Now we had finally found a satisfactory way to reflect a train of tiny
flashes—much too faint to be seen by the naked eye—from Telstar as it
passed across the sky during the night. But our main aim was to record
the exact times when these flash bursts occurred. With this information,
we could, using the method we described above, tell very accurately both
the satellite’s spin axis and its rate of spin. We do not have space to
describe the many problems that had to be solved in setting up the
equipment to record the flashes. Let us merely outline the procedure
that we finally devised:
1. _To pick up the satellite’s flashes we use a 12-inch-aperture
photoelectric telescope mounted on a radar trailer (shown in
illustration below). It is pointed by means of prediction drive tapes
produced by an electronic computer; these are based on data from
previous passes._
2. _On clear, dark nights when the satellite is at relatively short
range, we can see it with an auxiliary finder telescope, and then adjust
the large telescope precisely. Or, if the satellite’s high-frequency
beacon has been turned on, the Holmdel microwave antenna can
automatically point our large telescope._
3. _When flashes of light are picked up by the telescope, they fall
directly onto the cathode of a photomultiplier tube. They are then
filtered out from the random light in the night sky and amplified._
[Illustration: _Twelve-inch telescope and electronics box mounted on
a radar antenna pedestal at Crawford’s Hill. Three-inch sighting
telescope mounted on top has since been replaced by six-inch
telescope._]
4. _Rather than make a continuous recording of the output—one night this
would have produced a record twelve miles long for us to pore over—we
use an electronic trigger. This is the time base of an oscilloscope,
whose sawtooth output is set in operation only if a signal of four volts
or more is received (photo below)._
5. _A pen recorder makes a continuous line on a revolving drum, with a
heated stylus connected to a galvanometer marking a permanent record on
heat-sensitive paper. Any signal output from the oscilloscope is picked
up by the galvanometer and causes the pen to make a sawtoothed mark;
when the paper is unrolled from the drum, these marks are clearly
visible as notches in a series of otherwise straight lines._
6. _A synchronous timer marks the chart every ten seconds, and we are
able to time individual pulses with a precision of one tenth of a
second. Because the beginning and end of a train of pulses are not
always distinct, we can only determine the center of a burst of
flashes—which we use as our most important time indication—to within two
seconds. However, this is accurate enough, for a change of only one
degree in the orientation of the satellite’s spin axis would change the
time of the flash burst center by about half a minute (see below)._
7. _We use a second oscilloscope to check on whether the signals we
receive are genuine flashes or just accidental stray light. This
oscilloscope has a long-persistence screen, which we use as a temporary
memory. The pulses traced on its cathode ray tube are automatically
photographed by a 35-mm camera while they persist on the screen. We can
then examine the photograph to see if the pulses are genuine, which we
ascertain from (a) their shape and size and (b) the intervals between
successive pulses. Looking at the photographic record also confirms
whether we are observing flashes from the 68° plane mirror or the 95°
faceted mirrors. We can calculate the satellite’s spin rate by measuring
the intervals between individual flashes._
[Illustration: _General view of amplifying, monitoring, and
recording gear that picks up glints of sunlight at the Crawford’s
Hill observation station._]
[Illustration: _Enlarged portion of a typical pen record of flashes
of sunlight from Telstar mirrors, showing a burst of 21 glints from
the 68° mirror recorded at 03:40:58 Greenwich Mean Time on August 9,
1962. Synchronizing vibration mark seven lines above the recorded
burst indicates the time 02:59:00. Measuring the horizontal distance
between consecutive sawtooth marks tells us that the spin rate is
between 163 and 164 revolutions per minute. (Precise measurements of
the oscilloscope traces fixed the exact spin rate at the time of
this burst at 163.64 revolutions per minute.)_]