Re: Speaking of Ritchey-Chretien Scopes

From: John Savard (jsavard_at_excxn.aNOSPAMb.cdn.invalid)
Date: 01/10/05 

Date: Mon, 10 Jan 2005 01:29:41 GMT

On Sun, 09 Jan 2005 22:35:05 GMT, "Hilton Evans"
<hiltonevans@dont_spam_me.net> wrote, in part:

> I think that's why I put *unsuccessfully* in parentheses
> above. My question was might it have made sense for
> Criterion, or some other manufacturer, to try to compete
> with Celestron with an "easier" design.

The Meade RCX400 hardly qualifies as easier, and the same goes for a
straight Ritchey-Chretien.

A Maksutov isn't easier - the corrector uses way too much glass, and is
steeply curved. The surfaces may be spherical, but the aspheric surface
on a Schmidt-Cassegrain happens to be easy to make... if you know the
trick. (Hence, the *lawsuits* in the Schmidt-Cassegrain field.)

If you want something easier than an S-C, something like the Cape Newise
or Ralph W. Fields' design might be considered, and, perhaps someday a
Chinese firm will go in that direction.

Unfortunately, I can't seem to get Ralph W. Fields' design to work well
with small central obstructions. My current thinking is going in this
direction:

radius thickness material
 414.146 5 N-BK7 full aperture corrector
 402.814 300 air
-810.947 -265 mirror to air
  94.669 -10.935 N-BK7 Fields corrector
  96.045 -1.5 air
-397.620 1.5 mirror to air
  96.045 10.935 N-BK7 Fields corrector, second pass
  94.669 458.944 air

Note that the full aperture corrector bulges *out*, and so one can't do
anything like a Gregory-Maksutov... unless one wants to go Gregorian
instead of Cassegrain, but that would juggle the corrections completely.

This sort of thing is a telescope "easier" to make than an S-C, and it
still offers a sealed tube, without the element sealing the tube being
merely an expensive but inactive flat.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html