Originally posted by BarryE In complete ignorance, I've never thought about this. I'd have guessed that focussing at infinity was all that was ever needed for astro photography. If there's any question of where the lens' infinity point is I'd have thought this could be set using a distant earth based object. I suppose at wide apertures, a remote object on earth and a planet may be a fractionally different, but there's always the moon. Focus on that obliging body then what's a few million miles to mars and onwards. Guess I'm missing something, As I say I'm ignorant of astro stuff (and much else ...).
Almost …
The problem with using (or relying on) the moon is that, when it's up, and especially when it's a week either side of full, it tends to "wash out" the more interesting detail in the sky, even more so if the air in your area is less than "crystal clear"
Finding a bright star or planet to focus on is often the only available option when working under "ideal" conditions.