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RE: [ATM] Ross null test?
From: steve beccue [mailto:lsteve@Beccue.com]
>As long as we are considering both. Do both tests
>have the same tolerances? That is, are the results
>equally sensitive to the physical position of the lens or
>mirror?
>What are the tolerances involved?
Good question!
I contacted John Upton about the Maksutov/Waineo null test, and here is one
analysis he did:
Tom Krajci
Albuquerque
=============================================
Tom,
I finally got enough free time to look at tolerances for the Waineo /
Maksutov null test. I've only run one sample mirror. It takes a couple of
hours to manually tolerance the set up.
On Tue, 10 Aug 2004 19:45:27 -0600, lou krajci wrote:
> Would you be interested in setting this Maksutov/Waineo null test
> up in OSLO and seeing how accurate and touchy this null is for the
> amateur?
Here is the sample set up I used. It is a 10" F/5.0 paraboloidal mirror
and uses a 6" F/6.0 reference null sphere.
Waineo/Maksutov Null 10" F/5.0
SRF RADIUS THICKNESS APERTURE RADIUS GLASS SPE
NOTE
OBJ -- 25.500000 2.5500e-05
AIR
AST -72.000000 -69.450479 2.786457 AS
REFL_HATCH
2 100.000000 73.390926 S 5.000000 S
REFL_HATCH *
IMS -- 0.000243 5.7468e-05 S
This particular set up results in the following null quality.
PKVAL OPD RMS OPD STREHL RATIO
0.003489 0.000849 0.999937
I independently changed one variable at a time to determine the point at
which the null drops to a Strehl ratio of 0.800. Here is what I found.
Changing OBJ Thickness (The Source to Reference sphere distance.)
-4.4% to +3.9% tolerance
Changing AST Thickness (The Reference to Mirror under test distance.)
-9.5% to +12.6%
Changing the AST Radius (The ROC of the Reference sphere. This is how
accurately you must know the value.)
-3.0% to +3.8%
Changing the 2 Radius (The ROC of your Mirror under test. Again, how
accurately do know the value?)
-6.1% to +7.5%
Conic Constant of Reference
-0.542 (0.173 wave surface) to +0.542 (0.173 wave surface)
My overall conclusion is that the test is relatively easy to set up and
tolerances seem wide enough to get good results under a wide range of
conditions. The spacing tolerances seem easy enough to achieve. The main
downfall is that the reference sphere should really be at least half the
size of the mirror being tested. In my mind, this may limit it to mirrors
under 18 to 20 inches diameter since few amateurs (other than the hard-core
fringe element) would be willing to make a 10 inch or larger spherical
mirror for this special purpose.
The reference sphere doesn't seem to have to be extremely accurate but
it must, of course, be quite smooth. Any zones on the reference would show
in the test and be hard to separate out from the mirror being tested. Most
advanced amateurs wanting to use this test in the first place would e
capable of makeing the reference to sufficient accuracy.
As with any null test, the key advantage of this method is the ability
to directly see the size and location of the defects on the mirror without
the need to refer to a graphic representation. My past use of the Dall null
test showed this to be a significant advantage in visualizing exactly where
to work the surface.
Best Regards,
John D. Upton
Georgetown, TX
http://www.atm-workshop.com/ The ATM's Workshop
http://members.aol.com/ronwin20/ Ronchi For Windows Software
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