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Re: [ATM] correction -+

You are assuming that the mirror truly matches the
conic section exactly and that is the only error. It
is not a good figure of merit by itself.  The RMS and
strehl are much better. It is unreasonable to believe
that the surface exactly matches any conic.  Instead
it will have some close fit to a particular conic and
will have an RMS error relative to that conic. Most
Foucault reduction programs state the RMS error in
relation to the -1 conic constant of a parabola or
other user selectable conic. Many also tell you what
conic you are actually closest to, however the Strehl
number or RMS is the figure of merit you should be
using to determine when to quit.  

Dale Eason


--- KR <kreid@primus.ca> wrote:

> Hello list,
> 
>     Twice I've asked myself, what value of
> correction is on the level of a 
> RMS value of 6, a PV of 1/13, strel .988?
> I don't recall any discussion on the matter. When
> you approach very good 
> readings, the temptation arises; I can't make it
> around the barrel again 
> without loosing good Foucault results, so the
> good-enough becomes relevant.
>     This is plate glass, f/6. If I do a set of
> readings in the morning when 
> it is warming up, they are different than an evening
> set when cooling. I 
> will continue to average readings, so far -1.06, but
> they have ranged 
> from -1.065 down to -1.06 (not really an average).
> If plate glass gives such 
> wide ranging results, I ask what value of correction
> weighs in with good 
> Foucault readings?
>     Star test will tell in the end. Does anyone have
> thoughts on this?
> Thanks   KMR 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



 
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