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. These should be as close to your "science frames" before actual flat fielding, since this program is actually designed to determine the illumination correction, which flat fielding is designed to remove.

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.

Corrections:

• Nonlinearity: good to keep twilight flats to the 10,000 or lower count level, otherwise we would possibly need to correct for non-linearity effects, which adds another step to the process.