Re: ATM 21" f/6 report: status and questions

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

Jeff,

Thanks for the tips on fine-micron grinding.  5 hours to fully polish a 22"
with a 14.5" lap is very impressive!  Somebody hand polishing a large mirror
should definitely take the time to develop the skills required for 5 and 3
micron grinding  -- preferably on a much smaller mirror!

I suppose it all boils down to tradeoffs.  Polishing time versus superfine
grinding, number of grit step versus number of cleanups, etc.  One's skill
level, experience, tools, and overall resources need to be considered when
choosing a technical path to a completed mirror.

Can't resist: here's my personal best polishing setup, a 20" mirror on top
of a 23" lap.  Starting with a 9 micron surface, after a non-stop 3hrs30 it
reached "flashlight test" quality on the whole surface, and "laser test"
quality by 4hrs30.   The tradeoffs: the lap takes over an hour to prepare,
requires lots of practice to press, must be exactly the right hardness, and
is relatively useless for figuring (except to quickly return to a sphere).
Also, the push-resistance is way beyond the abilities of a single mortal
man.  I actually prefer a truly full size 20" lap for a 20" mirror, because
it offers more figuring options.

http://www.kupercontrols.com/misc/20top23.jpg


Bill T.