Re: ATM Foucault tester

["Ken Hunter" <kb7h@onemain.com>]

Andrew...

If you're rebuilding your tester, you'd be better off if you could get
your eye as close as possible to the KE... 20mm will limit the
allowable view for short focus mirrors and you would have to rebuild
it again if you were to want to test a short fl mirror.

Ken Hunter

----- Original Message -----
From: "Andrew Melville" <a.melville@mailbox.gu.edu.au>
To: "Atm (E-mail)" <ATM@shore.net>
Sent: Thursday, August 01, 2002 7:24 PM
Subject: RE: ATM Foucault tester



Hi

Thanks all for your, as usual, very helpful replies. I am looking
forward to fixing it all up this weekend. As I see it I need to make
sure the order of components, heading towards the mirror, is,

1)  Source

2)  Diffuser

3) > 3mm (1/8") gap

4) KE

And I need to get my eye, for a 200mm (8" or there abouts) f6, about
20 mm from the KE.

If this is correct I have a bit of rebuilding to do.

One thing about this hobby, doing it with no personal supervision, is
that nearly everything goes wrong first time so one needs to come up
with fixes. I guess thats why I like it so much. Having the ATM list,
with its experienced and helpful members makes it a lot easier.

Andrew

-----Original Message-----
From: Dwight K. Elvey [SMTP:dwightk.elvey@amd.com]
Sent: Friday, August 02, 2002 9:24 AM
To: atm@shore.net
Subject: RE: ATM Foucault tester



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>From: "James Taylor" <u2pilot@satx.rr.com>
>To: "Dwight K. Elvey" <dwightk.elvey@amd.com>, atm@shore.net
>Subject: RE: ATM Foucault tester
>Date: Thu, 1 Aug 2002 15:15:22 -0700
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>>The diffuser should not be mounted directly
>> on the back of the razor blade and should have a little
>> distance between the two. A couple of pieces of thin cardboard
>> should do ( rob the tops from a cereal box to shim with ).
>
>Dwight:
> Maybe I'm thick here, but I'm having trouble conceptualizing why the
>diffuser can't be right behind the edge.  The tester I built and used
last
>Fall had the diffuser directly under the knife edge and it "appeared"
to
>work just fine through all of my testing.  I'm wondering what
problems one
>might have if the diffuser is close to the edge.
>
> If you like, you can see photos of my tester and some of the digital
>Foucault images I took while testing the mirror at
>http://home.satx.rr.com/thejts/foucault.html
>
>James Taylor
>
>
>
>
>

Hi James
 I saw your photos and I could, nor could you, tell from them
if you were seeing problems of the diffuser being too close
to the source edge. What you take a picture of is the shadow
of an edge. I could not tell you if that edge measured slightly
closer to the diffuser or the true edge if there is an interaction
between the two images, diffuser surface and source edge.
 Remember, things are already fuzzy to your eye and camera.
 When we use a Foucault tester, you'll notice that the depth
of field is about how finely you can take a measurement, regardless
of the measuring scale. This sets some limits on how
close the diffuser surface can be. While 1/8 inch is surely
conservative, placing it right at the edge is most likely
pushing it. For Foucault testing, depth of focus is most
likely limited by diffraction effects. You only want the
effect of one edge and not the interaction of two. Are you sure
that the surface of your diffuser is smooth and uniform at
a microscopic scale?
Dwight