From: Bill Marriott 
To: Don Bates 
CC: atm{at}shore.net
Reply-To: Bill Marriott 


Don;

I am a strong advocate of replacing pitch laps, if there are figuring
problems as you finish polishing out, so as you clean things up, the new
lap can break in, yet still be soft and pliable for following the parabola
to its finish.
Many, many times at the Chabot Science Center Telescope Making workshop, we
have seen people struggle and struggle with figuring with a thin too hard
lap. New lap, problems evaporate, life is good.

However, we have lots of pitch and advice available, so its easy for us to
say that, if  you do not have easy acess to pitch, then that's another
story.

With a f6.6, you will hopefully not be figuring for long. You could wire
brush the surface, then wave a torch over the surface to bring up new
pitch,  you just might make it, if there is enough pitch left.
Thin pitch acts harder
Thick pitch acts softer.

One may also say, that if you are not experiencing problems, then why fix
something that is not broken?

There, a  nice straight forward answer.  8*)


Don Bates wrote:
>
> After reading several posts in the archive, I am
> convinced that there may not be a "right" answer to
> this question. But like any good ATM'r, I'll ask
> anyway:
>
> Should I pour a new lap prior to beginning the
> figuring phase?
>
> I am almost done polishing my 10" f6.6, and the lap
> has become woefully thin. (probably somewhere around
> 1/16" thick.) I still need to remove a few more pits,
> so it could get a bit thinner still before I am ready
> to figure. One camp advocates pouring and breaking in
> a new lap before finishing the polishing phase. That
> way you have a nice thick lap to figure till the
> parabola God shows you the right strokes for a 1/20
> wave mirror.
> The other camp says "Absolutely not! The new lap will
> ruin your figure, ripple the glass, and never really
> mate properly with the mirror. Since you won't be
> using the lap (hopefully) for long sessions of
> figuring anyway, as long as there is still a shred of
> pitch on the surface of the tool, keep on strok'n!
> Any thoughts?
>
> Don
>
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--- BBBS/NT v4.00 MP