Re: Simple telescope design question
From: Vladimir Sacek (vladis.2_at_juno.com)
Date: 06/30/04
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Date: 29 Jun 2004 19:11:16 -0700
Robert Maxwell Robinson <max@u.washington.edu> wrote in message news:<Pine.A41.4.58.0406281400280.81110@dante67.u.washington.edu>...
>
> My question is about a variant of a Newtonian reflector. A Newtonian
> reflector has a parabolic primary and a flat secondary that is placed
> on the optical axis some distance shy of the focal point. The light
> reflected off the secondary goes to the eyepiece.
>
> Would it be a Bad Idea to reverse the order of the two mirrors? The
> flat elliptical mirror would have to grow to have the same diameter
> (along it's _shorter_ axis) as the parabolic mirror, and would be
> similar in position to what I think is called a Steering Mirror.
> Light would hit the steering mirror, then the parabolic mirror, then
> pass through a hole in the steering mirror and go directly into the
> eyepiece, like this (only longer):
More viable arrangement is one with the first surface as a round flat,
tilted at a small angle (less than 10 degrees) reflecting incoming
light to a concave mirror, which focuses through a hole in the tilted
flat. Such arrangement has refractor-like configuration, in having the
eyepiece placed at the tube end opposite to the sky. It is also
spider-free, although requires additional small diagonal for
comfortable observing. Good part is that flat doesn't have to be
really flat - a very mild sphere would be much easier to make, and
would induce entirely negligible amount of astigmatism. Collimation is
nearly as simple as in the arrangement with large diagonal, because it
can be done without the small diagonal at the eyepiecen location. In
other words, only the primary needs to be adjusted (of course, star
diagonal itself needs to be well collimeted).
Vlad
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