Re: Simple telescope design question

From: Martin Brown (|||newspam|||_at_nezumi.demon.co.uk)
Date: 06/29/04 

Date: Tue, 29 Jun 2004 10:12:34 +0100

In message <Pine.A41.4.58.0406281535490.84304@dante67.u.washington.edu>,
Robert Maxwell Robinson <max@u.washington.edu> writes
>
>Thanks for your help; I'll look at websites on solar scopes. The idea
>that large, flat mirrors are harder to make than large parabolic
>mirrors sounds *way* strange to me; I thought you practically started
>with the one to make the other!

Rough flats are easy - window glass is roughly flat. But making optical
flats is another matter altogether. The classical method requires three
identical pieces and working each one against the others in a systematic
way to get them all exactly flat and polished. Its a lot of work. Most
ATMs buy their flat since they only want one, and it is tedious to do
well.

If you want to look at it another way. Making an optical flat is rather
like trying to make a mirror with a specified radius of curvature (R ~
infinity). ISTR For a lambda/8 flat of diameter D it comes roughly to
R>10^7 x D

>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.

That isn't how it works. The wavefront must be unmolested on the way
into the scope or the resulting image will not be diffraction limited.
>
>At any rate, I'd like a second opinion about how much trouble the flat
>mirror would cause in this design.

It can be done, but it would be more expensive than the standard method.

Typically it is used for some professional scopes that are too bulky or
fragile to be pointed at the sky. Try a Google search on siderostat or
heliostat. Some of the optical interferometry systems use flat mirrors
to collect the starlight to feed into the optical bench for combining.

Regards, 

-- 
Martin Brown