Posted 12 August 2015 - 07:06 PM
I used to stick with a single ISO setting myself, but in practice, there is more to be concerned about than simply how much electronic noise you have in total. It also does not really take all that much to swamp all of the electronic noise in a camera with background sky, even at a decent dark site (i.e. ~21mag/sq") when doing RGB imaging.
In practice, I think dynamic range, particularly how limited it may be at a given ISO, becomes a bigger source of concern in practical situations for RGB imaging. I like ISO 1600 on my 5D III as it is nearly devoid of pattern noise and has really low read noise. However it is also pretty short on dynamic range, and it is extremely easy to clip stars. I find myself using lower ISO settings lately in order to get more dynamic range and preserve star quality as much as I can.
I sometimes find myself imaging at ISO 400, which on the 5D III DOES have some banding noise, and aiming for 1/5 to 1/4 histogram rather than 1/3rd histogram, in order to keep my stars from clipping. In my case, I am only using a 150mm aperture 600mm f/4 lens. It's a big-ish aperture, but far from the largest, and far from the fastest scope (say compared to an 8" or 9.25" hyperstar.)
In general, I do not find that any amount of electronic noise poses a problem for my DSLR RGB imaging, even when I am at a 21.5mag/sq" dark site (about 13-16x darker than my back yard, which allows me to expose 3-4 stops longer per sub.)
I think the area where read noise, quantization noise, etc. become a more important factor is when you are either at an exceptionally dark site (22mag/sq" or so, which can be ~40x darker or more than the average red zone), or imaging with narrow band filters. In both cases, you can expose for very long periods of time, and contrast can increase significantly. With RGB you can still swamp electronic noise with airglow, but you might need 12-15 minute subs to do it. With narrow band, especially with 5nm or 3nm filters, testing done by a number of guys in the BII forum have shown that you can expose for 75-90 minutes before your background sky noise swamps read noise under heavy LP, and none have ever fully exposed the background sky at any degree of dark site.
Most of us don't have access to pristine 22mag/sq" dark skies, and since the topic is on the effective Unity Gain ISO for DSLRs, narrow band does not really apply (and if you try, I think there are significantly greater concerns about the consequences of doing so with an RGB DSLR than read noise). In practice, I doubt that small differences in quantization noise, even if they are up to or more than the 2-4e- read noise at higher ISOs, are going to matter most of the time. In practice, I think that clipped stars due to the much more limited dynamic range available in most DSLRs is more of an issue...at least, it has been the largest issue I have run into with my own imaging with a 5D III and 7D.