Re: Simple telescope design question

From: Bill Nunnelee (zosma_at_earthlink.net)
Date: 06/29/04 

Date: Tue, 29 Jun 2004 14:01:33 GMT

Your idea has been used to build binocular mounts. The first surface
mirrors found in photocopy machines are sufficiently flat for low power
viewing.

http://www.backyard-astro.com/equipment/skywindow.html

"Robert Maxwell Robinson" <max@u.washington.edu> wrote in message
news:Pine.A41.4.58.0406281944480.94458@dante73.u.washington.edu...
>
> Thanks, James! This puts the nail in the coffin.
>
> To sum up the responses I've received, a pre-primary flat surface
> isn't a good replacement for a small flat secondary. The flat would
> need to be bigger than the primary (the problem I already knew about).
> It would need to be quite flat over its entire surface, and there is
> no known process for grinding something optically flat that approaches
> the ease with which a parabolic mirror can be ground; the best idea
> involves grinding three blanks against each other, which takes half
> again as much work.
>
> All of this leaves the idea of using a curved mirror instead of a flat
> mirror, and that puts the question firmly into a different category.
> If I keep going on this idea, I am certain I'll end up reinventing the
> classical Cassegrain design, or something else that was discarded in
> favor of the classical Cassegrain.
>
> I was sure there was a good reason I haven't seen that design; turns
> out there is.
>
> --Max
>
> On Tue, 29 Jun 2004, James Horn wrote:
>
> |Robert Maxwell Robinson <max@u.washington.edu> wrote:
> |
> |> One comment I was going to make was that I don't think the large
> |> "flat" mirror would need to be nearly as flat as the flat secondary of
> |> a standard Newtonian, since the flat secondary is put at a place where
> |> the image is already highly magnified.
> |
> |Actually, it's far worse. The secondary in a Newtonian only needs to be
> |accurate over an area as large as a point in the final image appears on
> |it. So, for instance, my 2" diagonal on my 10" f/6.5 Dob needs to be
1/10
> |wave (or whatever you're going for) accurate over each 1.25" area of its
> |surface.
> |
> |A pre-primary flat has to be that accurate over the *entire* surface - or
> |over 10" in my case. And do it after a hole has been put in it, with the
> |change in stresses that yields. And do it at the front, unprotected from
> |dew and thermal changes.
> |
> |It does eliminate spider (secondary mount) diffraction though.
> |
> |Best to you!
> |
> |Jim Horn
> |

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