Posted By Tom Mengel
I know some of the Celestron 6" reflectors (HD-150?) have a small "corrector lens" in front of the secondary that makes the F4 primary into a F7 overall system. I'm assuming this is a small Barlow/corrector that also cuts down on the secondary size as it allows the secondary to be "pushed back" in the optical path.
Doing some preliminary ray-trace work ups with a F5 6" mirror (I have a partially polished mirror I'm debating on finishing) In the ray-trace I come out with a 1.2' dia. "Barlow" in front of a secondary of the same size (minor axis) that yields an unobstructed 1.8" dia. field at the focus. This would end up with about a 20% obstruction and would yield a F ratio of about F8 in a tube about 36" long.
Why is this design not more common? It would seem the maker could do some nifty things off axis correction wise (and maybe even use cheaper spherical mirrors) modifying the small lens and you end up with an OTA about 1/2 the length (and weight) to boot.
So ... would any of you much more knowledgeable forum lurkers care to shoot some holes into this idea before I put any more time in on it??