From: "Richard F.L.R. Snashall" 
To: Mike and Sara 
CC:  atm{at}shore.net
Reply-To: "Richard F.L.R. Snashall" 




Mike and Sara wrote:
> Hello!
>
>
>     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?

I haven't tried this yet, but... I asked a co-worker.  He suggested get an
"unaluminized polycarbonate capacitor" from Radio Shack, and take
it apart.  He stated that the laminate therein was on the order of a micron
or so thick.  That should make the fringes easier to see.

                Rick S.

>     Thanks and best regards.
>
> Michael Heald
>
>
>

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