From: "Mel Bartels" 
To: 
Reply-To: "Mel Bartels" 


>>>
Another promising class of supports is the +-45 deg point support. This
placement cancels astigmatism, even with forces out of the plane - but you
can get "trefoil" astigmatism and coma instead.
<<<

Perhaps an observation of mine might apply.  Back when testing my 1 5/8
inch thick 30 inch using the two block at 90 degrees separation for
convenience's sake on Polaris, I noticed a stubborn astigmatism at highest
powers that remained in position despite rotationg mirror and changing
diagonals and using a large wall electricity fan aimed at the mirror face
to cool the glass.  Since I didn't intend to use this edge support for the
real McCoy, I pursued it to the point of convincing myself that it was not
intrinsic to the primary's figure, but not further.  I wonder if I was
observing the trefoil astigmatism.  The star shapes were oval at a 3:2
ratio at 25x per inch of aperture and higher.

Mel Bartels

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