On the command line in a terminal window
an ls command will remind you of the file names. We need
to remove the dark background by subtracting from each raw spectrum image
an image taken - with the same exposure time - with the shutter closed. If the
directions were followed early, these will all be the same
100 second ``dark'' frame. It records the bias in each signal element that
results from the thermal transfer of charge. These losses very nearly
are independent of exposure to light, so we can simply subtract them and
get an image that is free of dark signal.
There is a program on the system that will subtract two fits image files.
On the command line, type fits_subtract and you will be prompted
for the name of the first image file 
of the pair to be subtracted to
make a difference file 
. The
important thing is to use a naming sequence that is easy for you to
remember. The system does not care what names you use, but do not
overwrite your original files.
In the following you will use only the dark-subtracted files.