ATM 21" f/6 report: status and questions

["Ken Bertapelle" <kenb@cqg.com>]

All,

For those interested, below is a recap of what has happened on my mirror
project.  Here are a few questions for the more experienced members out
there:

Questions:
==========
I ordered 2 kilos of gugloz #64 pitch, and I'm planning on using a full size
pitch lap, MOT to do the polishing.  That should result in a nice spherical
surface for figuring.  I suspect the mirror is going to be hard to push over
the lap with these dimensions.  Do you all think this will be a problem?  If
so, what are my alternatives.

I plan to make another smaller lap for figuring, but what's a good size for
this mirror?

Recap:
======
1)  Bought a blank, 1.55" x 21" diameter, both sides blanchard ground.
2)  Beveled edges on both sides.
3)  Rough ground one side to a depth of about .02", but then decided I was
grinding the wrong side.  Re-flattened the ground side, turned the blank
over, and started grinding the other side.
4)  Rough grinding with a hydrocal tool, 5" thick, 14.5" diameter, faced
with metal washers.  Using 46-70 grit, heavy pressure, MOT, stroking the
center of the blank over the edge of the tool.  21 hours of grinding this
way.  Measuring sagitta with a machinist's ruler.  Stopped when reached
desired depth of .2165".
5)  Made a new full size tile tool.  Switched to 60-90 grit.  Grinding with
light pressure, MOT, 1/2 diameter W stroke.  5.5 hours of grinding this way.
By the end of that time the entire tool is making good contact, and the
entire mirror surface seems to be wearing evenly.  The grinding stroke are
smooth, with no grabbing.  I think the surface is developing a nice sphere.
The f ratio is still a little long compared to my f/6 target, but I prefer
it to be longer rather than shorter.
6)  Switched to #120 microgrit.  Same stroke as in 5.  Grinding for 5 hours.
Mirror wear appears to be uniform.
7)  Switched to #220 microgrit.  Same stroke as in 5.  Grinding for 2 hours.
Did a sharpie test and the ink wore away from the center in an even way,
starting at the center, and moving outward.  This is where I am now.  I
think it will require at least 3 more hours of #220, before moving on to
#320.
8)  Before starting #320, I'll make the hydrocal bases for the polishing and
figuring laps.

Thanks in advance for your suggestions.

- Ken Bertapelle