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ZL2VAL > SETI 21.04.03 22:45l 109 Lines 5147 Bytes #-7332 (0) @ WW
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Subj: Arecibo Diaries: Narrowing the search
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Arecibo Diaries: Narrowing the Search
By Seth Shostak
SETI Institute
posted: 07:00 am ET
18 April 2003
In the early 1600s, small telescopes appeared as novelty items for sale
on the streets of France and Italy. They were simple, two-lens devices,
mostly useful for spying on the neighbors, but they caught the attention
of Galileo. It didn?t take this famous Venetian long to figure out how
they worked, and he was soon constructing telescopes for himself. By
1611, Galileo had built a yard-long, tubular instrument fronted by a
small, bubbly, one-inch lens. It boasted an unimpressive 20x
magnification. Peering through this toy-like device, he was able to see
the four large moons of Jupiter. This discovery changed our paradigm for
the solar system, and earned Galileo endless column inches in astronomy
textbooks.
For Galileo, setting up for observing was pretty straightforward: (1)
Take telescope outdoors, (2) position eyeball near the small end, and
(3) make groundbreaking finds.
Needless to say, for today?s high-precision research telescopes, setup
is more complicated. Arecibo is no exception. Unlike our radio
astronomer colleagues, we can?t count on signals that will be heard "all
over the dial." Such broad-band emissions, the type spewed into space by
natural cosmic broadcasters (e.g., quasars), are not much affected by
terrestrial interference ? the forest of signals that sprout from local
radar, GPS satellites, telecommunication birds, etc. Sure, these
appurtenances of modern society produce nasty static at a few
frequencies, but for most radio astronomy their effect is no greater
than the disturbance caused by an (admittedly unusual) convention of
flautists at the beach. The notes get lost in the wide-band roar of the
(quasar) surf.
For SETI, it?s different. We?re hunting for narrow-band signals ? the
very same type as the man-made interference that fills the airwaves.
This RFI (Radio Frequency Interference) can clearly frustrate our search.
To avoid this problem, we take a cue from Charles Messier, the 18th
century French astronomer who tried to help comet seekers by cataloging
all the potentially confusing fuzzy objects in the sky. On our first day
out, the Project Phoenix team points the telescope overhead, and locks
it down. We then do "RFI scans" by slowly stepping up the microwave dial
and noting all the narrow-band signals, and even some (such as GPS
broadcasts) that are a bit less narrow. These are cataloged into an
on-line database that can be used during the search to identify (and
quickly toss out) persistent terrestrial signals. During the course of
observations, additional "earth-bound" signals are found, and the
database grows. We don?t bother to try and identify these signals ?
whether they?re spy satellites or airport radars is of no concern. We
log ?em and leave ?em.
Another class of disturbing signals that could confuse our search system
are the internal "birdies" caused by the endless racks of electronic
equipment that bulge the walls of any radio observatory. These, too, are
narrow-band signals (they?re called "birdies" because if you listen to
them on a radio, they sound, well, like avians.) We catalog them to make
sure that these chip-based chirpers don?t get mistaken for
extraterrestrial transmitters.
Once we?ve done our electronic reconnaissance, it?s time for an
end-to-end test of the whole ball of wax. In previous runs of Project
Phoenix, this was accomplished by picking up the transmitter from the
Pioneer 10 spacecraft ? an "extraterrestrial" broadcaster that provided
us with a handy test signal. Unfortunately, this distant (and
thirty-year old) probe recently went radio silent. However, the SETI
League, in New Jersey, regularly bounces a 200-watt signal at 1,296 MHz
off the moon. They do this not for the benefit of the lunar radio
audience, but for radio amateurs (hams) here on Earth. Two nights ago,
we aimed the Arecibo dish moon-ward, and with a bit of fiddling and
adjustment, soon found a signal that had traveled a half-million miles
from New Jersey to Puerto Rico. It was a great and gratifying way to
show that the system is truly attuned to the sky.
Telescopes were the first instruments to extend human senses, something
they now do with unprecedented power. As night falls, we will turn
skyward the largest telescope ever built. The street vendor?s novelty
has grown up.
* Arecibo Diaries: Narrowing the Search
<http://www.space.com/searchforlife/shostak_arecibo_2_030418.html>
73 de Alan
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