It would be ok to take pictures of the moon, but you won't use it for something like the milky way. Astrophotography is typically done with a wider angle lens (35mm or less) and something like a 1.4 or 2.8 max aperture would be better. You might pull off other large celestial objects, Venus when it was closer, but even 800mm isn't going to fill the frame so you'll need to crop quite a bit in post. This isn't going to be like shooting through a telescope if that's what you're after.