Re: ATM RE: 12" refractor

["Tom & Lou Krajci" <krajcit@3lefties.com>]

>From: astrojoe@erols.com

>Hi Roger,

>snip>
>> Personally, I suspect that a very well-made Cassegrain of f/25-30,
>> employing a small secondary, aluminum tube that's well ventilated, and
>> capped with an optical window will perform as well or better than your
>> proposed refractor.

>Why an optical window ? What benefit would the OTA gain. I have owned
>both a  C-9.25 & C-14 and found the trapped air to be a major problem in
>cooldown time. Wouldn't the additional diffraction from a spider be the
>lesser of two evils ?

I strongly recommend that IF you use a window, it be antireflection
coated...the lower the reflections the better.  Rutten and Van V. in their
book Telescope Optics   http://www.willbell.com/tm/tm6.htm    mention on
page 232   "Internal reflections in catadioptric telescopes can sometimes
lead to a considerable loss of contrast."  There can be halo/ghosting
problems as well as an overall loss of contrast.

Tom Krajci