Re: [ATM] Pitch lap

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

Christian, the only part of the process that you left out (from what you
have said in your post) is to put the mirror on top of the pitch lap while
it was warm and fresh.
First, always use a mixture of Cerium Oxide (CeO), water and a drop or two
of dishwashing detergent or liquid hand soap (the kind that the wife likes
rather than the stuff that is for getting car grease off of your hands- that
stuff tends to have grit in it) and apply to the mirror freely.
After you pour the pitch onto the lap, you then make the primary curvature
with the mirror.  Then remove the mirror and use a stick (metal sticks work
best with some release agent like butter or dishwashing detergent on them)
and press in some grooves to make blockings in the pitch rather than a
single surface and just as soon as you have done that, put the mirror back
on the pitch and press some more.  Repeat as necessary while the pitch is
hot, remembering to insure that there is water between the pitch and the
mirror at all times, preferably with some CeO in there also.  If the pitch
sticks to the mirror at all, recover from that sticking above all and put
some more CeO and water mix on the mirror before putting it back on the lap.
If the mirror is warm to begin with (maybe 130F), the cooling action of the
mirror won't be as strong as it would if the mirror was cold and will allow
more working time with the lap.  This is the basic John Dobson way of doing
it and it works well and is quick.
FWIW, I also wipe my tool that I'm putting the pitch onto with some
turpentine just before I pour the lap.  This does two things - cleans the
tool of grease and fingerprints and allows the pitch to be a little thinner
at the surface which promotes adhesion.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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