Re: segmented mirror for personal telescope
- From: Louis Boyd <boyd@xxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 19 Apr 2006 22:47:49 -0700
apeneck@xxxxxxxxx wrote:
Apologies if this isn't a good group for this.
Let's say I wanted to build a personal telescope with a segmented
primary mirror system. There are a number of possible designs that
come to mind, e.g., duplicate the design of the one of the already
existing very large optical scopes on a much smaller scale.
However, I have in mind a much simpler design. I know next to nothing
about optics, and so am probably unaware of some of the issues that
might arise -- which is why I'm posting this here.
Anyway, the hypothetical design might consist of 3 circular (as opposed
to hexagonal) mirrors of 8 inches in diameter. They would be organized
in a triangular (equilateral) fashion. All three mirrors would reflect
off the same secondary, which would send the image to an eyepiece
centered between the three of them.
Assume for the moment that a spherical curvature is used for the
primary mirrors. Would not such a design enable the construction of a
scope that functionally (in terms of light grasp) is nearly the size of
a 16" scope, but with a focal length more typical of an 8"?
Also, I suppose that this advantage (shorter focal length) would cease
to be if one were to use parabolically curved mirrors vs. spherical?
If anyone has any thoughts they'd care to share, or recommendations for
good elementary optics texts that would get me started, I'd appreciate
it.
There are several obvious problems. The first is that you have to keep all of the mirrors in alignment. You need some kind of substrate to hold the mirrors without them tilting with respect to each other. A tilt of an arcseond in one of the mirrors will produce an obviously bad image in a telescope of that size. You need something as rigid and having as low of thermal expansion as a big slab of low expansion glass. You might as well just make the mirror out of that piece of material.
If you use multiple small mirrors they have to be accurately matched for focal length. Mirrors cost more to get tightly matched focal lengths.
Even if you were able to get perfect 8" mirrors in perfect alignment, star images would not have the clean round diffraction rings of a single mirror. For some applications that might not matter.
There are ways to use multiple mirrors which don't have tight alignment requirements. An example would be to put a ccd camera at the prime (or Newtonian) focus of each mirror, then combine the images using software to compensate for small image offsets. Software to do that is available and inexpensive. I don't know of any reasonable way to handle the problem for visual observing which would be satisfactory.
Lou Boyd
http://www.fairobs.org
.
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