Observing Procedure

Follow these steps during the twilight period of night to obtain the necessary frames.

What are twilight frames? See Phil Massey:

Then, "reduce" these frames in a standard manner using a program such as IRAF (or this could be done with custom routines written in IDL or Python). The reduction means that each twilight frame should be corrected for bias (pixel-to-pixel variation), overscan (variation of count "pedestal"), and dark current.

Twilight flats should have a decent amount of counts (> 20% saturation level), since this determination is difficult (impossible) to do accurately with an underexposed image.