[Fwd: [Raytrace] Spherical mirror telescopes]

[Hermit]

Peter and all.  The mail program is set so that replys go to the
original poster.    This is 'protection' from accidently sending a
private email to the group.  On my mailer, I must hit 'reply to all' if
I am using the header from another post instead of starting 'fresh'.

Ken


-------- Original Message --------
Subject: [Raytrace] Spherical mirror telescopes
   Date: Fri, 07 Dec 2001 12:36:58 -0500
   From: Peter Chen <chen@stars.gsfc.nasa.gov>
     To: <mailto:@poph-f.gsfc.nasa.gov>

Hi:
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
http://www.focussoftware.com/file_exchange/
which I did not know about.

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.

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.

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.

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.

Regards,
P.C. Chen
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