artificial star

[stuw2750 (Stuart)]

Hi
Im trying to collimate my telescope with a hubble artifical star
i find i cant get the airy disk just the donut only - wont focus any further
its rougly abt 20 mters away
has anyone experience with these - My telescope is an 11 inch celestron CPC GPS
im thinking i might be too close would that be correct ?


any pointing in the correct direction would be appreciated.

[multiweb (Marc)]

Still too close. From memory my C11 needed at least 26m to come to focus. Or you could put a lot of extension tubes at the back to reach focus. But it's best to keep the separation between primary and secondary close to the ideal spacing which from memory is around the 41cm ball park.

[5ash (Philip)]

There is a formula for calculating the distance between the artificial star and telescope (suiter's book would have it in). It depends on the size of the artificial star and resolution of the telescope among other things. I do remember making such a calculation for my 9.25" some years ago and it was about 43 metres. For a c11 it would be more , as a rule of thumb about 20 x the focal length. It has to be this sort of distance to give a star size better than the resolution of your telescope otherwise your looking at a round hole not a starlike point.
Philip

[yoda776 (Matt)]

I have a COMA by www.adrianololli.com
One I have is the blue one - http://www.adrianololli.com/listino.asp

First time I have attached in a forum outside of selling, so hope it works.

It appears to depend on the size of the artificial star, particularly for my artificial star.

The table indicates the size of the scope on the left, then distance from a 0.019 size star, 0.035 and a 0.050. The distance increasing with artificial star size.

It works on my Meade LX200, Celestron NexStars I had and a friend's LX90 10".

not sure if this helps you but thought worth a shot.

[75BC (Brendon)]

I'd love to know an answer to this as I have the same scope and thought of getting an artificial star but read vastly different figures of the distance required.

Unless I was to take my scope somewhere, which would be inconvenient, the most I could get at home would be about 25 to 30 metres.

[Satchmo]

If you are just collimating all that counts is that you can at least focus on the artificial star. If you are trying to evaluate optical quality thats a whole different ball game . Distances at which the proximity of the artificial star won't induce some spherical aberration become much longer as the aperture increases, but spherical aberration is not a great hinderance to collimation.