This requires clear and stable skies with reasonable seeing.
Good transparency is nice but stability is more important.
Extinction Extinction measurements are observations of red and blue stars, preferably the same ones, at low and high airmass, ideally at 1.0 and about 1.8 airmases. Observations above about 2.2 airmasses are probably not as useful as those slightly under 2.0. Flats You should get at least three and preferably five good twlight flats in u' every night you observe. It's easiest to do these at sunset, but if you miss, or if it's cloudy, rainy, etc., you can get them in the morning. You should also get at least three sets of dome flats every night.