In general, the actual flux of an object is the difference between the flux observed towards the target, which is the sum of source and background flux at that position, and a background flux level derived from an observation nearby the target postion. For chopped mode observations the background flux level is obtained from the chopper off-position(s).
For staring observations separate observations are necessary to determine the background flux level. It is recommended to perform the background observation before the target observation in order to minimize the signal transient due to the flux step which increases the photometric stability.
The instrument settings of the background observation should be accomodated to the background flux level in order to optimize the dynamic range of the read-out electronics. This is done by setting in the background observation the requested ``source flux'' to the background flux level and the requested ``background flux'' to zero. The background flux level at any given position can be estimated using the IRAS or COBE all sky data.