Re: ATM Polishing 8" and not sure whats up..

[lindberg <lindberg@olywa.net>]

J Davis wrote:
> 
> Ive began polishing my 8" that I started a year ago. I poured a new
> lap today with the Willmann Bell Pitch medium hard.  I cold press for
> about an hour and contact with the pitch lap seems good,  and my first
> 3-4 strokes are very stiff "major pull" and then the lap lets go to a
> medium to light pull. If I increase my stroke past appx 2" the
> resistance usually drops to almost nothing, and returns soon as I have
> returned the mirror inside the 2" overhang. During rotation of the
> strokes it seems to pull harder at some tangents but I have been
> unable to tell if it always at the same place on the mirror or the
> lap.
> 
> The mirror is an F4.7 and is fully polished out and ready for
> figuring, will the foucault show if I have a defect or do I need to
> use the Ronchi Test.

        What you are describing sounds like normal cranky lap behavior. Warm
the lap up with a heat lamp (or soak it in warm water), press, and try
again. This sort of thing is not caused by any defect in your mirror...
And yes, you ought to plan on using both tests because they compliment
each other in the information they provide. But you don't need testing
to solve this problem.
> 
> It seems that I might have ended up with a little bit of an astigmatic
> curve?
        
        No. Not unless you persisted in doing something grossly wrong while
grinding. The presence of astigmatism can't be determined by lap
behavior.
        
> If its possible to polish should I use a 1/3 W stroke or continue with
> a 1/3 center over center?

        Use either to polish the mirror. If you are polished out and are now
figuring, the stroke to use depends on what you are trying to
accomplish. What does the figure look like under test? Might be time to
hit the books... For beginners I recommend the Sam Brown book published
by Edmund Scientific. My son is using it. Hope this helps you, Jeff.
        Steve.