The trended flux is determined on a bias subtracted calibration lamp
 exposure in LSS mode, generally obtained with the 300V grism, and
 stored in the keyword QC LAMP FLUX. After the background subtraction
 the total signal is integrated and divided by the exposure time and
 by the total number of CCD original pixels (keeping in account a
 possible rebinned readout). In the case of FORS1 data, where overscan
 regions are missing, the background is evaluated as the median level
 of the first 200 CCD columns for flat field data, while for arc lamp
 data a background map evaluated from the regions without spectral
 lines is computed and subtracted.