Re: ATM Sub-Diameter Tool/f2.5 6"

[btk@pacbell.net]

> > Alfredo Neves wrote:
> > >So far I got a nice sphere, but the ronchi test shows a 1/4" turned
> > >down edge.

Bill replied;

> >
> > Assuming at this point you are using a full size lap, have you done  > > much work with tool on top? Is your edge fully polished out?
> > If not, give it a try, using a 1/4 center over center stroke 
> > (3/4" overhange front and back).
> >

Alfredo further questioned:

 
> I've been working with MOT and full size tool. The
> edge is almost polished out (there are still a few
> pits). Just to make sure I understand, I should use a
> 1/4 stroke with the sub-diameter tool on top, right?
> 
> Alfredo

And Bill further replied:

No.

Save the sub diameter tool for later, during figuring, when (if) the
full size tool will no longer suffice.

If your edge is not fully polished out, it is consistent with your
turned down edge. In actuality, what is going on is the fully polished
out area is of a shorter radius than the edge, so the edge appears to be
turned. As the edge gets polished out all the way, this longer radius
zone should disappear. 

Working with the FULL sized lap on top, with the short 1/4 coc stroke,
will accelerate this process, continue till the zone is almost gone,
then flip back over to MOT, continue with a 1/4 coc stroke, and you
should end up with a smooth consistent curve which is hopefully
spherical.

This of course is based upon my experience which is 99% with f4.25 or
slower mirrors, though by my calculations, the corrector for my MakNewt
is F1.3, and these basic principals still applied.

Good luck!

Bill Marriott
btk@pacbell.net
http://www.starlightoptical.com
telescope/optical stuff