Re: [ATM] Seeking advice polishing an 8" F/6

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

I didn't see a reply to this.

That looks pretty good. It shows some roughness but that's not too bad for
just finishing polishing. Slowing the stroke a bit should help that.

If you have Texereau's How to Make a Telescope, look at figure 42 number 7.
do that stroke across the up-sloped zone on that dinner plate should take
care of it quick. Do that one time around the barrel, 3 to 5 minutes maybe,
and test. This is a tool on top method but it can be done mirror on top
putting pressure on the other hand.  

Another way would be to start parabolizing and work that central flat area a
little more than the turned up area. Do that by using the Classic Method
shown in figure 43 stoke number II deepening the center. Picture the width
of the turn up on each side of the center of the lap. As you do your W
stroke as the center of the mirror enters that zone in the middle of the
lap, reduce the number of strokes. So you have more stokes on the left and
right side of the W's and fewer in that center. Again go slow and adjust
where in the W You concentrate more strokes by what you see happening on the
surface. When the center and outer zone merge to one you adjust the W to
work the entire curve uniformly.

Another way is Method C number II in figure 43. Tool on Top and that little
W shown would be the width of the up-turn. That looks to be about 1 1/4
inch. Needless to say this works really fast. You have to be careful and not
let that large overhang rock the lap off the edge. And again as the center
and outer zones merge to one curve adjust the stroke. Method C is a two step
technique. Parabolize by alternating working the center and outer zones.

It is interesting that you can choose whichever way to start. And you can
see how various advice on "what should I do?" can appear contradictory yet
be valid. You could rid yourself of the turned up outer zone by wearing it
to the longer radius of the center area, or you can wear the center deeper
and shorten its radius to that of the outer zone. And you can do some
combination of both. You can carry that right into parabolizing your mirror.
You can try to make it a sphere and then parabolize, but it is much the same
strokes to work the surface in the same direction. So it is reasonable to
say just start parabolizing. You will pass through a sphere on the way to a
parabola.

I will add that I have had good fortune with a shape like yours doing
Classic Method A stroke I except doing it Tool On Top. It will concentrate
work on the outer zone and roll it down. If you get the width of the W and
the length of the stroke right the roll down will match the parabola and
maybe leave a small amount of easy work in the center.

Jerry 






-----Original Message-----
From: davidtrumble@comcast.net [mailto:davidtrumble@comcast.net] 
Sent: Saturday, February 04, 2006 2:30 PM
To: ATM List
Subject: [ATM] Seeking advice polishing an 8" F/6

Hi all,

A small group of us in Boulder started mirror making about a year ago. A few
of 
us are now in the middle of polishing.

Mine looks like ...
http://www.atmlist.net/contrib/davidtrumble-at-comcast-dot-net/02-02-06%20Al
an%20Dave/Dave3.jpg
(Knife edge on the left.)

I'd like to hear what people think of this "dinner plate", and how to best
proceed from  here.

I using a dental stone tool with Gugolz Pitch #64, formed with a
PitchLaps.com mold.

Thanks!

--
David Trumble
Boulder, CO
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