Re: [ATM] Nearly polished...but...

["Jerry" <wa4guu@verizon.net>]

It is in Texereau how to fix a hyperbola, but don't do that now. You won't
have to. 

The most important thing to fix is the unpolished edge. Whatever odd shape
you have will turn into something more normal by the time you have polished
the edge. That is if you maintain channels in the lap and use something like
a normal 1/3 to 1/2 diameter W stroke.

A W stroke something like 1/2 diameter high and 1/3 diameter wide TOT will
work the edge faster and give you something near a sphere. It will probably
be near a sphere long before the edge is finished polishing. 

Since your temperature is warm, don't make the channels in the lap too deep
and wide. About 1/4 inch wide and deep will do. You will have to trim the
lap frequently because the channels will fill in quickly. Don't let the
channels fill in anywhere on the lap. Re-trim before they fill in. You don't
have to trim the entire lap every time you trim. If you see a place where a
channel is pinching closed, trim the pinch with a razor blade to the width
and depth of the rest of the channel. When the channels have closed about
half way, (to 1/8") it will be time to trim the entire lap again. That will
be frequent because of the warm temperatures, but it does not take long to
make 1/8 inch channels into 1/4 inch channels. I use a razor blade, but it
takes a knack. Find what works for you.

If a big flake comes off of a facet and it bothers you, just fit it back in
place and press lightly with your thumb for about 10 seconds. It will stick
back in position.

I like to keep the lap wet while trimming and have a sprayer handy. The
pitch chips will not stick to the lap as much. I rinse the chips off the
face of the lap occasionally with the spray and that keeps the lap wet too.
The longer they sit on the lap the more they will stick. It is pitch so it
won't hurt if they stick and you can't get them all off, but you have to
press them into the surface. The more chips you have stuck to the surface of
the lap, the longer it will take to press them in.

That brings you to what comes after trimming. Pressing. Depending on how
well you did keeping the chips off the face of the lap and not otherwise
raising up high spots on the lap, pressing after the trim might take 45
minutes or it might take several hours. Paint the lap up good with rouge
slurry or whatever else you might be using and press.

Jerry





-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Norm Prince
Sent: Saturday, July 07, 2007 12:17 AM
To: atm@atmlist.net
Subject: [ATM] Nearly polished...but...

Okay so I've got 8 or 9 hours of polishing in on the 12" f/5 project, and 
its looking pretty well polished...(i've been looking at it with my "laser 
line", my ghetto version of the laser polish test notion)...the last 1/4" of

edge is being a bit reluctant to take a polish along with the rest of the 
mirror...and its bugging me...but that's not my big question.

So i gave it a look-see under the KE, and it appears to be hyperboloid (I 
could be wrong but that's my best guess, I can try to take a feeble picture 
for anyone curious)...wow what an odd shape...I've never actually seen that 
shape under a KE so it took me a bit of time with nose in books and playing 
with "Ronchi for Windows" to figure out what the heck i was looking at in my

own mirror.  Okay, so I wasn't able to find any understandable advise as to 
how to fix that shape in Howard, or a copy of a book by Jean Texereau.  Any 
suggestions as to how to proceed and get back on a better track???

Let me add tho that my pitch lap has totally had it...at least sort of...the

channels are completely gone on about 80% of it from center out, and I'm 
freaked out about trying to rechannel it (and not sure how to do it short of

a saw) for fear of getting something really scratchy in it at this point. 
Part of the problem is the temperature locally...75 F and up most of the 
time, and i got no AC....

Any advice on how to proceed from here given my sorry set of circumstances 
would be greatly appreciated?!?

-norm prince-
GA, USA (land of the second mirror learning curve) 


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