Cumulative distribution of Planck hit counts on the sky (here at 857 GHz with N_side = 2048), with the corresponding integration time per sky pixel (given on the top axis). The three sky zones used in the analysis are defined at 857 GHz: shallow (50 to 75% of the hit count distribution, short-spaced lines, blue); medium (75 to 95% of the hit counts, medium-spaced lines, green); and deep (95% and above hits, widely-spaced lines, red).