A bias level is present in every CCD--image and arises from an electronic offset which is added to the signal from the CCD before being converted to digital values (the bias prevents a negative output signal). Under the present operating conditions the bias level has a value of about 200 data units.
A special purpose command has been generated to subtract the bias from a series of frames. Note, however, that this procedure is not necessary if you are not going to take a weighted average of frames. Only in this case, will the inclusion of a bias effect the weighting given to an individual frame.
To subtract a bias value for a series of frames, execute the command:
BIAS/CCD bias_value [catalog]
where catalog is the name of the catalog containing the series of frames. The default is the general catalog. To understand how to create and use catalogs, see the description of catalogs in Volume A, Chapter 3.