At this moment I am thinking that this issue hasn't a global answer. Many others things can affect the ideal time and ISO.
Some people consider as same thing ISO and gain of amplifier. Since digital camera is the main camera sold, why we haven't a potentiometer to adjust linear or logaritimic this gain ? Similar to audio amplifyer. Why it is in step: 100, 200 , 400 , ... , yet ? Would be because ISO isn't only gain ? There is some more work with the signal to correspond with the old ISO from film results ?
No matter. I will see my Canon as a black box. I have few steps of ISO to use: 200, 400, 800. ISO 1600 with my normal camera temperature about 35 degrees celsius is not useful. Therefore my main option will be 400 and 800.
I have a limit of signal on the histogram. The ideal would be around 25 % to 50 %. But my normal sky glow can kill it. So the maximum time of exposition with those two ISOs can be found fastly. Perhaps with 2 or 4 captures, since I work with steps of time: 30s, 60s, 120s, 180s, 240s ... Less dark files to store in my hard disk.
Therefore, do what I did: take 2 or 4 photos. Analyse the shape of stars or details from a cloud. Analyse the colors of stars or clouds. And ... the end ! It's done.
note: amount of noise can be reduced by dithering and dark files subtraction.
Undoubtly with CCD this is different. With them we have linear control of the gain ! And, perhaps, here is an other misunderstanding. Some people think the DSLRs as CCD. "you need more dark, you need more time of expositon, you need more ISO (gain), ..."
CCD and DSLRS are two different worlds ! Mainly if you are talking of
stock color DSLR.
