 Type 621 Mann Comparator
General Information
The Type 621 Mann Comparator was manufactured in 1957 by David Mann,
Incorporated (Lincoln, Massachusetts), and used for 20 years by the
US Central Intelligence Agency. 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 an inventory of 800,000 photographs which
gave them detailed 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 headquarters in Charlottesville.
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 (µ) could be reached with the instrument, and plates
as large as 14 × 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.
Operation
The X- and Y-coordinate stages can be moved by use of handles (1, 2)
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 (3) 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 reset to the engaged position, re-engaging the stage 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 handwheel (4) 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 (5).
There is a microscope (6) pointed at the observer at an angle of 22°
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×, 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. Finally, the microscope was installed with a bakelite
focus knob which allows the observer to change 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 (2) is on the
right side of the machine, and the one for the Y-direction (1) is on the
left/front side of the machine. A small window (7) 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 is equipped with an aluminum plate holder (8) which
can be used with up to 14 × 17 inch 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. For this to happen, a special roll film
stage can be mounted instead of the plate holder.
Alignment
In order to make precise measurements of stellar positions on
photographic plates, one needs to make certain that the plate is
perfectly aligned on the measuring machine, such that moving the machine
in the X-direction also moves the plate in this direction and only in
this direction. To align the plate on the machine, several steps must
be followed. First, one must disengage the stage handles so that the
stage can move freely. Second, the microscope must be set on a fiducial
marking or other point to be aligned with the ways of the measuring
engine. Next, bring any pair of points of interest on the plate
parallel to the ways by using the free-sliding motion of the stage and
the rotation adjustment of the rotary table. Finally, re-engage the
stage to the precision screws. The plate should be aligned perfectly.
Electrical
There is an optical encoder and stepping motor on each of the X- and
Y-axes. Each motor is attached to a precision screw, but the motor
disengages when the motor is not driving so that the operator can make
fine adjustments to the position by using the handwheels. NRAO later
installed a small control panel (9) on the front of the engine and then
interfaced the whole system-control panel, stepping motors, and
encoders to a microcomputer, which was then connected to a VAX (a line
of minicomputers produced by Digital Equipment Corporation
popular in the 1980s), so that the motions made on the measuring engine
were sent straight to the VAX for reduction and analysis.
NRAO also replaced the microscope eyepiece on the machine with a
G.E. CID solid state television camera (10), which displayed 244 rows of
248 pixels each for a total field of view of 2.4mm × 2.0mm (or
2.7' × 2.2'). Viewing plates on the television screen was easier
on the observer's eyes than looking into a microscope, particularly for
long measuring sessions.
Company Information
This measuring engine was manufactured by the David Mann Company.
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