read noise vs dynamic range (and exposure)

[ChoJin]

Hello

I have a kind-of math question about the tradeoff between the read noise and the dynamic range for the Canon 6D.

I have a hard time deciding between shooting at ISO800 vs ISO1600

according to http://clarkvision.com/reviews/evaluation-canon-6d/index.html

800  0.74      4.6      9900        11.1
1600  0.37      2.9      4900        10.7

(the value are slightly different at http://sensorgen.info/CanonEOS-6D.html but the idea is the same)

at ISO800, the 6D has a read noise of 4.6 with a maximum signal of 9900
whereas at ISO1600, the read noise drops further to 2.9 with a maximum signal of 4900.

After ISO1600, it becomes roughly linear.

If I understood properly explanations at http://dslr-astrophotography.com/iso-dslr-astrophotography/ (and some other sources), it is therefore best to shoot at either ISO800 or ISO1600 to improve the SNR.

Now I'm puzzled to decide which one.

I also started to wonder if there were a way to mathematically compute in advance a proper exposure for a given target.

Let's say I want to image M31. ISO800 or 1600?
At ISO1600 I would be more likely to saturate the core since the dynamic range is reduced, but how long could I expose (ignoring sky fog from the light pollution at first)? Is there a way to compute (even roughly) that exposure or even decide which ISO to use without having to just experiment a lot?

In summary, what I'm basically asking is whether or not maths could help here to make an educated choice instead of pure experiments.

Any insights?

[pfile]

there is a script in PI called CalculateSkyLimitedExposure which can help, as long as you know the gain, read noise and dark noise for a sensor (all expressed using electrons).

there might be a thread in the forum about the script if you search, can't remember (the tool has been around a very long time.)

rob