Masks and complementary selected large regions that retain fractional coverage of the sky f_sky from 0.8 to 0.3 (see details in Sect. 3.3.1). The grey is the CO mask, whose complement is a selected region with f_sky = 0.8. In increments of f_sky = 0.1, the retained regions can be identified by the colours yellow (0.3) to black (0.8), inclusively. Also shown is the (unapodized) point source mask used.