Re: ATM foucault

["Richard Schwartz" <richas@earthlink.net>]

More questions?   What is the purpose of the added edge of the slit?  When
the knife edge is cutting off the light, it is up against its own image.
That is long after the image of the opposite side of the slit has been cut
off.

Likewise, what is the point of worrying about pinhole size, when only one
edge of the pinhole matters when the knife has almost cut off the light
cone?

Except for these points, for once I am in substantial agreement with Bob on
the need for the knife edge and the slit to be parallel.

However, I once found that I could get a fleeting glimps of the figure of my
mirror with NO knife edge!   This is because the edge of my eye pupil was
acting as the knife edge with a pinhole source.  Definitely not recommende
for beginners!

. . . Richard

----- Original Message -----
From: "Bob May" <bobmay@nethere.com>
To: <atm@shore.net>
Sent: Wednesday, December 26, 2001 11:21 AM
Subject: Re: ATM foucault


>
>
> My preference is for the slitless type tester where the blade of the KE is
> used both to provide a sharp edge to the light beam and also to block off
> the returned beam.  If you draw out the light path, you will see that this
> method actually produces a nice set of shadows but also allows for an
easier
> finding of the returned beam (the part that can be completely frustrating
> for even the expert at times) as the light is brighter.
> If I had my druthers of other designs, I'd put the light source still at
the
> blade edge of the receiving KE and then put a blade on the other side to
> make the slit that is desired by some.  The thing that this does is makes
> sure that the slit is parallel to the KE at all times and this allows for
> the tilting of the moving table without the loss of contrast from the KE
not
> being parallel to the slit.
> After that, the light source in front of the approximate place of the KE
is
> next best.  The reason for this is that the measurement of  the vertical
> (assuming that the KE is a vertical edge!) ellipse error can't be measured
> as the test is insensitive to that direction of any errors.
> The worst case is the placing of the light source way to one side as this
is
> how you measure gentle ellipses on the surface as you put the light source
> at one focal point of the ellipse and do the reading at the other focal
> point of the ellipse.
...
> Any more questions?
> Bob May
>