[Raytrace] Spherical mirror telescopes
Peter Chen
chen@stars.gsfc.nasa.gov
Fri, 07 Dec 2001 12:36:58 -0500
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Hi:<br>
<x-tab> </x-tab>Just like
to express my thanks to John Upton for his post on raytrace
origins. The post included a message from Peter John Smith which
contained a pointer to the site
<a href="http://www.focussoftware.com/file_exchange/" eudora="autourl"><font color="#0000FF"><u>http://www.focussoftware.com/file_exchange/<br>
</a></font></u>which I did not know about.<br>
<br>
<x-tab> </x-tab>I went to
the site and downloaded the files for the Shafer telescope and Dan
Schroeder's (author of Astronomical Optics) files. Dan's files
contained, as PJ Smith mentioned, examples of four mirror telescope
designs by RN Wilson. These are exactly the examples I was
looking for.<br>
<br>
<x-tab> </x-tab>For those
interested, the designs by RN Wilson are for the next generation of
extremely large (20-100m) ground telescopes. Telescope mirrors this
big cannot be made in one piece (for economical and practical
reasons). Hence they must be made of many segments. For
reasons of affordability and manufacturability, it is best to make all
the mirror segments identical. Hence the segments are all spherical
(spheres and flats are the only possible geometrical figures), resulting
in a giant spherical primary mirror.<br>
<br>
<x-tab> </x-tab>The
problem with a spherical primary mirror is that, unlike a parabola, it
does not focus parallel rays from infinity to a point. Therefore a
minimum of three mirrors are required to make a telescope that has good
resolution and an acceptable field of view. Three mirrors, however,
end up sending the light back towards the front of the scope.
That's why four is used in Wilson's design. Having an extra mirror
also permits some optimization, I'm told.<br>
<br>
<x-tab> </x-tab>I loaded
the sample files into Zemax and looked at the 3d layout. They look
good. I need to learn more Zemax and raytracing to understand the
details. But thanks to this list, it has saved me a lot of
effort (as I have done in the past) trying to measure the diagrams with a
ruler and infer the optical parameters from the original published
articles.<br>
<br>
Regards,<br>
P.C. Chen<br>
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