Gaertner Single Screw Measuring Engine
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.
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.
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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