Re: Simple telescope design question
From: Mike Jones (jonesmi_at_airmail.net)
Date: 06/29/04
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Date: Mon, 28 Jun 2004 23:22:35 -0500
Robert Maxwell Robinson wrote:
> 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.
>
> 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):
>
> pppp......................S
> ppp ... S
> pp ... S
> pp ... S
> p ...
> p.....................|= Eyepiece
> p ...
> pp ... S
> pp ... S
> ppp ... S
> pppp...........S
>
> I can't believe noone has considered this simple variant on a
> Newtonian before; so does anyone know the name of this design? Also
> I've never heard of one being constructed, so there must be some
> significant problem with it. Can anyone tell me what it is?
You have described (very well) the telescope Charles Fundingsland
invented, built and patented in the 90's, called the "Fundyscope". Mr.
Fundingsland published his 6" aperture design in S&T, but I don't recall
the year and month - maybe someone can look that up. The George B. Wren
II Supernova Search Telescope (SNST) at McDonald Observatory is the
largest Fundyscope in the world, with a Galaxy Optics 18" f/4.5 primary
mirror and 24.25" diameter steering flat made by Mike Marcario at High
Lonesome Optics. I derived the tracking equations and algorithms for
SNST, and Wayne Rosing (also a VP at Google) implemented the tracking
equations and made the thing work, and it worked very well. Bill Wren
used it to discover several supernova. See
http://hej3.as.utexas.edu/~www/SN/.
The Fundyscope steering flat has to be VERY flat to prevent image
astigmatism, on the order of 1/20 wave peak-to-valley, and must be
supported by an edge/back flotation system that can maintain that flatness
over the full angular pointing range. Achieving 1/20 wave P-V precision
on a 24" flat right up to the edge requires a truly skilled optician such
as Mike Marcario. The hole in the flat must also be a tapered 45º cone to
prevent vignetting the field at maximum mirror tilt. Making the steering
flat is the main drawback to Fundyscopes.
Mike
- Next message: Martin R. Howell: "Re: Nuts, bolts, etc"
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- In reply to: Robert Maxwell Robinson: "Simple telescope design question"
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