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Re: [ATM] Lateral Wire Test
Alfredo,
I haven't used the test for a while, but I have been experimenting
with two setups - one remote-controlled via a Dos laptop:
http://web.telia.com/~u41105032/tester/tester.htm
and also a tester with a geared-down motion via a fulcrum and a
micrometer.
With the former on a "normal" table, I could get credible and
consistent readings - with the latter, nothing like it. My conclusion
is that even touching that setup at all will move it by a micron or a
few - I don't know. Maybe clamping it down to a really sturdy table
would have worked, but the remote-controlled, hands-free approach is
just fine.
This should give my answer your last question - I believe the main
problem is that the setup must be immune to disturbances by touch.
>>>I've been playing a bit with the LTW to test a 12.5 F4.7 mirror but
I am not getting consistent results. Could some of the list experts on
this test give me some advice? My doubts are:
>>>How far inside focus should I take my readings?>>>
This can be tested - try a position with a fairly wide band at the
center, it will be a little narrower towards the edge. I don't think it
is too critical, closer to "focus" (or actually paraxial COC!)should
give less influence of other factors, but perhaps more difficulty
reading close to the edge.
>>>Is a wire diameter 0.08mm good thin enough?>>>
The wire and source widths are not terribly critical - I prefer to
make a thin line on a piece of transparent plastic using a sharp
scalpel blade, as it is much easier to handle than an actual wire, and
easy to try different widths - or an etched line on a glass reticle I
have frome some microscope equipment. As a light source I used a laser
diode at low current - very narrow. Either way, you should see the
bright center line of the diffraction pattern - if not, the "wire" or
source is too wide.
>>>How critial is the measuring of the mirror to wire distance and the
ROC?
As in Foucault - a few mm should not matter much. But once you get
some readings to try, you can recalculate adding and subtracting a few
mm from what you measure to see how much it changes the estimated
figure.
>>>What are the main source of error to watch out for?>>>
See above - touching and disturbing the setup while moving the stage.
Good luck,
Nils Olof
----
I am using a digital micrometer with a 1 micron resolution on a home
made aluminium caustic tester, but I am not sure of the actual movement
of the stage correspondding to the micrometer reading.
Regards,
Alfredo
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