Re: ATM TEX vs Figure34.exe?

[Aplanatic@aol.com]

Hi Lawrence:

> When I input the same Foucualt readings which give me 1/5.9 wave for my
>  13.1" mirror in TEX into the program called Figure34 I get a 1/7.54 wave
>  miror.  Which would you trust more?  Why might they differ?

The two programs use different assumptions about the slope of the mirror 
between measurement points.  In Figure, I assume that the surface is a 
quadratic spline, the simplest and smoothest curve that has the correct 
bilateral symmetry.  The mirror may or may not be closely approximated by a 
such a surface model depending on the degree of high frequency structure 
present.

Because of measurement "noise" and potentially unmodeled high-frequency 
surface structure, Foucault testing will never yield an exact answer, but 
should, if done carefully, yield an answer that is good enough for our 
purposes.  

It is much more valuable, in my opinion, to use and quote the Strehl and/or 
RMS than it is to quote the P-V wave rating.  If you want a very good mirror 
shoot for a smooth mirror with a Strehl above 0.9 and then star test.

Dave Rowe