Re: [ATM] Understanding Foucault

["Richard Ozer" <rozer@pacbell.net>]

Sixtests is a DOS program where you input your foucault readings and it will
return the estimated accuracy of your mirror surface.  There is a windows
interface to sixtests available at:

http://www.atmsite.org/contrib/Holm/frontsix/frontsix.html

RO

----- Original Message ----- 
From: "Paul Johnson" <bigatm@mn.rr.com>
To: <atm@atmlist.net>
Sent: Monday, June 28, 2004 7:16 PM
Subject: 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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