[ATM] pitch lap

["David Williams" <dwilliams1128@earthlink.net>]

Hello ATM list

I am making a 20" mirror 1" thick and I have used a 12" sub diameter pitch lab tool solid pitch tool.  The way I made the tool was I took a compass, set it to the maximum setting (12" diameter) and then cut out a piece of 3/4" plywood with a saber saw.  Then I took masking tape and made a dam around the edge of the wood, then I mixed some plaster of Paris (I found out that some of this stuff dries as fast as you mix it, but some of it dries in about 15--20 minutes: it's this latter one that you want).  The prices are somewhat different too.  Then I take the mirror and turn it curved side up and wet it and put soap on it, then put aluminum foil down on it.  After that you take the wooden tool with the slightly thickened plaster and invert it over the mirror and slightly press it down.  Then check to see if the plaster runs out.  Press until the plaster just starts to run out around the edges.  

After the plaster has dried then you work with the pitch, in a similar maner that you did with the plaster, but you have to add heat to the pitch in order to get it to melt and run.  I tried to cut the wooden strips and nail and lay them over aluminum foil and then pour the hot pitch in the channels that way.  But that didn't work.  The pitch stuck to the aluminum foil and made a big mess.

I talked to a friend of mine named Jerry Martensen who has made telescope mirrors before and he said that I should have painted a heavy layer of CEO (cerium oxide)  and water onto the aluminum to prevent the sticking.  

He said that a sub diameter lap is good for polishing, but for parabolozing, you need to have a full diameter lap.  So I'll have to order more pitch from Willmann Bell.  I got 5 lb. for about $25.  

How many of you out there would recommend the method of pouring the lap all in one piece and then cutting the channels with a saw?  Or the other way where you cast the pitch out into squares?

Does anybody know of a good deal on a cheap foucaolt tester?

Thank you for your help.
dwilliams1128@earthlink.net
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