Use ds9 to look at each of the dark frames you took at different times and temperatures. Sample around the images with the cursor, find a region that looks reasonably uniform, and for each image note an average signal level.
5. For the series at 2 C with different exposure times, note the average signal level in each frame. Plot a graph of the dark signal versus time. Is this signal linear in time? Is it zero if you extrapolate back to an exposure time of zero? If not, what does that mean?
6. At each temperature, calculate the number of electrons/sec/pixel that result from the dark current. You can do this simply by dividing the measured signal in ADU by the exposure time in seconds, and multiplying by the approximate CCD sensitivity scale in ADU/electron. Plot a graph of this function.
7. How would you describe the dependence of the dark current on temperature? Is there an optimal temperature for this CCD?