Re: Simple telescope design question

From: Chris L Peterson (clp_at_alumni.caltech.edu)
Date: 06/28/04 

Date: Mon, 28 Jun 2004 22:57:13 GMT

On Mon, 28 Jun 2004 15:42:00 -0700, Robert Maxwell Robinson
<max@u.washington.edu> wrote:

>
>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!

You start with a flat piece of glass, but not flat in an optical sense (that is,
to a fraction of a wavelength).

When you make a Newtonian primary mirror, you start by making it spherical. That
turns out to be very easy, because when you grind two pieces of glass against
each other that is the natural shape that results. It is a low energy solution.
But you don't worry about the exact degree of sphericity- that would make things
very difficult. You just get close. Nobody usually cares if their mirror that
was intended to have a 2m focal length ends up a few cm on either side of that.
But if you are trying to make a flat mirror, there is only one solution,
radius=0, and no easy natural physical processes to get you there. Optical flats
are much more expensive on an area basis than spherical mirrors. (Converting a
sphere to a paraboloid is a more manual operation, but again, there are an
infinite number of solutions, so the cost isn't very high.)

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

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