Re: ATM Pre-polishing Question

[Aplanatic@aol.com]

Hi Gary:

> I wonder if anyone out there has tried pre-polishing their mirrors?  By
>  this I mean going at the mirror with a pitch-lap (or pads) and the final
>  abrasive used in fine grinding, after fine grinding is finished.  For
>  example, if you use 5-micron as the last step in fine grinding, do you then
>  do some polishing with this before going to cerium oxide (or whatever
>  polishing agent you would normally use)?  If so, have you found that it
>  helps?  Does, for example, a 1/2-hour of pre-polish save you an hour of
>  polishing?

Yes, tried it once on a 10" mirror and found that the time you save in 
polishing is more or less canceled by the time it takes to make another 
lap.  In my case I pre-polished with 3 micron after grinding with 5 micron.  
Surface looks sort of polished after this treatment but is still very gray 
under a flashlight.  I polished out the 10" in perhaps half the time, about 
four hours, but it probably took close to that much time to make the 
substrate for the extra lap and then pour and press the lap.  I was too 
chicken to try the pre-polish lap with CeO, but it just might have worked 
OK.

Since then I have found that if I score the lap really well with an exacto 
knife, use a thin mixture of CeO, use lots of downward force, and let the 
lap dry out I can polish out a 10" from 5-micron in three or four hours, so 
I don't see any reason to go to the trouble unless you have a spare lap 
hanging around, in which case it might significantly shorten the polishing 
time.  Otherwise, just make that polishing lap squeak and scream.

Dave Rowe.