the Euclid signal, from pixel-to-pixel sensitivity variations (bottom right, highlighting a quadrant covering 205ʺ × 206ʺ, with a zoom on a 30 by 30 pixels area showing the pixel-to-pixel sensitivity differences) to the scale of the FoV.
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