From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


I think what the test does show is that the radii are different.  The
surfaces are closer at the center, so the convex surface is likely the one
of shorter radius.  Pressing away from the center reduces the spacing
there, and fewer fringes show.

If you press down on the center of the disks, the spacing gets closer and
you see fewer fringes.

With so many fringes, I'd be hard pressed to determine how straight any
are, or if there are minor wiggles or odd concentrations which show zones. 
A band is 1/2 wave, So you need to be working with very few bands.

I'd say that when the elements show fewest fringes, and slight wedge
straightens out the fringes (finger pressure to move the fringes) i.e. not
concentric rings, you may get a hint of how relatively smooth the surfaces
are by the smoothness of the bands.

Other than a guess, I'd say you need to match the radii better by some
polishing, then look for few straight or smoothly curved bands.  Can you
figure how many waves difference there is?

colin


-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Mike
and Sara
Sent: Wednesday, December 11, 2002 5:23 AM To: atm{at}shore.net
Subject: ATM Interference Testing Technique and a D-K Secondary



Hello!

    After the previous posts, I've reground my secondary to the correct ROC
(65mm diameter secondary with ROC=310mm).  I have a tile tool.  I've
polished both the secondary and the tile tool.  The tool seems to have
taken polish well, and looks fine on foucault testing.
   I've treid the interference test with a green laser and red laser
pointer.  I used tissue paper to diffuse the light, and tried the source
with and without a lens.  The lens did not seem to help, so I am not using
it.
    Without paper shims, I can get fringes - Usually a bull's eye pattern
with some aberation around the corners of tiles at the center.  The spacing
between the fringes is farthest apart ner the center, and get closer
together toward the edge.  There are many fringes, but easily seen.  I
don't see any change in the pattern near the edge.
    I didn't see any fringes when using paper shims.  Finally, I got the
thinnest tissue I could get, and I started to see fringes.  Again, there
are a lot (more than when I simply mated the the curves together without
shims), and the fringes seem to be straight, and the edge looks fine,
though with so many fringes, I loose the pattern easily.  Pressing down on
the secondary around the thick shims seems to space out the fringes, though
the pattern changes obviously.
    Is the difficulty in seeing fringes using shims because of the short
ROC?  Or do the two curves differ substantially?  Since the ROC isn't as
important as the sphericity of the secondary, even if the radii differ
significantly, does this result indicate good sphericity?
    Thanks and best regards.

Michael Heald

--- BBBS/NT v4.00 MP