ATM 20" BVC - part 1

["bprewitt" <bprewitt@neo.rr.com>]

First, some observations on the 20" f5-generated BVC blank I'm grinding:

The blank is very dark, but not completely opaque. A strong flashlight makes
a very dim glow as seen through the blank.
It was apparently made by fusing/vitrifying a stack of disks. From the back
(non-generated side), I can see the remains of a few small patches of some
residue, squashed bubbles, or something that is lighter than the surrounding
material. All appear to be at the same depth, roughly 1/4 inch. At the edge,
the remnant of the line of the edge of a disk is visible, with surrounding
material flowed/fused to it.

The surface of the back was not flat, and had a bit of wedge to it. One edge
was 2 1/8", and the opposite was a bit under 2". It looked like the blank
had rested slightly off-level in the annealing furnace after having
partially solidified, causing a depressed crescent shape on one side of the
back face, and a slight dome on the other. The depressed area was perhaps
1/32 below the edge.

I started by grinding the back of the blank flat. It took approximately 30
wets with 80 carbo and a piece of plate glass on a 30 rpm turntable with
hand pressure on the glass, and blank on the bottom. This gave an indication
of the hardness of the material. It seemed to work fairly quickly.
My only previous experience was in hand-grinding a pair of 8" pyrex mirrors,
so it's hard to separate the hand-vs-machine effect from the material
hardness, but it seemed to go "quicker" than I remember with Pyrex. It is
definitely easier to bevel this material than Pyrex.

I've been delayed for a few days by a bad bag of cement grout, and had to
start over with fresh material to make the tile tool. The epoxy under the
tiles is curing right now, with the 2 1/2" by 16" tool resting on top of the
generated surface (separated by papers). In a couple of days, I'll seal the
tool with a couple of coats of polyurethane varnish, and begin making
contact with the tiles with 120 carbo and gentle strokes.

As soon as the tool comes off the blank tomorrow, I'll try to measure CTE,
and compare the width of a scratch on plate glass to a scratch on the blank,
made by dragging a similar weight,  to try to determine relative hardness.

I sure hope this works out, since this 20" blank cost about the same as 15"
of Pyrex.

Worries at this point are :
1. Will the wedge in effect when the blank was generated cause the optical
center to miss the physical center ?
2. Will there be any astigmatism due to the off-level annealing ?

(I'm not really too worried - I know to grind more and worry less.)
             Bill Prewitt

My webpage of CCD imaging, Telescope making, and Caving:
http://home.neo.lrun.com/imaging