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I'm not sure what the relationship between temperature and mean pixel value in a dark frame for a given exposure but there is a linear relationship between dark count and exposure time.
So I'd suggest the taking and averaging of some bias frames (zero exposure time darks). They can be done only occassionaly if you want to save time. Then take some darks of resonable duration (whatever you determine is tolerable) - these have to be done in the field for this method. Subtract the bias and scale the darks to the required exposure.
So if you had a one hour exposure you could take a ten minute [bias corrected] dark and multiply all pixel values by 6. Maybe not all S/W packages allows for this but some certainly do - eg AIP4WIN.
If you can measure the temperature fairly consistantly then you can also build up a library of bias-corrected darks and scale from those - thus saving more time - but there will always be some tempeature variation between temperature that any given library frame was taken at and actual temperature in the field.
Mark C.
Last edited by mjc; 25-07-2010 at 12:57 AM.
Reason: [biased corrected] dark
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