Re: Simple telescope design question

From: Robert Maxwell Robinson (max_at_u.washington.edu)
Date: 06/29/04 

Date: Mon, 28 Jun 2004 20:07:17 -0700

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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