Should I in fact be taking a dark frame for each observing session,
and if so does this indicate there's something wrong with the camera?

Tom


I think you need to take frequent dark frames. Probably more often that just
once every observing session.

The dark current is a function of the temperature of the chip and the
temperature of the chip is not stable. The dark frame used should be taken at
the same temperature as the image, the best way to do this is to use a dark
frame that has been taken within moments of the actual image so the
temperatures of the two are as close as possible.

Also, I believe it is necessary to take individual dark frames for each length
exposure. I know that some more sophisticated cameras with thermostatically
cooled CCD chips use multiple dark frames and average them.

Also, make sure you are saving both the original image and the dark frame in an
Uncompressed Format, either some sort of RAW file or as a TIFF. If you save it
as a JPG, the "lossy" compression will not retain the accurate pixel by pixel
infomation necessary for the dark frame subtraction to be effective.

Best wishes, clear skies...

jon