Re: ATM Starfish shaped lap for parabolising

[Bill Marriott <btk@pacbell.net>]

Hugh J. Masterson wrote:
> 
> A question on laps for parabolising:
> 
> I've read about trimming the outer squares of the lap before
> parabolising into a mild starfish pattern, or something similar. This
> supposedly reduces the chances of getting a TDE.
> 
> What if you mess up on the parabolising first time. Are you then
> left with a lap that is useless for other stages of corrective figuring??

Hugh;

I have become quite fond of star laps over the
past couple of years, they allow accentuated
correction in certain areas, with out the zoning
that may be associated with sub diameter laps. (I
have also been using a lot of sub diameter laps,
with good success )

Just recently, over at Chabot telescope workshop,
I helped a gentleman finish a 16" f5, made from
3/4" plate, which up until the past year or so I
would of said was worthless to even try, (but
comments on this list have started me to believe
otherwise). Was polished out, but very oblate when
I first saw it.  We used a sub diameter lap to
knock the oblateness down, but astigmatism arose.
Went back to full size lap, which eliminated it.
Alternated between full and sub laps during
parabolizing, the full size lap would put
correction on the edge, but not much in the
center, so back to sub diameter, which always
seemed to generate astigmatism, though very
effective at knocking the center down.

We ended up at one point with edge almost done,
center way to flat, and significant
astigmatism.....

Chopped the full size lap into a big star, five
minutes of a medium W stroke, and things were much
better, five minutes more, with a wider W, looked
pretty darn good, let it sit, tested, and called
it done.

Not perfect, but ideal for the guy's desire for a
reasonable light bucket that he could take back to
his village in France to allow the local kids to
get a taste of a big scope..


Several months ago I finally finished a 12.5" 
f4.6 mirror I have been working on for quite a
long time, using a reverse star lap, i.e. with the
center cut out into a star, which solved my
problem of the edge not picking up enough
correction compared to the center.

I also have finished a mirror using a lap with the
70% zone cut out to reduce the overcorrecting in
the 70% zone of a mirror.

When done with the particular problem being
attacked, I then paint pitch back on the cut out
areas, repress well, and am back in business with
a regular lap...

Just my long $.02...

Bill
--

Bill & Tracy Marriott
Woodacre, Ca. USA
btk@pacbell.net

http://www.starlightoptical.com
Telescope/optical stuff