Re: ATM Diffraction ring or thin TDE

["robert allen" <rallen@iland.net>]

M. Hugues writes:
        How does one distinguish between a very thin TDE 
and the normal diffraction ring? [NOT a stupid question]
        It's not as easy as some would think. The classic
test for TDE is: 1. set your knife edge to read the outer
zone of your Foucault test. 2. Place a straight edge [ a school
child's ruler is fine] so it stands vertically across the center of
your mirror. 3. Cut the knife into the light cone [sorry, remove
the mask] As the mirror darkens you will see the difraction 
lines, both the arcs of the left and right edges of the mirror
and the edges of the ruler. Cut the knife in untill the entire
surface goes dark.[just barely, don't overshoot] 4. now compare
the 4 lines. They should all be equal in brightness and width.
        If the mirror edge on the same side as the knife enters
is invisable, it might be necessary to regrind. If this side is there
but dimmer and thinner the TDE significant and must be cleared
up before refiguring. If the side oposite the knife has a fuzzy or
 feathery look on it's inside edge you have a very narrow TDE
and it might have little effect on performance.
        My impression is that this is a more rigid test than the
Ronchi [altho that is a very fine tool and you should try to learn
to use it]
                Bob Allen ASKC