ATM BIG Secondaries

["Michael Burr" <mtburr@msn.com>]

Greetings:

At the risk of posting a semi-commercial message, ... I've recently
completed a proof-of-concept project to demonstrate that I can cut
elliptical secondary mirrors from large surplus flats. I hope this might
prove to be a valuable resource for ATMers who need large secondaries in the
5"+ size range.

Specifically, I bought a monstrous (11.5" x 16"), 1/10th-wave (reportedly)
elliptical flat from Surplus Shed. My intention was to trepan two 5.25"
secondary flats out of it for my binoscope project, but I didn't want to do
it by hand. After getting some suggestions from the list, I shopped around
and found a waterjet cutter who will work with me. It wasn't easy, as most
of them want to do large jobs only, and they charge exorbitant prices for
small runs. But eventually I found a five-axis waterjet cutter that doesn't
mind doing small jobs.

My first set of diagonal ellipses has now been cut -- one 6" minor axis and
two 5.25" minor axis. (I've already sold the 6".) They seem to have turned
out well, and the exercise proved that the concept can be cost-effective.

(Technically speaking I don't know how the figures might change as a result
of the cuts. Arguably for a 32" light bucket, the figure of the secondary is
less critical than it might be for a planetary scope. Personally, for my
binoscope, I plan to mask off the outer 0.05" to cover Twymann effect
distortion, and then forget about it.)

At any rate I'm now prepared to supply large elliptical secondaries --
generally in the 5" to 6" range -- until Surplus Shed runs out of monster
flats. For more info, see: http://www.mtburr.com/research/bigsec.htm

Comments and suggestions are welcome.

Thanks,
MTB