[ATM] Astigmatism in round robin mirror C with Foucault.

[Douglas S Angle <Douglas.S.Angle@can.dupont.com>]

First off, thanks again to all who have organized, participated in, and 
commented on the mirror round robin testing.  There's a real wealth of 
information there, and I don't think we are done with it yet!

There's been a number of questions and comments on this list about the 
ability to measure astigmatism using foucault.  I think the consensus is 
that, since it can't be seen in the test, it can't be measured.  There 
have been some dissenting views on this, sometimes from the same people 
making the statement in the first place (myself inculded)!!!  See for 
example:  http://www.atmlist.net/pipermail/atm/2004-March/001124.html

I was asking about the use of the fitted COC value to estimate 
astigmatism, and was told yes (and no), it can (can't) be done. 

Now that we have both interferometry and a number of foucault sets for the 
same mirror, I wonder if we can get a more definitive answer.  So I took 
my readings for mirror C and ran the two diameters through Sixtests. 
Diameter 1, I get  1647.820mm, and diameter 2 1648.737mm, for a difference 
of 0.9mm.  From the formula Jim gave (in the reference above) I get 0.03mm 
as maximum acceptable.  Is this in fact a demonstration of the detection 
of astigmatism using foucault, or am I misinterpreting the fitted COC 
value?  Is it worth calculating these values for the other mirrors and 
tester?

Sixtests outputs:
Mirror C diameter 1
150.5   0.0     Mirror, obstruction diameters (mm)
Foucault
0.0     Source distance (0=moving source)
y, mm   X, mm
24.2026 5.761
42.1132 6.315286667
53.721 6.594686667
63.246 6.93547
71.3994 7.25297
*
0.000   1647.820        Conic targets:  b, R (mm)
0.0338  Measurement std deviation, mm
1641.2  Longitudinal reading bias, mm
 

Mirror C diameter 2
150.5   0.0     Mirror, obstruction diameters (mm)
Foucault
0.0     Source distance (0=moving source)
y, mm   X, mm
24.2026 6.66369
53.721 7.53364
71.3994 8.27659
*
0.000   1648.737        Conic targets:  b, R (mm)
0.0338  Measurement std deviation, mm
1641.2  Longitudinal reading bias, mm

In case you've lost it, raw data for mirror mirror testing is at:
http://www.atmlist.net/contrib/srychnov-at-chem-dot-ps-dot-uci-dot-edu/Mirror%20C%20Data.pdf 
 

-Doug


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