Re: ATM Foucault test accuracy

[ToobWiz@aol.com]

Just skimmed that fine paper hoping to find something related to bias toward 
undercorrection in the Foucault test. This is not to denigrate the paper, as 
the topic just happened to be one that wasn't addressed.

I'm still wondering why mirrors tend to be undercorrected on star tests 
(really I mean observing) compared to the expected results from the Foucault 
test. I've run a couple dozen checks on mirrors that measure good, and had 
them confirmed by a tester with better eyes than mine. Take them out and 
check extrafocal images and they are undercorrected.

I am not talking here about temperature anomalies, but measurements with 
well-adjusted mirrors that have had time to reach equilibrium. On low FR and 
large diameter mirrors, I've taken to intentionally overcorrecting by fudging 
the target parameters (not the measurement values) in ADMIR (Yup!! I paid for 
it and I intends to get my money's worth ;-). On a 22" f/3.7 we shortened the 
target ROC by 9% !!! The extrafocal images look as if they had been computer 
generated.

Every mirror that I've been acquainted with that "overcorrected" according to 
the Foucault gave fine extrafocals. We guess at the amount, based upon 
aperture and FR, but we've hit it pretty dang closely so far.

Still wonderin'

Cheers/Carron