Re: Simple telescope design question
From: Brian Tung (brian_at_isi.edu)
Date: 06/28/04
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Date: Mon, 28 Jun 2004 22:45:58 +0000 (UTC)
Robert Maxwell Robinson wrote:
> pppp......................S
> ppp ... S
> pp ... S
> pp ... S
> p ...
> p.....................|= Eyepiece
> p ...
> pp ... S
> pp ... S
> ppp ... S
> pppp...........S
Ooh. ASCII art. I like it. (Honestly.)
> 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:
It's not the large that concerns me; it's the flat. Much less expensive
to create a flat secondary a couple of inches across than one that's
40 percent again as large as the primary (and that's assuming you only
want 100 percent illumination at the center of the field of view).
Trouble creating the flat is why most Newtonian manufacturers would rather
use spiders than optical windows--especially in large sizes.
And it's not true that the secondary doesn't steal light. It most
certainly does--but instead of being an obstruction that steals the
light, it has a hole in it that steals the light. The light that would
have bounced off where the hole for the eyepiece goes is missing from
what gets sent on to the primary. That creates essentially the same
light loss and (more importantly) diffraction effects. You *could*, as
you suggest, put a finderscope (my recommendation) or a detector beneath
the hole, so to speak, but the diffraction effects are why you do not
want to just "add the second image" back in to the principal image.
Incidentally, using the finderscope reintroduces the precise alignment
requirement that was taken out of the collimation process. If you
change the orientation of the steering mirror, you will also remove any
alignment you had between where the main telescope and the finderscope
are pointing. (Unless you have some fancy mechanism for halving the
angle of steering.)
As far as the balance issues are concerned, if you were to mount it on
a German equatorial mount, say, the issue is not the weight distribution
along the length of the scope. That is corrected by putting the rings
(or whatever the support mechanism is) further up or down the telescope.
The counterweight is to balance out the scope around the fulcrum that
turns in right ascension--which will still be a problem with this design.
This can be solved for either design by putting the telescope in a
Dobsonian mount and using an equatorial platform.
> 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?
How is that different from a set of baffles?
> 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)?
Those correctors are there to correct for a spherical, rather than
paraboloidal, primary. I don't see how your design makes manufacture
and alignment of the correctors any easier or less expensive.
I don't mean to be discouraging. You're thinking about these issues,
and ways to solve them, and that's the right thing to do. I hope you'll
continue to try ideas out, and I for one am happy to act as a sounding
board.
Brian Tung <brian@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt
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