Personally, I'd like to see Dave build a nice Ritchey-Chrétien (hyperbolic primary, convex hyperbolic secondary) - so he can teach me how to build and test one. <g> After all, that's what all the big-boys (HST, Mauna Kea, MMT, etc. etc.) use. That way I can build by dream 30" RC ten years from now. Clear and dark skies, Dale.

-----Original Message----- From: Chuck Hards [mailto:chuckhards@yahoo.com] Sent: Friday, February 21, 2003 1:58 PM To: utah-astronomy@mailman.xmission.com Subject: RE: [Utah-astronomy] Mirrors

Dave, consider a modified Newtonian approach, where the diagonal is set at a much lower angle than 45-degrees. The light beam is shot back down the tube and exits at a grazing angle, basically as low on the tube as you want. A third mirror can then turn beam back out at a right angle to the tube.

Once you get above 16" inches or so, the light loss of a secondary mirror becomes minimal in proportion to total area. The major advantage of an unobstructed system is a lack of diffraction artifacts, but unless you get the correction JUST RIGHT, the lack of diffraction spikes & better energy distribution in the diffraction rings won't offset the astigmatic images. And even the best unobstructed systems I've seen don't produce textbook-perfect spot diagrams.

Also, typically the larger the unobstructed system, the more difficult the optical work, -AND- an absoulutely rigid tube assembly is mandatory.

My 6 cents.

C. --- David Dunn <david.dunn@albertsons.com> wrote:

...

I think that next year I will try making a 20" f/10. I plan to fold the optics and view from the ground. I am researching this year to see how I can correct for Astigmatism and Coma because I would like to put the secondary off axis. So far I think I will have the 20" ground to f/10. I will use a 10" secondary and f/8 and bring the light back down to a diagonal to put the light in my eyepiece.

Dave