From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


The biggest thing with the thin blanks (in the 15:1 range and up) is that
they do tend to get astigmatism if the glass is ground wrong.  This means
that you have to be able to see what the astigmatism is in the results from
a Ronchi (twisted lines) or Foucault (odd zoning that isn't centered on the
direction of testing) tests.
The Foucault test with a slitless tester may show a little astigmatism in
the exact vertical direction (assuming that the KE is vertical) but that
indication won't rotate with the mirror.  A horizontally seperated KE and
source will tend to misidentify the curvature of the mirror which is why
the vertical orientation is the right orientation for the souce and KE. 
Note that the horizontal seperation is actually testing for a ellipsoid
rather than a paraboloid as the source is at one focus point of the ellipse
while the tester is at the other point of focus.  The vertical orientation
still produces the same result but the errors are in the vertical direction
where they can be easily ignored by testing on the horizontal direction
across the center of the mirror.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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