Density plot of background surface brightness in the Hα band as a function of measured mean sky level in counts. The colour scale is logarithmic, with blue representing the highest density of points. The images used to build this diagram are the IGAPS uniformly calibrated set, which enables validated conversion of the sky counts to surface brightness via each image’s zero point. The black dashed line has a gradient of 3.32 × 10⁻¹⁷ ergs cm⁻² s⁻¹ arcsec⁻² per count.