[ATM] Problems With Central Hole Unchanged

[Mel Bartels]

You should be able to remove central holes fairly quickly - a few minutes to 
1/2 hr.  Certaining this is a figuring issue.  Don't go back to fine 
grinding.

Are you applying enough pressure?  The lap should be hard to move at the 8 
inch size if you are pressing firmly.  Move slowly.  TOT is best, and you 
can accelerate the process by scraping away pitch in the center that mimics 
the size of the hole.

> I am looking at my mirror on my Foucault Tester (Homebuilt Stellafane
> Type) with the razor edge and Ronchi grating. The blade is coming in
> from the left and I am assuming the light to come from the right (thus
> a hole not a hill). When using the Ronchi grating (100 lpi) the lines
> will hardly deflect when crossing the hole.

So it's not much of a hole then?  Maybe you are pressing with your hands in 
the middle of the back of the mirror and causing deflection or local 
heating.  How close are you to a proper paraboloidal shape?

Mel Bartels