Astrophotography ISO – what’s your best setting? well that really depends on your camera. For a one shot image it’s best to experiment to see what your camera is capable of. Finding the optimum signal to noise combination is about testing different settings and seeing what works best for your camera. Too high an ISO and the image will be too noisy and bringing it down won’t help. Under exposed and trying to recover the detail will make it noisy as well.
Generally speaking – recovering under exposed areas will result in a more noise. Severely under exposed areas may not include any detail. I’ve read (sorry – can’t remember the source) that recovered shadows will be 3 times noiser – however I think this will probably be camera and conditions dependent.
Canon 6d Tokina 11-16 at 15mm F2.8 at 30s with different ISO settings. Lens correction and colour balance only applied to the images.
Now having said all of that – it doesn’t necessarily mean too much unless you put it on an even footing. To the fisrt is ISO 800 and +4 exposure in lightroom. To the second is ISO 6400 + 1 exposure in lightroom. You can see the different effects by recovering the exposure.
Don’t wait to test until you’re out in the field – you can easily go to a dark location (including a dark room) and test how noisy it is at different ISO’s and exposures. Note that the noise will still be slightly different on the field – because the colder the temperature, the better the noise performance (generally).