[ATM] The .70 radius support - interesting article

[IAPDK@ADMIN.DRAKE.EDU]

I found an interesting reprint of an 1975 article by Charles Reeve: 
"The Calibration of an Optical Flat by Interferometric Comparison
to a Master Optical Flat".  He refers to an 1954 study by Emerson
that explored the effect of gravitational bending on flats that determined
that the support radius for three pads that gave the minimum bending
was 70% of radius.  He says: "this result is not suprising since
the circle of radius r0 = .7071r divides the mass of the flat
into equal parts".

So there's your evidence for the 70% radius being commonly adopted for
so many years.  And it is not so bad really, if you also stick
to the recommended 1:6 thickness rule.

The article can be found at:
http://ts.nist.gov/MeasurementServices/Calibrations/upload/75-975.PDF

Paul Kline
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