Flat field

A flat field is the response of the complete camera system to a uniform source of light. Ideally a camera system would record the same brightness at every point on the sensor when exposed to a uniform light source. In reality there will be limitations in the optics and in the sensor itself that causes each point to have some variation.

If the flat field is recorded and stored it is a simple matter of subtracting it from any subsequent pictures to remove these variations. Many digital cameras has flat field subtraction of the sensor built in and it should be enabled for long exposure times. The camera then records and subtracts the sensor flat field with the shutter closed after the the main exposure.

An artificial flat field can be constructed by different methods if a true flat field can't be obtained.

See Coping with light pollution using artificial flat field subtraction.