The comparator was built in order to make very precise measurements on very large photographic plates or rolls of film. In particular, it was used by NRAO for many years for finding coordinates of objects on the Palomar Sky Survey plates for possible optical matches to what the radio astronomers had found in the same regions of the sky. It was also used by astronomers at McCormick Observatory for producing finding charts of areas of the sky that they were interested in studying. An accuracy of 1 micrometer could be reached with the instrument, and plates as large as 14 inches by 17 inches in the X and Y directions could be measured. This measurement was done by moving the photographic plate beneath a microscope along accurately scraped ways. By reading the dial on the X and Y direction screws, one could measure very accurately the relative positions of objects on the plates. David Mann, in 1957, manufactured the Type 621 Mann Comparator. Incorporated and used for 20 years by the Central Intelligence Agency of the United States of America. During this time, in the heat of the Cold War, the first space photographic reconnaissance satellites were launched in order to photograph areas of the Soviet block countries and confirm their developments in strategic missile capabilities.

Photographs of 6-10 foot resolution were routinely taken of atomic weapon storage facilities, weapon test sites, missile launching sites, and command sites so that by the early 1970s, the United States had amassed a record of 800,000 photographs which gave them thorough information about Soviet capabilities. These photographs were then measured on machines similar to the Mann Comparator. The National Radio Astronomy Observatory obtained the comparator from government surplus in 1980 and brought it to their office in Charlottesville. The X- and Y-coordinate stages can be moved by use of handles attached to the two precision screws for small motions, or for larger motions the stage can be disengaged from the screw and thus moved freely. In order to disengage the stage from the precision screws, the disengaging handle must simply be turned one half turn towards the label FREE. To re-engage the stage with the screws, the stage must be slid until the pointer on the dial indicator flicks up to 0 and then falls back. As the pointer comes to 0, the disengaging handle should be placed in the engaged position, and then it will be re-engaged with the screws. In adding up 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 degrees using the handwheel on the main stage.

The position of this stage can then be read to a precision of 20 by the operator using an eyepiece and illuminated vernier scale. There is a microscope pointed at the observer at an angle of 22 degrees from the horizontal for easy viewing, and it is a penta prism which is mounted at the back of the X-coordinate stage which allows the microscope to view the plate vertically rather than at this angle. The optical system has a range of about 13-15 power, with the range due to the focus adjustment mechanism. There are reticle lines in the microscope, and these are conveniently parallel to the ways of the machine, although they may be rotated by a small amount if conditions warrant it. Lastly, the microscope was installed with a bakelite focus knob which allows the observer to alter the focal length of the microscope. At the end of each precision screw, there is a hand crank and dial system for reading the number of turns that the screw has made. The hand crank to move the plate carraige in the X-direction is on the right side of the machine, and the one for the Y-direction is on the left/front side of the machine. A small window exists for viewing the readings, and both X and Y dials can be rotated slightly if the operator wants to start off from some initial setting. Measurements must always be made with the stages moving in a single direction-toward the dials-such that successive motions result in larger and larger dial readings. This necessity is due to backlash between the precision screws and the stages, so that moving the stage backward a certain number of turns is not quite the same distance as moving the stage forward a certain number of turns. The system is calibrated only to be used in the forward direction, and so if it is used backward, the dial will read the wrong value. The Mann Comparator, equipped with an aluminum plate holder which can be used with up to 14 by 17 glass plates. The photographic plate is mounted against two clamps toward the rear of the machine, and then allowed to settle on the other two clamps toward the front of the machine. The front two clamps are then tightened so that the plate stays in place. The measuring engine is also equipped so that film rolls can be examined there. Thus a special roll film stage can be mounted instead of the plate holder.