In article ,
Dave Martindale wrote:
(Donald Nichols) writes:
I don't know for micrometers, but I have seen a Gertner (sp?)
traveling microscope (a microscope mounted on a carriage for a
lathe-bed style optical bench) with a long horizontal travel with a
micrometer leadscrew. But the nut is captive end-to-end, but free to
rotate, except for a radial rod which engages a groove in a bronze bar.
That bar has fine setscrews to adjust the angle relative to the axis of
the leadscrew so it introduces very small corrections into the pitch of
the leadscrew. All in all, a very precise bit of metalworking -- and
*very* expensive. :-)
Do you know if this was intended to compensate for errors in
manufacturing, or to allow temperature compensation, or something else?
I think that it was to tune out tiny errors in the leadscrew
pitch. The one I examined was set at a just barely visible angle to the
leadscrew axis over about a 50mm travel. (Or was it 100mm travel?) I
no longer have access to one to examine, so I'm not sure.
But the access to the adjustments was sufficiently restricted
that I think that this was a factory setting and not expected to be
tweaked in the field.
The groove in which the radial arm traveled was straight, giving
only the ability to add to or subtract from the motion on a per turn
basis, somewhat like the micrometer which started this discussion,
except that it was adjustable. IIRC, the readout was in 0.01mm
increments, and the pitch was about 5 T/mm (0.20mm/turn).
Reminds me of another compensating mechanism, this one in softwa
Astronomical telescopes need to move to stay aligned with a star as the
earth rotates under the telescope. This is usually done by aligning one
[ ... ]
Just the motor drive is good enough for visual astronomy, but not for
photography where you need to keep the location of stars from drifting
on the image plane during exposures that may last many minutes. The
usual solution to this is having an auxiliary high-power eyepiece that
grabs a bit of light from outside the camera frame. You align a star in
the field of view with the crosshairs in the eyepiece, and then monitor
its location during the exposure. As it drifts away from centre, you
use buttons or switches on a guiding controller to bring the star back
to the centre of the field. Since this eyepiece is operating at a much
higher magnification than the camera, if the wandering motions are kept
small in the eyepiece they will be invisible to the camera.
Hmm ... instead of projecting the virtual image of the star on
the crosshairs, why not project it on a four-quadrant optical sensor,
(actually two half circles would suffice, I guess) and have it drive the
correction motor (slowly) in the appropriate direction depending on
where the star's light was falling. The ideal is when the star's image
is equally illuminating both halves.
Enjoy,
DoN.
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