Locating slitlets and flat-field correction

The very first thing you should do now is to locate the limits of your slitlets because this information is needed for all further commands. Therefore you type
LOCATE/MOS

This should produce the table .tbl ( mos) with the columns

and write the total number of slitlets to ( 0). It may be that the threshold defined by ( 500) is either too low (e.g. below bias value) or too high for your data. Then you should have a look at your flat field and try again with
SET/MOS flatlim=new_threshold LOCATE/MOS

If you do not have FORS data the column :xoffset will be set to zero. To determine the offsets of your slitlets relative to the first one (which is not necessarily identical with the center of the CCD) use
OFFSET/MOS

This command will take the calibration spectra of the different slitlets and by correlating them with the arc spectrum of the first slitlet will derive their offsets.

Normally spectroscopic flat fields show the spectral signature of the lamp with which they were taken. You can take out this spectral intensity distribution with
NORMALIZE/MOS

By default this command will fit a polynomial of ( 3) order to the averaged (along the slitlet) spectral intensity of the flat field (separately for each slitlet) and divide it by these fits. The results are stored in the frame . If you have not written anything to this keyword it will derive the name of the normalized flat field by adding an `n' to the name of the input flat (e.g. nmosff1) and write the name to . To perform the actual flatfield correction type
FLAT/MOS

This command will take the frame .bdf, divide it by .bdf and write the result to .bdf. If you have not written anything to this keyword it will derive the name of the flat field corrected object frame by adding an `f' to the name of the input frame (e.g. ffors0001).