Re: [ATM] Cassegrain vs. Gregorian (secondary testing)

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Michael Peck wrote:
> The secondary in your .len file only has a couple waves worth of 
> spherical aberration relative to a sphere (if I did the arithmetic 
> correctly). You could probably get away with doing a simple interference 
> test against a spherical test plate, especially if you use FringeXP to 
> do quantitative data reduction.
> 

Actually, about 3 (1.77 microns from best fit sphere).  But that
would also require creation of a special test plate.  The tests
I have described can use OTS lenses.

However, I think one of the advantages of this test is that, whether
the single or pair of doublets is used, the test can be somewhat
self-calibrating.  One could run a single pass calibration by
removing the secondary and putting the light source at what would
be the CoC of that secondary.  Then, with the knife edge, measure
the zonal deviation using the same mask as would be used in the
final test.  Each zonal error would be equivalent to a corresponding
angular deviation from the perfect sphere.  This angular deviation
for each zone would be doubled in the test with the secondary
reinserted.  This doubled zonal calibration would then be
subtracted from the final zonal measurements.

In fact, I wouldn't be surprised if this kind of calibration wasn't
already available in the S/W tools used for standard Foucault testing.

> Mike Peck

-- 

Rick S.

http://users.rcn.com/rflrs

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