Re: ATM HELP! mirror making crossroads

["Bob May" <bobmay@nethere.com>]

John Dobson is a bit slapdash in the way he does things like this.
Personally, if you want a shorter FL scope, grab the 120 grit and get it
down.  80 grit could be used but will be a bit harder as you will have to
stay in 120 grit for almost as long as just staying in 120 grit.  After you
get the curve near (not all the way yet) go down to 320 and get about 1-3"
longer FL than you want and let the finer grits get you the rest of the way.
Please note that once you get to the finer two grits, you are more
interested in getting a smooth surface and make it spherical.  Shorter
strokes will do these things.
Now, as to a turned edge.  This is purely a function of polishing and
figuring.  I can take a mirror and put a TUE edge on it as well as a TDE
edge on it with a little bit of work.  It is all in the way that the mirror
is moved on the tool.  Fine grinding has nothing to do with the turning of
an edge and going back to grinding after you find a problem like this on
your polished surface is merely wasting your time.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net