ATM 8" achromat (another update)

[Roger Ceragioli <rogerc@dakotacom.net>]

Last night I finished polishing r1 of my achromat, the last surface to be
done.  Then I cleaned the elements carefully and oil-spaced r2 and r3 and
taped the crown to the flint, inserting them into their metal cell.

Then today, armed with my trusty Foucault tester and 3 dielectric filters
to isolate the e-, C-, and F-lines, I positioned the lens cell in front of
a master reference flat and began to test the sucker.

Here's what I found, straight off the polishing machine: the central 65% of
the lens focuses about .010" closer to the optic (this number takes into
account the doubling caused by the double pass arrangement) than the outer
35%, but each of the two zones is basically flat.  The dividing line
between the zones is not sharp but they shade more or less into one
another.  A crude PV calculation based on a 4 zone Couder mask gives a
wavefront error of about 1/6th wave, and all the zones focus their light
within the Airy disk.

The back focal length is almost dead on the mark versus my design
calculation.  And the F and C lines focus at almost the same distance
(about .060") beyond the e line, just as they're supposed to.

Overall, I'm pleased with the performance straight off the polishers.  But
of course I will figure to achieve a smoother wavefront.

Since the inner zone focuses shorter than the outer zone, it appears that I
can just figure the convex r1 on its polisher MOT (just as if I were trying
to parabolize the inner zones of a mirror), since I need to lengthen the
inner zones to cause them to focus further out at the same distance as the
outer zones.

Roger Ceragioli