[ATM] Why and how the Mersenne-Schmidt-Paul-Baker-Willstrop-Stevicktelescope works

[Michael Peck <mpeck1@ix.netcom.com>]

Just giving credit where it's properly due...

I think Stevick mentions this on his web page, but it's not at all obvious 
when you set up the design in OSLO. Part of the reason the so-called 
"Stevick-Paul" works is because it's not a tilted component telescope at 
all (which also means it's not a "Schiefspiegler"). Instead the two 
spherical elements have been rotated around their respective centers of 
curvature by amounts just large enough to create an unobstructed light 
path, and what they are seeing is off-axis light from the primary. The 
design is free of all third order aberrations except Petzval field 
curvature (maybe distortion too, which doesn't count). It's residual 5th 
order aberrations that account for the slight departures from theoretical 
perfection in the ray-traced designs.

There's a good explanation by Willstrop of the optical principles of the 
design here: <http://www.ast.cam.ac.uk/history/mmmt/>. Basically, the 
primary and secondary are confocal, which makes the combination an afocal 
beam compressor (i.e. Mersenne telescope). The tertiary is separated from 
the secondary by an amount equal to it's radius of curvature. In effect it 
is seeing an aperture stop at it's center of curvature, which is the basis 
for a Schmidt camera. The primary and secondary together have an amount of 
spherical aberration exactly equal (to 4th order) to a Schmidt corrector, 
so combined with the spherical tertiary you get an all reflective Schmidt 
telescope.

This design was discovered by Maurice Paul in 1935 (Rev. d'Optique, v. 14, 
p. 169), with variants developed by Baker and, much later, Willstrop (1984, 
MNRAS, v. 210, p. 597) 
<http://adsbit.harvard.edu/cgi-bin/nph-journal_query?volume=210&plate_select=NO&page=597&plate=&cover=&journal=MNRAS&db_key=ALL>.

This is an important design because of its potential for wide, well 
corrected fields with the fewest possible optical elements. For an example, 
see here: <http://www.lsst.org/lsst_home.shtml>.

Mike Peck

_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.netcom.com/~mpeck1/astro/astro.html 

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