Specifications

16 AUTOSTAR CCD PHOTOMETRY
Note: It is very important that the optical train is not changed
between imaging and taking the Flat Fields. This means no moving
or adjusting of the camera relative to the telescope and only minor
focusing. This is why it is usually best to take flats at the end of a
session. If the optical path is not identical for the Flat Field images
and the star images, the flat fields will be of no value, as they will
calibrate against the wrong pixels.
Take a stacked Flat Field image for each filter. Take at least 10
exposures (the more the better) and combine them for each filter.
Name these stacked images “FB”, “FV”, “FR”, and “FI”, for the B,
V, R, and I filter Flats, respectively. (The names are needed to
ensure the right Flat Fields are used to calibrate the particular filter
images later.) Figure 2-18 shows a sample Flat Field taken using
the evening twilight sky and I band filter. (Flats taken in other
filters will look very similar.)
Figure 2-18. Sample I Filter Sky Flat.