Re: [ATM] Milligan interferometry report

[Michael Peck <mpeck1@ix.netcom.com>]

At 08:17 7/1/05, Koehler, Steve wrote:
>I have finally finished my report on the interferometry part of the "round
>robin" testing of Scott Milligan's mirror that resulted from the threads on
>Foucault/interfometry discrepancies in both the ATM and atm_free e-mail

Steve:

Nice work. I'm glad to see you've settled on LaTeX for this kind of report. 
Some more or less random questions and comments, starting with a couple pet 
peeves of mine:

Pet peeve #1: The word "astigmatism" should refer to primary astigmatism 
and perhaps, if the context is clear, higher order astigmatism terms, which 
have two symmetry axes separated by 90°. Using the word to refer to all 
asymmetric aberrations is a bad amateur practice (IMO).

Pet peeve #2: Wyant's 2 dimensional Zernike indexing scheme is a trifle 
idiosyncratic, and he isn't even consistent on his web site if I remember 
right. It would help to at least number the rows in the table at the end 
with the one dimensional index and maybe give nomenclature for the lower 
order aberrations.

I'll refrain from mentioning pet peeve #3.

Question: Both you and Galaxy Optics' web site mention the use of "Phase 
Shift Technology" equipment, but he seems to be doing fringe tracing. 
Googling the phrase turns up a company by that name that was swallowed up 
some years ago. Am I correct to infer that the phrase refers to the 
company, and not the use of phase shifting interferometry?

Question: I'm not sure I completely understand what you did to Mulherin's 
data. You took the average of the A and B sets to eliminate "test stand 
defects", even though you have evidence that test stand forces weren't 
constant between the two sequences. Then you replaced the primary 
astigmatism (terms 4 and 5) terms with the ones from Eason's analysis? This 
is going to leave some uncorrected defects in the resulting wavefront map, 
although obviously those must be small.

Comment: I'm not really surprised that the results agree well for high 
order aberrations. Perhaps you'll recall my discussion of "quantifying 
precision" on the interferometry group a few months back (yet another post 
of mine that nobody quite knew what to make of), where I showed that the 
statistical precision of coefficient estimates systematically improves with 
increasing aberration order. Apparently this general trend holds between 
test sessions as well as within them. That's an interesting, and new result.

Comment: It has to be encouraging to us cheapskates that Eason's SA 
estimate appears to be as accurate as the others despite the lack of 
nulling optics. I'd like to see a discussion of the sensitivity of SA 
estimates to the various lengths that have to be measured when correcting 
SA in software, but that would take some dreaded math that you were trying 
to avoid.

Comment: It seems to me little was actually learned about "accuracy", and 
certainly not absolute accuracy. To do that we would need a reference 
standard, and those seem to be hard to come by. So, are we learning about 
precision or repeatability here, and what can we say about relative 
accuracy, if anything?

Looking forward to part 2.

Mike Peck

------
Michael Peck
mpeck1@ix.netcom.com

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