Re: [ATM] First Pitch Lap - any tips?

[Mike Lockwood <melockwo@uiuc.edu>]

Bill,

Texereau's method seems to give many people trouble.  I have used the 
standard "pour the pitch on the tool" method many times with eventual 
success.  It may take you a few tries, but with practice you will get 
a good lap (but don't expect it on the first try!).  I recommend 
trying it for the experience, and it may work very well for you.

However, I use an alternate method for making full size laps (same 
size as the mirror) for small mirrors.  It gives me nearly perfect 
contact with minimal pressing.  I'm not sure if someone has described 
this before, but I haven't seen it.  I came up with this method 
independently.

First, I clean the mirror, make sure there's no dust on it, and coat 
the mirror blank with a little vegetable oil, just wiping a small 
amount on.  I put aluminum foil over the face of the mirror, leaving 
as few wrinkles as possible, and then I smooth the foil down over the 
edge of the mirror.  Then I make a masking tape dam around the outside 
of the mirror, over the foil, about 1/4" or 3/8" high.  Take the time 
to get it the same height all around.

Next, I pour the pitch on the foil-coated mirror until it's about 1/4" 
thick at the edge (near the top of the masking tape).  I let it cool a 
little bit until it's the consistency of honey.  Before it gets any 
harder than this I put the curved face of the tool (which has been 
degreased) into the pitch.  The tool should contact the pitch at the 
center first (since it is convex and the pitch is level), and then 
sink into the pitch.  If the mirror doesn't sink on its own, push down 
on it until the pitch contacts the tool at the edge of the mirror. 
(Don't be afraid to put some weight on it!)  At this point, some of 
the pitch will contact the masking tape and perhaps overflow.  Level 
the tool (by pushing on the highest side) so that the pitch is the 
same thickness all the way around, hopefully still about 1/4".

Finally, let the pitch cool.  Peel off the masking tape (once the 
pitch is cool enough it should come right off), and break away the 
pitch that has overflowed.  Once the excess has been removed, tear off 
or pull up the foil up around the edge of the mirror.  The tool, now 
with pitch and foil attached, should slide off the mirror when you 
apply sideways pressure.  (You may have to apply some force to do 
this, but they will come apart.  Don't worry if a little pitch near 
the edge breaks off.)

Once the tool is separated, to remove the foil, simply run it under 
cold water for a few seconds and it will peel right off the pitch. 
The pitch will now be accurately formed to the curvature of your 
mirror, and you just need to trim the edge, channel it, and warm press 
(as detailed elsewhere) before it is ready for use.  (I channel pitch 
with a soldering pencil or iron.  With practice this can be done 
without leaving raised ridges.)

By the way, when the Stellafane page says to coat the blank with a 
thick coating of CEO, they mean coat it with a thick mixture of 
polishing mix, meaning WATER and CEO, and maybe a little (1 drop) dish 
detergent.

Some pictures of me and my mirror making class making laps with this 
technique can be seen at

   http://www.astro.uiuc.edu/~uias/astropics/2004/20040219/

Don't be intimidated by the pitch - you may have trouble with your 
first lap and it will be messy, but just strip off the pitch and try 
again.  Wear rubber or latex gloves to keep your hands pitch free.

	Mike Lockwood


Bill Mitchell wrote:
> I will be pouring my first pitch lap this weekend for a 6" f/7 mirror
> and was seeking any last minute tips.  I've read Tex and Stellafane's
> tutorial, but was seeking input from the experts here.  One question
> I have regarding Stellafane's is they say to coat the blank with a
> thick layer of CEO before pressing the blank onto the cooling lap,
> won't the CEO fall off in the process?
> Any other helpful tips to ensure a perfect lap the first time would
> be appreciated 8^)
> 
> Thanks,
> Bill Mitchell


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