Simple telescope design question

From: Robert Maxwell Robinson (max_at_u.washington.edu)
Date: 06/28/04 

Date: Mon, 28 Jun 2004 15:08:37 -0700

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):

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?

The obvious fact of this design that makes it look worse than Isaac
Newton's design is the large, heavy flat mirror instead of a small,
light one. But here are the advantages I see that make me ask:

1. The only real collimation required is collimating the eyepiece to
    line up with the optical axis of the parabolic mirror. If the
    steering mirror is slightly out of alignment, you see a slightly
    different portion of the sky, but nothing goes awry optically. In
    a Newtonian, the diagonal has to be correctly aligned to bounce
    light directly down the center of the mount for the eyepiece, and
    then the eyepiece has to be correctly aligned along that same
    axis.

2. Counterweights are often used to balance a Newtonian telescope,
    because its weight is predominantly at one end. In this design
    there is already weight at both ends, which should minimize the
    need additional weights.

3. I believe steering mirrors are often used by owners of large
    binoculars to put the eyepieces in a more convenient place, and to
    reduce the amount of weight that has to be moved to steer the
    field of view. The steering mirror in this design should provide
    both of those advantages, but without being an extra optical
    element that steals light as it is in other cases.

4. The "obstruction" is a hole rather than the back of a mirror. To
    use the obstructed light in a Newtonian reflector, another
    diagonal mirror would have to be used to divert the light before
    it hits the secondary; and that mirror would grab some of the
    light, and have to be aligned with the components that use the
    otherwise wasted light. In this case, the light passes through
    and can be viewed with an (on-axis!) viewfinder, or for digital
    astrophotography it can be focused, collected with a second
    CCD, and ultimately added back into the digital image. [Having
    two detectors on the same optical axis might allow for some fancy
    cross-comparison of off-axis light, for example from two different
    optical designs, allowing both to be corrected into a superior
    image...but I digress.]

5. The prime focus would be somewhere after the light had passed
    through the hole in the steering mirror. This presents a golden
    opportunity to use an iris to eliminate the farthest off-axis
    light and enhance contrast when viewing the moon or other bright
    objects, does it not?

5. A Maksutov or Schmidt corrector could still be used, placed
    in the light path before the diagonal mirror and out of the way
    of the light reflected from the parabolic mirror. If one is
    willing to place the corrector even farther from the flat mirror,
    I think a less curved corrector could be used; mightn't that make
    them cheaper (at the expense of ending up with an even bulkier,
    L-shaped telescope)?

All in all, it sounds like rather a good idea to me. So how wrong am
I?

Thanks,

Max Robinson
Seattle

(This is a rewording of a similar message I posted to alt.astronomy,
before I knew about this newsgroup. Apologies if you've read it
twice now).

Relevant Pages

  • Re: Simple telescope design question
    ... It's not the large that concerns me; it's the flat. ... change the orientation of the steering mirror, ... turns in right ascension--which will still be a problem with this design. ... > willing to place the corrector even farther from the flat mirror, ...
    (sci.astro.amateur)
  • Re: Simple telescope design question
    ... > similar in position to what I think is called a Steering Mirror. ... Fundingsland published his 6" aperture design in S&T, ... mirror and 24.25" diameter steering flat made by Mike Marcario at High ...
    (sci.astro.amateur)
  • Re: MAK-CAS
    ... > You can make both Maks and SCTs with or without spherical mirrors. ... > taking longer than a Schmidt design is not entirely true. ... > corrector, versus the SCT, but from what I understand, a BK-7 Corrector ... > So now it's on to the Primary Mirror. ...
    (sci.astro.amateur)
  • Re: Parametric models of two and three mirror reflecting telescopes
    ... >distance and primary mirror focal length. ... designed from these with ray tracing using your favorite design ... requirement that the third order Petzval Sum be zero. ...
    (sci.optics)
  • Re: Plasma, LCD, DLP which way to go
    ... On the dlp mirror issue maybe you can ... sure I understand the technology well enough to be able to design a dlp ... I'm not particularly worried about the color wheel. ...
    (alt.tv.tech.hdtv)