ATM Testing Flats ala Waineo

[Roger Ceragioli <rogerc@dakotacom.net>]

Thanks for archiving Tom Waineo's article!  It's packed with great
information.

A couple of additional ideas:  I've made perhaps a dozen flats now and if
you really want to get into flats, a low pressure sodium vapor lamp is
much, much better than the "bug zapper" lamps.  The latter are not
monochromatic and can cause you confusion with multiple colored lines.  But
an LPS lamp makes nice black and orange fringes with lots of contrast and
sharpness.

Second, a moving spindle IS very helpful.  But it may be possible to make a
flat using subdiamter and full-sized laps at a fixed post.  But it will be
harder.

Third, fused silica/ULE are wonderful for flats (I haven't used Zerodur for
a flat yet).  They're so hard that they tend to resist the minor zones that
pyrex flats get, if they're worked individually.  My best flat so far is a
6" ULE flat.  It's really smooth and close to flat.  Whereas I'm finishing
a 12.5" autocollimation flat of pyrex and it's much trickier to smooth out
the little zones and drive the whole thing flat.

The Ritchey-Common test is EXTREMELY sensitive, to the point where trivial
zones may look serious.  A master reference flat used as a figuring
standard is less sensitive, but more generally helpful and makes life less
alarming.  Alas, for an ATM it will be hard to acquire.

Roger Ceragioli