deletions | additions       diff --git a/subsubsectionImages_.tex b/subsubsectionImages_.tex  index fb7f63e..cf007e4 100644  --- a/subsubsectionImages_.tex  +++ b/subsubsectionImages_.tex 
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ccdproc} on (as the flat field is different in different filters).  Then run {\tt ccdproc} on the cluster and standard images again.  To %To  make sure which filter the image has been taken with, you can look in %in  the ``header'' -- this is the part of the file that contains all the %the  technical junk about what the telescope was, what (and where) it was %was  looking at, how long it looked for, etc. To read the header of the image %image  bob.fits, use the command {\tt imhead bob.fits}. (If you get just %just  a single line, use {\tt epar} to change the {\tt longhead} parameter %parameter  of {\tt imhead} to yes.) Is there any noticeable difference between image before and after bias/flatfield correction? What about the background levels (i.e. blank spaces between stars), are they zero now? Is the background the same for all filters? (Note: the exposure times for these images were not the same. The B, V, R and I images had exposure times of 60, 7, 5 and 3 seconds respectively.) The background is due to \emph{sky brightness}. During the day, the Sun excites molecules in the atmosphere, which re-radiate at night. Most of the re-radiation is at specific (quantised) wavelengths in the V, R and I bands and the near-IR. Hence, the counts in your CCD measurements of stars also contain counts from the night sky. Luckily, the night sky uniformly illuminates the CCD just like a flat field. Your flat field corrections should make this background uniform across the CCD, hence it too can be compensated for.