The final module in the simulation handles the output data file.
Once all the image arrays corresponding to each sky component
are available, they are added together to prepare the total image
data and written out as a FITS image file. The count rate for
each contributor is integrated over the image size and given as a
number. Because of the relatively low flux levels in the UV, most UV
instruments are photon counting and thus we provide the output
in counts per second (the count rate).