1st attempt at a Moon shot...

[astrostu]

There is nothing wrong with using ISO200, or even higher for this subject.

This is not correct. Higher ISO means more noise. More noise on a dark subject that can require longer exposures is bad. If you can do a longer exposure (as in tripod or bigger aperture), then you should do that instead of a higher ISO. Pretty straight-forward.

 

[xtort-]

True, a longer exposure is preferable, but from my experience, noise really doesn't come into play until 400-800 ISO depending on the subject.

This is not correct. Higher ISO means more noise. More noise on a dark subject that can require longer exposures is bad. If you can do a longer exposure (as in tripod or bigger aperture), then you should do that instead of a higher ISO. Pretty straight-forward.

 

[astrostu]

True, a longer exposure is preferable, but from my experience, noise really doesn't come into play until 400-800 ISO depending on the subject.

That varies by camera and by what the individual person thinks is acceptable. Should someone be afraid of using a larger ISO? No. Should they use the lowest ISO possible when doing astrophotography? In my opinion, yes. For the moon, except during a total eclipse, I always use the lowest ISO - 100 on my camera.

I only go to 200 when I'm taking 5+ minute shots or when I'm in a dimly lit museum and even my f/1.4 needs a 1/15th sec or longer shutter speed at ISO 100. I think I've only gone to ISO 400 once or twice, and that was to get a quick exposure of the stars to use to focus before I went down to 200 or 100 for the actual shots I wanted.

So, perhaps the first sentence of my initial reply was a tad strong. It is not not correct that there is "nothing wrong" with using a higher ISO. If you are willing to have more noise, then that's fine. If you want to minimize the noise and can do a longer exposure, though, then the lowest ISO setting is preferable.