ATM Re:TDE tolerance - was Ronchi and TDE

["Dan Chaffee" <dchaffee@blitz-it.net>]

Peter,

]>Of course, if there are other more reliable ways, great.  But people
should
>not be frightened off testing for TDE using Ronchi.  Something like 1/4
wave
>literally leaps out at you and just cannot be mistaken for anything else.
>And I believe it is possible to detect 1/20 wave on a sphere, despite
>diffraction effects.

This sounds especially reasonable to me now that I have had the
opportunuty to do a ronchi test on R3 of my doublet and then
fringe check it with a test plate. A .5 fringe bend (.25 wave surf)
at the outer 6 mm coinsides with a very strong hook on a 90 lpi
ronchi screen.  The easily quantifiable accuracy of interference fringes is
a facinating, if not sobering confirmation of what ronchi bands
can tell us. Not that it matters as much for R3&2 of an immersion
doublet, but my other radii also have  rolled or TDE to some extent--which
is no doubt partially responsible for allegedly spherical refractive
surfaces showing undercorrection before figuring.

Dan