How far away does a light source need to be to act as a false star...?

[AstroAdam]

I'll explain!

With the cruddy nights we have at the moment, I'm trying to get a QHY5v camera set up on my finderscope. Got the adapter, but have heard of issues with not being able to get focus. It's a 9x50 skywatcher finder.

Question is, I have a few light sources I can use out of my open window to test if I can achieve focus at infinity. I can focus on them, however, I;m not sure how far away can be regarded as infinity (i.e. would a light a few hundred metres away be okay, or are we talking a few miles?). I basically want to make sure I'll be able to focus on a star once the sky clears.

Hope you see what I mean...!

Cheers!

[username]

Off the top of my head 100 yards should be ok.

If you can focus on something a few hundred yards away you'll be able to focus at infinity , especially if you have some travel left in the focusser

[Photosbykev]

as above, but no reason why you don't try to focus on a far hill side in daylight, maybe the top of a distant transmission tower a few miles away

[kirkster501]

http://hubble-optics.com/artificial-stars.html

these are most useful. The smaller the light source the closer it can be as a general rule. Stil needs to be a good 50 meters or so ideally.

steve