Recently I had to Coliminate my 6" CR150 (trying for perfection) and in looking for a decent star I stumbled on the best one to use!
Betelgeuse!!!
Because it's red the center is more defined when you defocus the star and it's more apparent how centered the optics are. Must have to do with the fact that the achromatics have color and so red Betelgeuse is more defined and does not blend in like a normal white star however bright it may be as Sirius was not as good. Believe me it's the best! Use Betelgeuse and you will have a much easier time getting closer to perfection. Once you get perfect the color seems to go away on axis or is at least is less noticeable in this particular scope.
Regards,
Greg
Betelgeuse!!!
Because it's red the center is more defined when you defocus the star and it's more apparent how centered the optics are. Must have to do with the fact that the achromatics have color and so red Betelgeuse is more defined and does not blend in like a normal white star however bright it may be as Sirius was not as good. Believe me it's the best! Use Betelgeuse and you will have a much easier time getting closer to perfection. Once you get perfect the color seems to go away on axis or is at least is less noticeable in this particular scope.
Regards,
Greg