Re: [ATM] Testing Cassegrain Secondaries

["David Harbour" <scarab2@cox.net>]

Anthony, Cary-

Don'r forget the null test with primary by autocollimation. Many have
figured their Cass. secondaries this way, without the requirement for a big
flat, by suspending the OTA (with a diagonal, or, even, without one) above a
pool of oil or quicksilver. The method has this advantage: nothing to
calculate or measure (it is a null test, of course) and is therefore "idiot
proof" (nothing to quantify with calculations- just figure for a clean null,
the way G.W. always did).

Roger Tuthill wrote up his experiences testing Cass. secondaries this way (I
can't remember when). Of course one would obtain the most light with
Mercury, which, however, is more sensitive to rippling from passing freight
trains, etc; one author said overlayer the mercury with a thin film of
vegetable oil for dampening riples, I believe. Also I think it is expensive.
My 3 cents worh of contribution.

Dave

P.S. Also remeber that a full diameter (same as primary) flat is not needed.
That was the big lie the makers of the Hubble telescope told everyone who
suggested null testing by autocollimation. Either a subdiameter flat (the
diameter of required to be as large as from edge of secondary to rim of
primary of the OTA) or just another, smaller scope to use a a collimating
source. Big flat not required.

----- Original Message ----- 
From: "Anthony Stillman" <atmer@flash.net>
To: <atm@atmlist.net>
Sent: Saturday, May 07, 2005 3:53 AM
Subject: [ATM] Testing Cassegrain Secondaries


> 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+nul
l&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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> ATM mailing list http://www.atmlist.net/
>


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