RE:ATM Caustic Test

[andydtg@phoenix.net]

Don Clement wrote:

>   Has anybody done Caustic testing with a wire null and an 
>     accurate X-Y stage?

I've done this test several times as a proof test on commercial 
mirrors installed in scopes I've built (3 17.5" f4.5 Coulters and
one 24" f4.5 Star Instruments), about 15 yrs ago. I used 1" dia
zones spaced on 1/2" centers, and a .0002" diameter wire on
the face of a beamsplitter cube, another face had the slit mtd
on it. The stage was a 4 x 4" lab x-y stage specified repeatable
to a couple of microns.

The biggest problem was that the apertures look really small
at r of c & there are a lot of diffraction effects. Even with
a coated mirror the null was not easy to see. I set up a small
image intensified telescope (about 6x) behind the rig to make
the wire crossing easier to observe. Repeatability was about
+/- .0001" & I averaged 10 readings/aperture, getting an
accuracy in the +/- 0.00005" range. I figured the sensitivity
of the test at about .02 wave (surface) in green light.

BTW all 4 of these mirrors tested at 1/3 to 1/2 wavefront
at focus, this was later proved correct in star testing.
One of the 17.5's and the 24 were reworked to better than 
1/5 wave later.

The caustic wire test is very accurate if done properly but it is 
a *lot* of work to get even one set of readings; I don't reccommend
it for use in figuring except maybe in the final stages.

Today's technology using stepper stages and CCD cameras might
make it possible to automate this test, probably not worth it
for just one mirror or two.



 
Andy Saulietis / DTG Alt-Az-Fp Drive Systems
HDPE Worm Gears, Custom designs & Machine work
12617 Harriet Ln
Santa Fe, TX 77510 USA
409-925-8854 Voice/Fax
e-mail: andydtg@phoenix.net