Re: ATM Help: First Mirror, Polishing done?

["Bill T." <twentiethwave@hotmail.com>]

Doug B. wrote:

>>
Is it possible, polishing by hand, to be done with an 8" pyrex mirror after
only ~5 hours? Starting with an 8" channeled lap microfaceted by pressing
with
onion bag, working TOT, COC, with the cerium oxide that came with the
newport
kit (one tbsp CeO in one cup H2O),pushing the wets 'til i couldn't push no
more, the mirror looked uniformly bright after ~1 hour, examining with 8x
loupe it was easy to focus on the surface, but it looked completely uniform
from center to edge, the channels completely closed up after ~2 hours, and i
left them that way after reading about the benefits of unchanneled laps on
this here list, and, because i'm lazy, it sounded like a pretty good idea.
At
this point it i could no longer see haze on the surface, and again, the
shine
was uniform across the whole face of the mirror.  a laser pointer dot was
easy
to see on the surface at this point. after ~5 hours, in a dark room,on the
cleanest bits of the mirror face,  i can see a very faint image of the laser
beam. will the polish ever get to a point where the laser is totally
invisible
on the surface? and is there a way to get the surface clean enough to be
sure
it's not dirt, rather than pits, that is reflecting the beam? i know i
should
probably polish more and worry less, but polishing by hand  with the
unchanneled lap is killing me, and i already wasted about 4 hours trying to
get pits out with 320# grit that came out in 2 wets of 500#.
<<

Doug,

You're pretty much right where you should be, maybe even a little ahead.
The laser dot will not be visible at all on a fully polished mirror surface.
You need maybe another two or three hours.

You're definitely not seeing dirt, the pitch wipes the mirror very clean.
Brush dust away with the dry edge of your hand or a fine brush to be sure.
An uncoated mirror surface is amazingly tough, don't even be shy about
wiping it with paper towels

Your slightly unusual COC, TOT strokes are the reason the edge came in so
quickly.  This stroke also tends to parabolize the outer parts of the
mirror.  This increases the amount of pressure required to push the lap,
since energy must be expended to deform the lap as it passes over the
non-spherical surface. The W stroke is designed to polish a mirror to a
sphere, and requires much less pressure.  But from where you are now, you
should probably continue with COC, TOT.

Unchanneled laps work great for some people while others seem to get better
results with channels.  IMHO you need to press an unchanneled lap a lot more
frequently and for longer times than a channeled lap -- this is especially
true with not-quite-spherical COC polishing.  Keep the microfaceting going
with frequent scratches on the pitch with a wire brush, Xacto, or whatever.
If you wind up with zones that just won't fully polish try going MOT for a
while.  Re-channel if things just seem to grind to a halt.

But don't give up on polishing.  As far as I can tell you're nearly there.
It will interesting to see what the figure looks like when you're done.
Don't to try to judge your mirror with a Ronchi test before it's fully
polished.

Bill T.