It's not just a question of magnification, if it's detail you're after then you need to make sure you're exposing correctly. It's a very bright object in the middle of a pitch black sky, your camera is not going to do the right thing in that situation. There are probably calculations, but really it's just a case of trial and error with exposure time to see if you actually can see anything other than a point of light.
I needed almost as low as 1/1000 to see anything on the moon a few days ago.