I put an SQM on a 1 km grid across Lisbon and made a lovely contour map of something that cannot honestly be contoured. Four months, one admission.
Two years of SQM-L readings from one hole in my lawn, the night a floodlight went up next door, and what 0.33 mag/arcsec^2 costs in hours.
Three winters on NGC 891 from a Bortle 4 backyard. Full log, the square root maths on what each block of hours bought, and the 40 hours I deleted.
Where the fifth root of 100 comes from, why 5 log D is just Pogson again, and why every aperture table you have seen is a point source table.
You've read everything.