[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 Three-inch sighting telescope mounted on top has since been replaced by six-inch telescope._] 4.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._]