 Gaertner Single Screw Measuring Engine
General Information
The Leander McCormick Observatory, under the directorship of Samuel
Alfred Mitchell, purchased its first single screw measuring engine
from Gaertner Scientific Corporation in 1916. It was only partly paid
for by the $250 grant from the Henry Draper Fund of the National Academy
of Sciences, of which George E. Hale was the chairman. This was the
first grant that the observatory had ever received from one of the great
foundations. In 1919, the machine was completely paid for by another
$400 grant from the same source. Although these were small sums of
money even at that time to support the work of an observatory, the
second grant, at least, had come unsolicited to Mitchell, simply on the
merits of his work at the observatory. This measuring engine was used
from to time of its purchase through the 1960's, and thus a large number
of photographic plates at McCormick Observatory were measured using this
particular instrument.
The engine was designed by Frank Schlesinger, the man most famous for
coming up with a way to determine parallaxes from photographic plates at
the time of the turn of the century. He designed the engine with
several specific factors in mind, in order to create the easiest and
most efficient measuring process available. First, he chose to use
rectangular rather than polar coordinates, since polar coordinates are
much more difficult to measure and compute and they restrict the choice
of comparison stars. Second, he realized that if a star is near to the
ecliptic, its parallactic displacement mostly occurs in Right Ascension
and not in Declination, meaning that the star does not move very much in
latitude, only longitude. So to simplify the measuring engine and
produce it more cheaply, he chose to measure displacement in only one
direction--longitude. And third, Schlesinger had the choice between
using a screw to make the measurements or else a scale or reseau.
Although scales and reseaus have several important advantages, he chose
to use a screw for the following reasons. With a screw, the engine
operator only needed to make a single bisection with the microscope
reticle lines and then to read the dial to get his measurement. In
contrast, a scale required bisecting several times on scale divisions
and so was more difficult and time consuming. Another advantage of the
screw measuring engine was that only one kind of object was measured so
that the operator was less easily fatigued. The magnification could be
adjusted at will, also, while in a scale measuring engine the
eyepiece-micrometer had to be adjusted for whole runs. And finally, a
scale machine had large errors introduced when the operator had to
measure at a large angle from the optical axis of the microscope, to
obtain data from the edges of plates. With a screw, this is not an
issue because the microscope stays at the same angle from the plate
carriage.
There are also disadvantages to the screw measuring engine. For
instance, it is not very convenient to measure widely separated objects
on a plate, since turning the screw for the length of the plate takes a
lot of time. The guiding way for the plate carriage also needs to be
perfectly accurate, so that the motion of the plate with the screw
provides a precise determination of the distance between objects on the
plate. The microscope also needs to be kept at a constant angle from
the plate carriage for accurate measurements to be made. This is not
usually a problem, but this angle may vary a bit with temperature.
Finally, eventually the screw will wear down after being used. In order
to minimize this effect, Schlesinger designed a screw with a large
diameter, a rather large pitch, and deep cut threading. He drew up
plans for a machine considering all of these factors and then the
machine was built by William Gaertner and Company of Chicago, where the
University of Virginia finally bought it.
Operation
A photographic plate is clamped onto the carriage (8) with two equally
long clips, shaped in such a way that every plate comes into the same
plane. These clips are mounted so that smaller plates can also be held.
The plate carriage is moved vertically in order to position the plate
correctly underneath the microscope using the handwheel (2) on the right
of the machine. A star is then placed under the micrometer by moving
the microscope carriage (5) right and left using the handwheel (1) on
the left of the machine. Many measuring engines are built with the
precision handwheels on the right side, since most operators will be
right-handed, but Schlesinger figured that operators would be just as
adept at measuring with their left hands, plus then it left the right
hand free for recording data.
The carriage is oriented at an angle of 35 degrees to the horizontal,
and there is a counterweight attached to it by way of a cord threaded
through a sheave (6) such that the cord is parallel to the guiding ways
and the force of the weight goes through the center of gravity of the
carriage, so that the carriage won't twist from side to side as it is
moved. The guiding ways are built to allow the machine to move
extremely smoothly, as one of the ways is the regular rectangular shape
and one of them is V-shaped so that the machine encounters very little
resistance as it moves. Additionally, both the carriage and screw move
with little friction in their bearings.
The microscope (4) has a focal length of 8 centimeters to provide the
machine operator with a net magnification of the plate of about 7 times.
The micrometer screw (7) has a pitch of 1 millimeter, and four turns of
the screw translates to 1 millimeter of distance on the photographic
plate. Furthermore, the micrometer dial (3) is graduated into 100
parts, so each tenth of these spaces can be read to 1 micrometer.  This
would allow the operator to read down to a precision of about 0.003
arcseconds on plates taken on a 40-inch telescope, like the one
Schlesinger used at Yerkes Observatory.
A certain error is present when one tries to measure any kind of image
with width to it. One's tendency is to set the measuring reticle either
too far to the left or too far to the right, and so one's data will not
be very accurate. Schlesinger realized this problem and thus he
equipped the eyepiece of the microscope with a reversible prism. The
prism can be rotated 90 degrees so that the field is apparently rotated
180 degrees, and then the operator can do bisections from both sides of
any given image and then take the average of those measurements.
However, the reversing prism was never used by astronomers at McCormick
Observatory. Early astronomers at the University of Virginia decided
that it would be more reliable to measure the photographic plates
directly and then to flip them and measure them from the reverse
direction, so the reversing prism was actually removed from the machine
and then discarded.
Although the original rationale for designing this measuring engine with
only one screw was reasonable, it did allow only one coordinate to be
measured. It is also useful to at times be able to measure in the other
direction, so there needed to be a way to rotate the plate 90 degrees.
Most measuring engines had a fully rotatable stage with a graduated
circle and one or two micrometer microscopes to measure how far the
stage was rotated. However, this design makes the measuring engine much
more complicated and expensive so Schlesinger figured out a way to get
around this problem by not having the stage move at all. He placed four
posts (5 at left) at 90 degree intervals around the stage. Four small
round dots were then punched into these posts and thus formed the
vertices of a square. The circular casting to which the posts are
mounted into is rotatable by a screw (9 on top picture or 3 on bottom
picture) which is tangent with respect to the carriage. Within this
casting, a second circular casting, also with a tangent-screw, is where
the plate is clamped. How is this then used? The line joining post 1
and post 3 (seen as a wound piece of string-2 on bottom picture) is made
parallel to the guiding way. The inner circle is then rotated using the
tangent-screw to get the plate in position to measure the new
coordinate. According to Schlesinger, "As compared with the usual
method, in which a graduated circle is employed, this device has the
disadvantage of not permitting the measurement of position angles. It
has the advantages of simplicity, cheapness, and, above all, greater
accuracy."
This measuring engine was digitized (10 on top picture) in 1967,
allowing the position of the carriage to be fed directly into a computer
for easier computation and analysis of the data.
Company Information
Gaertner Scientific Corporation
was founded in 1896 by William Gaertner, a
precision instrument maker who worked for scientists including A.A. Michealson
and A.H. Compton at the University of Chicago.
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