Re: Simple telescope design question
From: Chris Rowland (chris.rowland_at_dsl.pipex.spam.com)
Date: 06/28/04
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Date: Mon, 28 Jun 2004 23:32:37 +0100
I don't know much about this but as that doesn't stop anyone else;-)
Large flat mirrors are much more difficult to make than parabolic
mirrors. This is probably the principle objection to this idea.
However scopes are made with this sort of arangement, particularly
solar scopes.
Another advantage is that the scope can be fixed and only the flat
miror moved to aim at and track objects.
Chris
On Mon, 28 Jun 2004 15:08:37 -0700, Robert Maxwell Robinson
<max@u.washington.edu> 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.
>
>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).
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