[ATM] Testing Cassegrain Secondaries

[Anthony Stillman <atmer@flash.net>]

Cary,

>good, convenient, QUANTATATIVE, and PRACTICAL solutions


"A New Test for Cassegrainian Secondaries"  Gleanings Nov '57
Testing through the convex-plano secondary to a flat mirror then back
through the secondary again.  Index limits the values of m (3.5-5) (BK7 m~5)

"A Test for Figuring Cassegrain Secondary Mirrors" Gleanings Jan '70
Testing through the back after radiusing it.

 "Null test for hyperbolic convex mirrors" AO Vol. 22 No 1 / Jan '83  p12,13
Testing through a plano-convex null lens in front of the secondary.  The
null lens is easy to make at the cost of limiting the allowed values of m
(1.7-3.5)

"Self-null corrector test for telescope hyperbolic secondaries"  AO Vol.
22, No 4 Feb. '83 p520,521
Testing through the secondary's flat back at conjugate foci.

"Null testing convex optical surfaces"  Szulc AO Vol. 36 No 25 / Sep '97
p6274 ..77
Through a hole in the secondary onto a larger (1.2-1.8 times) concave
mirror which has been figured ellipsoidal.  The light is then reflected
back onto the secondary and then retraces it's path.

Of course there is the hindle sphere null test.  The light passes through a
hole in the center of the sphereoidal mirror and strikes the secondary.  It
then reflects and expands back onto the hindle sphere.  It then retraces
it's path.  There is the concave proof plate approach favored by Texereau.
There is the through a trough of turpentine, through the back of the
secondary with its face exposed to the air solution presented in ATM II
(the analysis is wrong.)  There's the immersion null
<http://astro.umsystem.edu/atm/ARCHIVES/SEP00/msg00518.html>  There's the
internet
<http://www.google.com/search?hl=en&ie=ISO-8859-1&q=Cassegrain+secondary+null&bt
nG=Google+Search>

There are a lot of convex asphere tests.  All have challenges.  A lot of
people object to the Hindle sphere because it will likely be as big as the
cassegrain's primary and it's another surface.  A lot of people object to a
null lens because it's a lens and an extra two surfaces.  It seems, and
this is one of these general rules for which I can immediately think of an
exception, that an instrument with 'n' surfaces needs at least n+1 surfaces
made to make it.  Remember, one needn't make a secondary or a null lens
from scratch.  You can start with a simple technically specified lens
bought from an optical supply company or on the surplus market.  Choose
your design wisely (Dall-Kirkham) and you may not need to do anything.


Anthony


inertia invidia est
sloth is blind


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