From: Mark Holm 
To: scottythefiddler , atm{at}shore.net,
        ulhas{at}nagpur.dot.net.in, scottythefiddler{at}cogeco.ca
Reply-To: Mark Holm 




scottythefiddler wrote:

>>>My Strehl ratio calculator (using the formulas from Jim Burrows web
>>>site) is
>>>at http://pong.telerama.com/~mdholm/atm/cells/rmscalc.html
>>>It gives a Strehl ratio of 0.8804 for an RMS of 14.2 nM.
>>>
>>It also gives a Strehl ratio of 0.8385 for an RMS of 16.7 nM.
>>
>
>This is one of the reasons that I still use the Millies-Lacroix method.   I
>know what the limits are.   In this case, the same data has been put through
>three different pieces of software, and 3 different results have emerged.
>Apparently each has different underlying assumptions about the figure of the
>mirror.  What are they and when should each be used?
>
>
It turns out that in this case, the basic difference between my numbers and
Sixtests, for the Strehl was the choice of reference wavelength.  I just
posted the explanation to the list.

Millies-Lacroix is also subject to the exact same problem, though it may be
hidden in the common derivation.  You have to assume a reference wavelength
to make any sort of tolerance decisions.  There isn't a choice about
 whether or not to use a reference wavelength, because light behaves
(for most atm purposes) like waves.  The
 only choice is the value of the reference wavelength.

 Wavelength is a fundamental property that shows up in nearly  all the
equations.    You can't say 1/4 wave or
 1/8 wave or 1/20 wave without specifying what the wavelength is, and as
I said in the post I just sent to the list,
 the choice of reference wavelength for a lot of atm purposes is a bit
arbitrary.

I just screwed up in not remembering that my slightly arbitrary choice
wasn't quite the same as Jim Burrows' slightly arbitrary choice.

Mark Holm
mdholm{at}telerama.com

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