From: "Herb Kasler" 
To: "Michael Burr" , "ATM list"

Reply-To: "Herb Kasler" 




Mike,


Well,  I guess that depends on what you mean by strict lateral motion
control, and what kind of wire test we are talking about. My understanding
is that for the caustic test, which I think is also performed with a wire,
you need highly accurate motion control in both X and Y dimensions. I
actually made my test rig with thimble micrometers on both axes, but I
never tried to do a caustic test with it. What I'm talking about when I say
"wire test", is just using a wire (or a painted hair, in my case)
to null the zone on the mirror instead of a knife edge. Oh, and you have to
use a pinhole or a slit. The slitless concept does not work here. The wire
(or hair) has to be just big enough to cover the image of the slit. It
creates a shadow that looks like a Greek letter Phi, that is, when
everything is lined up just right. If the phi is round, and it stays that
way through your whole micrometer X travel, and the stem part is all one
piece and straight, then everything is lined up right (and your mirror
doesn't have really bad stig). The round part of the Phi is the null zone.
I guess you do have to have the hair held steady at just the right lateral
displacement so that it covers the image of the slit, and it also has to be
perfectly parallel to the slit for the test to work right, so it would
certainly help to have a screw adjustment for the lateral displacement of
either the wire or the slit, but it doesn't have to be a micrometer like
mine was. A spring-loaded 1/4-20 screw will work. You could probably even
manage just by holding it in some kind of clamp and tapping at it, but that
might drive you crazy. In order to get the hair parallel to the slit, I did
just tap at a paperclip bow holding my human hair, which in turn was held
to the balsawood KE mount on the y-axis slide of my tester by a small
binder clip. I was not a metalshop kind of guy in school, so my tester did
not look pretty. In fact, it looked like it was built by a chimpanzee. But
it worked! Just like my telescope, but that's another story.

Anyway, I guess the short answer is no, you can't just pop a hair in the
tester you have. I don't remember exactly what the Stellafane tester looks
like anymore, so I'm not sure how easy it would be to modify it  to work
that way. It might be that all you have to do is 1) Laminate some aluminum
foil to a microscope slide with spraymount adhesive, cut a slit in it with
a fine X-acto knife, and clip it over your lightsource to make a slit; and
2) stick a cheap dial caliper to the back of your tester, so that the
mobile jaw is as close to the light source as possible. Then clip your KE
or hair to that mobile jaw, and presto! you have lateral motion control
enough for what you want to do. I don't know if it matters if your
lightsource and hair move together. Mine moved separately. I think it might
be easier to keep your KE or hair right on the optical axis if they move
separately, unless you mount the hair directly above the slit, in which
case I guess it makes no difference. I also like the added precision that
you get from doubling all the offsets when only the hair moves. I did spend
QUITE a lot of time futzing with my alignment, though....



Good luck,

Herb.

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