you can still use the simple formula of
http://www.iceinspace.com.au/forum/s...d.php?t=117010
TargetADU = BiasADU + 10*RN*RN/camera_gain
with the following qualifications:
- the formula assumes negligible dark current, but the DSLR will have dark current that increases as the chip warms through an exposure (ie, it isn't negligible and it isn't a constant effect)
- the DSLR pixels look at the sky through Bayer filters, so the measured sky brightness will depend on which filter the test pixel is under.
From your measured data, it seems reasonable to take the RN and add a bit to account for dark current fluctuations - and use that as the noise term in the formula. Assuming a system noise of ~6e yields a TargetADU of a bit over 3000, which is what you could aim for. When measuring the sky background to see how it compares with the targetADU, suggest you move the cursor around and try to estimate the miniumum sky level, rather than the average and then vary the exposure so that the min is at the TargetADU (this gives good SNR in the least sensitive colour channel). It would be better to use something that returns the actual ADU when measuring the sky background, rather than normalised values like PI gives. The whole-image histogram is not an ideal way to measure sky background if you have much vignetting. However, if you choose that approach and the field is fairly evenly illuminated, setting the sub length so that the left hand edge of the histogram is at about 3000 should give you a reasonable exposure.