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Re: ATM 8" f3.8 -- Worthy project?
At 14:44 10/25/2000 -0700, Dwight Elvey wrote:
>"Matt Terry" <mterry@tampabay.rr.com> wrote:
>
> > A couple of quick questions: How many folks have made sub f/4
> > smallish mirrors?
I've made a 10"f/2.6 RC cass primary, surface RMS ~ 18 nm (barely
passable); it ain't easy, but possible. Mirrors this fast and hyperbolic
generally can't be figured with full-size pitch laps; I went with HCF strips.
> If you are going to use a Foucault type tester, it is
>generally accepted to use a wire tester rather than a knife
>edge ( although it is still not determined if the readings
>that you get are in fact better). My feeling is that it
>is still the best method for fast mirrors...
Lots of people on this list use the wire test. The rule of thumb is that
Foucault starts to fail faster than f/4 - maybe f/3.8 is close enough to
give it a try.
> In the wire test, you normally use a pin hole source
>and a wire for the detector. The wire tends to make a ring
>at the RoC for that zone. On fast mirrors, this ring is
>quite distinct. The disadvantages are that you need a
>bright point source as small or smaller than the wire
>and like I said, it hasn't been shown by any of the diffraction
>experts on this list as to the validity of using the center
>of the wires shadow as the measurement point ( although
>I'd think it was vary close to it ).
If you have a really small source, diffraction theory says there's a bright
line at the center of the wire shadow (like Poisson's famous bright spot at
the center of a circular shadow). First glance says that's where to make
the zone measurement. Sometime in the - near? distant? - future a sixtests
update will come out which will estimate the test accuracy in terms of the
number of zones, measurement accuracy, etc.
-- Jim Burrows phone 206.244.2933, fax .0294
-- mailto:burrjaw@halcyon.com
-- http://www.halcyon.com/burrjaw/
-- Seattle N47.47233, W122.36620 (WGS84)