From: "Mike and Sara" 
To: 
Reply-To: "Mike and Sara" 


Hello!  I've just finished my DK secondary.  I am afrain I bit off a bit
too much, making the 90mm disk with ROC of 310mm!  The hardest part was
getting the test sphere spherical.  With such a deep curve, I kept digging
in at the edges during polishing, giving a large Turned up edge.  IN any
case, with a lot of effort, I got close so the central 60mm was good.  I
have just finished trepanning that section, so its off for aluminizing!

    As far as fringe testing, I could never get a good fringe pattern using
shims.  I even tried Saran wrap, and I still had too many fringes, though I
had a good idea where zones were (especially when they matched up with the
edge problems of the primary as seen in Foucault!)

    Previously, someone suggested I put the mirror and the test plate on top
of each other and observe the number of fringe circles.  With one to two
fringes, I would be cose enough with ROCs to judge spericity.

    With a lot of fringes, by pushing in on the center, if the fringes
appear to move toward the center, then the ROC of the convex mirror is
longer than that of the concave test sphere.  Polish the convex mirror with
the concave tool on top.  If the fringes move out with pressure, the
reverse polishing position is what is needed.

    For my 90mm disk, I had about 13 circular fringes when I first tested
it.  It took me three to four hours to get down to two fringes.  Be
careful, the first time I did this, I ended up scratching the mirror, and I
had to start over!

    Even when I got down to two fringes, shims did not work.  I found an
interesting effect though.  When I placed the convex mirror on the concave
test plate, as I got the ROCs close, often I hade half a dozen straight (or
nearly so!) lines.  I think what happened was that the air wedge functioned
as small shims!  By touching one edge of the mirror to the test plate
first, then gradually lowering the other end, one side had a larger air
wedge than the other side, so I was able to get the few straight fringes I
needed to judge sphericity.  I think that the paper and Saran wrap shims
did not work for me is the low F number of the mirrors I was testing.

    So, test ROC by pushing in on the mirrors when they are in contact and
adjust the polishing stroke depending on how the fringes act under
pressure. Use the air wedge to your advantage by making it perform the work
of paper shims, especially if the F ratio of the mirrors is low!  Best
regards.

Mike

----- Original Message -----
From: "Dominic-Luc Webb" 
To: "Thomas Janstrom" 
Cc: "atm" 
Sent: Thursday, April 03, 2003 12:09 AM Subject: Re: ATM Fringe testing....


>
> On Wed, 2 Apr 2003, Thomas Janstrom wrote:
>
> >
> > Ok I now have my DK secondary ready to test, at a first glance (no
wedges
> > used) I get about 26 fringes neatly centered. With wedges I get arcs,
and I
> > mean lots of them, far too many to count. I have tried several wedging
> > options, some with as small a difference as 2.5microns, the largest was
> > 0.1mm, in both cases there were more fringes than you would want to have
to
> > count.
> >
> > What should I do to bring the fringe numbers down (this question is only
RE:
> > using an air wedge) Tex suggests a shallow wedge, but I tried that....
> >
> > Secondly what is the recommended polishing technique to steepen a convex
> > curve?
> > Clear skies, Thomas
> > http://www.tjanstrom.com
> > "Don't make me set the laser printer to stun"
>
>
> I am working some F/1.0 to F/1.5 glass (Schmidt primaries) in 310 mm
> range. My polishing tool is a disk with triangles cut on the edges,
> so it looks kind of like a gear. This pretty much completely eliminated
> my TDE problem, which was a lot of waves of error from beginning. I went
> back to grinding to get the glass round again, and did fine grinding
> and polishing on this polishing tool. I have an optimistic opinion of
> cutting into tools. Aside from the general routine of 1/3 diametral
> strokes, I could suggest considering holes at strategic positions,
> perhaps in the middle in your case. I even use multiple tools to optimize
> contact at the surfaces I am most interested in with decent success. The
> final stages requires good pressing to make sure tool and mirror have
> good contact. Some people might dismiss my method, but I would urge people
> to experiment with cutting into their polishing tools (not plural, tools).
>
> Dominic
>
>

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