From: "Martti Koskimo" 
To: 
Reply-To: "Martti Koskimo" 


Hello,

>  Is the difficulty in seeing fringes using shims because of the short
> ROC?
 The difficulty is the changing obliquity from the middle to the edge and in
curvature difference.
Usually a collimating lens is used so that the light paths are square to
the surface being tested.
Sometimes even complex collimating lens system must be used.This is not
always a simple matter.
 The Obliquity error gets much smaller if the surfaces are very close each
other.
Obviously the secondary  mirror itself acts as a sort of collimating lens
but not as a perfect one.

> 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?

You loose accuracy if there is much difference in the radii. If the fringe
spacing is only a few millimeters
and you have previously mentioned angle error then is questionable if you
can have significant estimation of the errors even if the fringes seems to
be straight.      sorry ;-(

The cure to your problem is  :    Make a separate reference mirror
preferable at least 100 mm in diameter and get the curvature close to the
curvature of the secondary . You should do this also because the stability
of your polished tool might be questionable.


Martti Koskimo







From: "Mike and Sara" 
To: 
Subject: ATM Interference Testing Technique and a D-K Secondary
Date: 11. joulukuuta 2002 13:56


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





----- Original Message -----
From: "Mike and Sara" 
To: 
Sent: Wednesday, December 11, 2002 12:22 PM 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
>
>

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