Re: Simple telescope design question

From: Clif (WA2GUF_at_netscape.net)
Date: 06/29/04 

Date: 29 Jun 2004 08:25:51 -0700

Robert Maxwell Robinson <max@u.washington.edu> wrote in message news:<Pine.A41.4.58.0406281400280.81110@dante67.u.washington.edu>...
> Hi, I'm new to the group. I have been learning about telescope
> designs for a month or so, and have a question that I haven't been
> able to find the answer to; I thought one of you might like to answer
> it.
>
> 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):
>
There is a very good example of this idea at Stellafane: The Porter Turret
Telescope:
http://www.stellafane.com/public_relations/public_relations1.html
(and subsequent web pages)
Nothing wrong with the idea at all, except for the difficulty of making
a big flat or the expense of buying one. Porter was willing to spend
the effort because he was able to achieve a comfortable, enclosed,
relatively fixed observing position with the design. Think of it, you walk
into a cozy little building a bit bigger than an outhouse, close the door
behind you and look out through the wall of the turret through the eyepiece
and you can see the entire sky out of the wind and the 10 below Vermont
winter weather. Definitely worth the added effort and expense. But
if what you want is a portable telescope or one for use in a conventional
observatory, forget it. The telescope just gets bigger and heavier and
more expensive.
                         Clif Ashcraft