Artificial star questions

I'm getting ready to make an artificial star using a flashlight (Mini Mag) and thin metal preforated with a pin.
I have a couple of questions for those with experience:

Should star tests and collimation be done at night with such a set-up, or will this work in daylight as well?

What is a good distance to set the scope for the star?
I'm using this for a 9.25" SCT f/10.

If I stack sheets of the metal and preforate them on a hard surface to get sheets with holes of different diameters, how do I know which is optimal? Trial and error?

David

I found that a regular sheet of Reynolds Wrap aluminum foil from the kitchen wrapped around a cheep 2 "D" cell flashlight has quite a few micro pores that make great stars for collimating my refractor.

To get bigger/brighter stars I'll slightly crinkle the foil.

The flashlight works well indoors during the day and great at night.

Jerry

David Pitre said:


What is a good distance to set the scope for the star?
I'm using this for a 9.25" SCT f/10.

David

David,

The distance doesn't matter much for collimation. However for a star test it has to be far away so that the rays are almost parallel, or else the star test will show sperical aberration. Suiter's book on optical testing has a table listing the required distances for various diameters and f-ratios, but I'm at work and the book is at home. If I remember correctly it's around a hundred meters for a 10" f/10.

One thing to note: an artificial star will usually be mounted low, so your scope will be almost horizontal. This means you'll be looking across a long stretch of ground, which will cause lots of heat distortion unless you wait until early morning. If you can locate your star across a grassy field or a body of water you can minimize this. Also your scope and mount may be flexed or distorted differently when looking horizontal vs when looking up, meaning your perfect collimation may not be perfect any more.

I messed around with artificial stars for a while (I used a single optical fiber lit by a green LED). I found the drawbacks too numerous...now I just use Polaris.

Tim Iafolla