[ATM] Ultra Thin 16" f3.5 kinda long

[vorblesnak@peak.org]

I posted some questions regarding pie plates awhile back.  I was in the
learning curve towards an ultra thin large apature mirror.  I had the
opportunity to work on the plates during a grinding class put on by the
Eugene Astro Society and taught by Mel Bartels.  I broke two, got one
done, (with astigmatism), and am finishing two more.  One of these days I
will put that adventure to print.  The goal here was to work on thin
stuff.  I had worked on an 18" f6 done in 3/4 inch glass.  I finally got
that one figured out, yet another history to write.  During the class Mel
offered me a project he had started, a 16" slumped blank done in 1/4 inch
plate glass.  Like a trout to a fly I snatched it up.

The blank was already slumped to roughly an f4 curve.  I got both the
blank and the slumping tool.  The first step was to convert the slumping
tool to a grinding tool.  I used epoxy and glued on pieces of standard
double strength window glass.  The pieces were irregular, because that is
how they came out of the trash can at the glass shop.  Once the epoxy was
set I began to smooth the surface.

The glass had been slumped over the mold and there were alot of little
bubbly artifacts in the surface .. almost like a magnified lemon peel
surface.  I started the grind with 220 grit to remove these.  I don't have
my notes available but I think it took about two hours of work to smooth
those out.  I ground until I had two deep ones left at the edge and they
were mostly gone.  I then switched to 400 grit and ground until those two
were gone and the surface was smooth.  Next was an hour of 500 grit then
about an hour of aluminum oxide.  I don't know what the size it is, as it
is a pre polish material from the local lapidary shop.  I put two
tablespoons in a pint of water and shook the hell out of it. I let it
settle for ten seconds and then sucked the top half out with a turkey
baster and that was the material I did the pre polish with.

The first lap I did was with hard pitch on a 12 inch tool.  This is nasty
stuff to work with and I don't like it.  I polished for about two hours
with this lap using cerium oxide.  An occasional glance at the surface
with a coarse ronchi tester told me I was getting a turned edge and a hump
in the middle.  All of my mirrors get a hump in the middle.  I finally
broke down and cast a full size tool out of hard grout and then made a lap
on it.  This was also done with hard pitch and I never did get it the way
I wanted it. (I had a lot of hard pitch).  I finally got a routine worked
out.  I would scrape the channels down to the grout and then warm the
pitch and shape it.  Then let it cool for a bit and cold press.  Finally
charge the mirror and start polishing.  As much as I fought it, it did
allow me to nearly polish out the mirror.  Surprisingly, no zones. 
Towards the end I had prepped it to re shape it and set it in the sun to
soften,  and forgot about it.  It was a mess to clean off the deck and the
lap was shot.  I switched to a 12" lap left over from a previous project
and finished polishing.  The last step was figuring.

The curve was oblate with a pretty good hump out to about 30%.  I started
working it with a long stroke, center over edge, mirror on top.  I was
going to hog it out to remove the hump.  After about three minutes I
cleaned the surface and had a look with the ronchi grid.  I had a nice
parabola, not deep enough, but a nice curve to the lines.  Hmmn, piece of
cake!  I went back to the lap for another three minutes and had a look and
had a crater.  OH ^%$&!!  I went in the house to get a drink of water and
to cool off and came back out in about an hour.  I had a slight oblate,
with a hump in the middle.  Now this is odd.  It is completely backwards
to anything I had seen before.  I put the glass back on the lap and hogged
some more with the same results.  I didn't panic so much this time, just
let it cool for an hour and looked at it.  The hump was getting smaller,
the lines were still straight.  Hog Heaven, pardon the pun.  After about
two hours of this kind of polishing I had a sphere but still didn't
understand what was going on.  Up to this point I was willing to hog and
be damned, but now I was getting close and did not want a hyperbola.

I thought about it for a day or two and hit on an idea.  The shape of the
blank is very much like a meniscus lense.  Maybe I was warming the back
enough to deepen the curve on the front.  I was working the glass bare
handed.  To test this I set the mirror, back side down on a basket of warm
clothes right out of the clothes dryer.  After two minutes by the clock I
put the mirror on the test stand and had a nice parabola.  I let it cool
for an hour and had a sphere again.  Now I sat down on a chair and put the
mirror on my lap and rotated it.  This simulated warming the back with my
hands but left the front to the air.  After five minutes I put the mirror
on the stand and had a look.  Nice curve with a big hump in the middle. 
This fits, the middle was never warmed so it never expanded.  Now I know
what causes the weird look after grinding, the back is warmer than the
front due to the heat of my hands.  I am going to wear gloves to finish
the figure.

That is about it for right now.  I have designed a simple ring cell to
mount it.  I have not seen any distortion and I think this is due to the
menicus shape of the blank.  Using this and pie plates as a basis my next
one will be a 24" in 3/8ths inch thick glass.  It too will be an f4.

Stats:

16" diameter
1/4" thick glass
Slumped / precurved to approximately f4
Actual f number  3.5
Focal Length 55.5 inches   1400 mm

David Davis
Toledo, OR  97391



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