 Brashear Blink Comparator
General Information
The Brashear Blink Comparator was designed and built by The Brashear
Company, and loaned to McCormick Observatory by Peter Van de
Kamp at Sproul Observatory (Swarthmore College) in 1949. It was
used by observers at University of Virginia during the 1950's.
The machine is really a microscope which optically superimposes two
photographic plates by "blinking" between them so quickly that the two
plates look like only one. The astronomer can then compare the two
plates and look for differences between them. The viewed image quickly
switches between the two plates so that the background stars, which are
fixed in their sky positions, stay in one place and thus don't appear to
move. However, sometimes there will be an object which does appear to
move or change in brightness, and those are the objects of interest to
astronomers. At the time when this comparator was used, astronomers at
McCormick Observatory would have used it to detect double stars,
meteors, comets, variable stars, parallax movement, and proper motions.
Operation
The comparator is arranged so that one plate can be mounted on the left
side of the machine and a second plate can be mounted on the right side
of the machine. The plates are mounted on a white translucent plate
carriage (1), which is illuminated from behind by two separate light
sources. The plates are held in position by four clips (2) on each
carriage, and they are brought into the same plane of view with respect
to the microscope.
It is of great importance for the two plates to be brought into exact
alignment with each other in the microscope, and so there are several
devices to help in this endeavor. The left plate carriage has two knobs
for moving the plate in the X- and Y-directions. The X-motion knob is
on the left side of the carriage (4) and the Y-motion knob is on the
bottom of the carriage (3). The right plate carriage has only a
handwheel (5) which allows the plate holder to move 360° in a
circle, so if two plates are off by an angle rather than a distance, the
astronomer could turn one of the plates that angle and have them
aligned. Two plates were aligned in the following way: First, the right
side plate was set in the clips in its place. Second, the left side
plate was set in its clips and then moved using the X- and Y-handwheels
to get it exactly aligned with the right side plate. If there ends up
being a small angular difference between the two plates, then the rotary
stage can be turned that angle to get them aligned.
The microscope apparatus (6) as a whole can be moved vertically using
the knob at the bottom center of the engine (7, or 2 on the closeup
below) and horizontally using a double-knob to the right of the vertical
motion knob (8). The horizontal motion knob is divided into two parts.
The larger wheel allows for large motions (for instance, between
different parts of a plate) and the smaller wheel is for small motions
(within a very small section of a plate).
An eyepiece (9) sits in front of each of the plate carriages, and the
two eyepieces are arranged so that light coming into each is directed
toward the central microscope (6, or 1 on the closeup below). In back
of the microscope, a mirror takes the light from one (and only one) of
the eyepieces to the microscope, where a measurer's eye would be
located. This mirror is hinged so that it can flip back and forth
between the right and left plate carriages, so that alternately the
microscope views the right and left plates which are mounted there.
Earlier comparators had knobs or switches which flipped this mirror so
that the astronomer could blink between the two plates manually, but
this machine is equipped with an electronic blinker so that the
astronomer could switch between plates at several different rates
without actually doing the switching himself. This was a great
convenience, since it freed the astronomer's hands for recording data or
moving the plates on the measuring engine. Additionally, the microscope
contains a horizontal reticle for the purpose of precisely determining
the positions of objects on the two plates being compared. This reticle
can be moved slightly in the vertical direction using a knob (4 on the
closeup) on the right side of the microscope. The microscope is
focussed using the knob (3 on the closeup) above it.
The control panel (10) at the lower left of the machine allows for the
automatic control of both left and right illumination lamp intensities
and the speed of blinking between the two plates. The lowest blinking
speed is about 2 blinks per second, and the fastest is many blinks each
second. An astronomer would normally choose to use a blinking rate of
3-5 blinks per second so that he could see clearly what had moved
between the two images. The control panel also allows the observer to
hold still on one of the plates rather than blinking between them, for
more careful measurement or observation of an object.
Company Information
John A. Brashear was a self-taught telescope maker who established his
own telescope manufacturing business in 1881. His most famous
contribution to the industry was a reliable and easy technique for
silvering glass surfaces, which was widely used by makers of telescope
mirrors until 1933 when vacuum aluminizing became feasible. Although he
was only educated through the eighth grade, Brashear was elected as
vice-president of the Association for the Advancement of Science,
director of the Allegheny Observatory, chancellor of the University of
Pittsburgh, president of the American Society of Mechanical Engineers,
and founding director of Carnegie Institute of Technology (now known as
Carnegie-Mellon University). He was also a great humanitarian, aiding
in almost every charity in Pittsburgh.
The Brashear Company was
established in 1881 by John A. Brashear and manufactured some of the
largest telescopes in the world during the late 1800's. After the
second World War, the company moved into instrumentation, engineering,
and manufacturing of optics, mechanics, and electronics.
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