Re: ATM Foucault test accuracy

[lowther@att.net]

I've not found this to be the case.  I know many people say they like to
take the measurement just as the zone starts to change.  Perhaps that is
a systematic error.  On some zones I tend to make 'different'
judgements.  I like to do at least a half darkened image.  Sometimes I
like to nudge the tester and see if both sides 'wink out' at the same
time.  On steeper zones I've noticed you can see the shawdow start
coming in from the outside at the same time.  Sometimes I think I just
obsess too much on my readings?  Bottom line, you can have several tests
on a zone other than when both sides just start to change.  


ToobWiz@aol.com wrote:
> 
> 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

-- 
Ken Lowther
Youngstown, Ohio
http://www.atmsite.org