Some or all of the images may already be dark-subtracted. Usually the image name will indicate if it has been processed in this way. The raw image might be ``m51.fits'' and the dark subtracted image might be ``m51d.fits''. If not, on the command line in a terminal window and in your image directory (use cd to navigate), an ls command will remind you of the file names. You may look at the notes.txt file to see information about each image. We need to remove the dark background by subtracting from each raw image of the sky an image taken - with the same exposure time - with the shutter closed. This is a ``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 
. Use the dark frames that are closest in
time to the image files and make a dark-subtracted image file for
each region we recorded. It may be helpful to use a similar file name
such as ``neptuned.fits'' for a dark-subtracted neptune image 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. Often these will have been done for you and you can go immediately to the next step.