Re: ATM Computerized Foucault Test

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

Marcus E. Hennecke wrote:

>idea: Basically, in the classic Foucault test, one has to compare the grey
>level of two areas opposite of one another. This is usually done through a
>Couder mask. Comparing them and making sure that they are the same is not
>always easy. If the video camera were connected to a computer, could one not
>let the computer make the decision which zone we are currently looking at?
>This web cameras are getting cheaper and cheaper and should probably suffice

<...>

>The testing procedure could then look as follows: Move the knife edge until
>it is at the ROC of the center zone. Let the computer do a reading of the
>position. Then move the knife edge a bit and let the computer do a reading
>of the position and finding the corresponding zone. Keep doing this until
>you reach the edge zone. With the right math it doesn't matter too much if
>you measure the same zones each time since the computer knows which zones

I've done pretty much exactly what you suggest, and even wrote up an 
analysis of some early results. Follow the atm link in my sig to read about it.

I don't think I wrote about it, but I did look at finding the minimum 
variance over arcs as well as just comparing brightness levels averaged 
over arcs on the left and right sides of center. The results are about the 
same using either criterion, but comparing averaged brightness levels 
seemed more intuitive to me so that's what I used in my data reductions.

Since writing that report I used the method to figure a larger f/4 mirror 
with some success, but for various reasons I've concluded that you should 
look elsewhere if you need a true quantitative test.

Dick Suiter described a very similar approach in ATMJ #13. There was also 
an article published in S&T several years ago, I think ca. 1994.

Mike Peck

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Michael Peck
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