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Re: [ATM] Understanding Foucault


Thanks, all, for the response.  Sixtest needs some sort of frame-grabber, no?  would a digital camera suffice?  I'm a wiz with Photoshop and so could get just about any results i wanted!  


Paul Johnson
St. Paul, Minnesota USA
9:12 PM (UTC -6)

Monday, 6/28/2004 @ 6:42 PM; mdholm wrote:

> Dave Harbour's description of the Foucault test is very good.  The only quibble
> I would have is that he shows what is known as the Millies - LaCroix method (or
> the "tornado" graph) for analyzing the data.  Careful work by more than one
> mathematically inclined atm has shown that the M-L method is weak.  Now that we
> have computer programs, they are the way to go.  I am a big fan of Sixtests.  It
> uses about the most sophisticated math that can be applied to Foucault results,
> and since Jim Burrows knows his math pretty well, we can be reasonably sure it
> is correct.

> If you have to do it by hand, the best is probably to steel yourself to using
> Texerau's analysis.  It's not all that hard with a calculator, and, so far as I
> know, gives correct results.

> About moving source versus fixed source.  Moving source works well and has some
> advantages.  The drawback is that knife edge movements are approximately halved.
>  This means that the requirement on precision of measurement (how many
> thousanths of an inch toward and away from the mirror) is twice as tight.  Since
> you can buy a fair dial indicator for less than $20 from Harbour Freight, this
> isn't as big a problem as it once was.  Most ATM's find that their readings are
> uncertain to more than the 0.001 to 0.002 errors that may remain in a cheap dial
> indicator.  The source of uncertainty is difficulty reading zone nulling and is
> independent of the measurement tool.

> With moving source testers, it is common to cover only half of the light source,
> and to use the same knife edge at the source as for the probe.  Many atm's find
> that they can get more sensitive shadow readings by blocking most of the source
> with a second knife edge, leaving a slit, as commonly shown in moving source
> Foucault testers.  The "main" knife edge still serves both for the source and probe.

> Mark Holm
> mdholm@telerama.com
> _______________________________________________
> ATM mailing list http://www.atmlist.net/

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