 Mann 422F13 Measuring Engine
General Information
The Mann 422F13 Measuring Engine was purchased in 1967 from the David
Mann Company. This machine is still in use today, as astronomers from
the United States Naval
Observatory visit the McCormick Observatory several times a year to
utilize the machine.
Unlike most previous measuring engines, the Mann 422F13 allowed the
precise measurement of positions in two directions, both X- and
Y-coordinates, using two precision screws rather than just one. It
could make measurements with a precision of 3 micrometers (µ).
Operation
The X- and Y-coordinate stages (which move the plate carriage right/left
and back/forward, respectively) are movable over small distances using
the two precision screws, controlled by handwheels on either side of the
measuring engine. The X-coordinate handwheel (3) is on the right side
of the machine, and the Y-coordinate handwheel (2) is on the left/front
side of the machine. Both have windowed vernier scales (5, 4) with which
the positions of the two screws can be read. For larger motions the
stage can be completely disengaged from the screw and moved freely. To
disengage the stage from the screws, the disengaging handle (to the
left/bottom of the plate stage) should be turned one half turn towards
the label FREE. To re-engage the stage with the precision screws, the
stage should be slid until the pointer on the dial indicator flicks up
to "O" and then falls back. As the pointer comes to "0", the
disengaging handle should be placed in the engaged position (one half
turn away from FREE), and then it will be re-engaged with the screws.
In addition to the linear motion of the stage in the X- and
Y-directions, the rotary table which supports the plate stage can be
moved a full 360° using the handle (6) on the main stage. The
position of the stage rotation can be read to a precision of 20" using
the scale around the rotary table.
A binocular microscope (7) is pointed at the observer at an angle of
22° from the horizontal carriage of the machine (1), where the plate
is mounted using two clips. The two clips allow every plate to come
into the same plane of viewing with respect to the microscope. The
stage on which the plate is mounted is made of glass and illuminated
from underneath, for easy viewing. The Mann measuring engine can measure
plates of up to 9 × 9 inches in size. It has a stage
range of 250 × 265 mm. A second model of this type encorporated a
viewing screen rather than the binocular microscope which would have
been much easier for long measuring sessions, but the model owned by
McCormick did not have this capability.
At McCormick Observatory, the plate was usually measured from left to
right (in the increasing X-direction). If the plate was a trail plate,
then it was aligned such that the trail was parallel to the X-axis of the
measuring machine. This was achieved by aligning a trail on one side
of the plate with a horizontal cross-wire on the machine. The
horizontal cross-wire connects two flat screws on either side of the
plate stage. If the plate was not a trail plate then it was usually
aligned such that the base of the plate was parallel to the X-axis of the
measuring machine. Two measurements of X- and Y-positions were made for
each image, and they had to be made made while moving in the increasing
coordinate direction and measuring the first image encountered for each
star first. The star whose parallax or proper motion was to be found was
measured first, followed by all of the reference stars, and then
finishing with a second measure of the parallax or proper motion
star. In order to achieve greater accuracy and consistancy, when all of
the stars had been measured in this way the plate was rotated 180°
and the path was followed in reverse order. These measurements were
recorded on the Datalogger machine, on punched paper tape, which could
then be read into a computer and analyzed from there.
Company Information
This machine was manufactured by the David Mann Company.
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