Artificial Star using a ball bearing

[ggreene1]

I was wondering if anyone has used a ball bearing as an artificial star for collimating a SCT in sunlight ? I had read somewhere that this is possible. Can anyone tell me if this is true and if so what type (i.e. chrome or stainless) and what size ball bearing. Where you able to find them individually ? If so, where ? Thanks for any input.

[AlBoning]

It is true and it has been done. All the information can be found in Suiter's "Star Testing Astronomical Telescopes." The important factors are that it is far enough away and the reflection be small compared to the resolution of the scope. So think smaller is better. Some people have used Christmas ornaments. At night one can use a flashlight for illumination. However, considering the price of a Hubble Optics Artificial Star I can't imagine why anyone would want to use anything else.

[Peter Drew]

I've done this myself in the past, a 6mm ball is adequate. You need to have it set up at least 100 feet away and make sure that the Sun, the telescope and the ball are in a straight line. If the Sun's image strikes the ball too off axis it will introduce an astigmatic appearance. The downsides are getting a sunny day and low level warm air turbulence.

[ggreene1]

AlBoning, thanks... I wouldn't have thought small. I'm glad I asked.

Peter, Thanks for the info on set up and the need to keep all in a straight line.

Man I'm going to learn a lot here...