A Pseudo-interferometer
So, we now routinely use two telescopes, widely
separated, in what we call a “two-element pseudo-interferometer.” It is a
pseudo-interferometer because we never actually bring the signals together and
change-delay the phase of one and multiply them together. But it does work like
an interferometer and it works very well.
We were in Australia in 1995 and spent a year and
a half at Green Bank, West Virginia. We are now at Arecibo, our second
telescope being in the United Kingdom, the Lowell telescope in Jodrell Bank.
At Arecibo we observe within plus or minus 6
hours of local midnight. We do this not because the sun is a strong radio
source, but because of the solar wind. The charged particles leaving the sun
can actually destroy the coherence of a narrow signal. So we are observing at
night and every day we detect an extraterrestrial signal.
We have detected the transmitter on board the
Pioneer 10 Spacecraft, which has now left the solar system. It moves on the sky
like a star. It has a transmitter of a few watts broadcasting essentially 6
billion miles away and we detect it very well.
What have we learned to date? For about 400 stars
at frequencies between 1.2 and 3 gigahertz, we have not found any transmitters
with at least 1012 watts of effective isotropic radiated power (that
is equivalent to a strong terrestrial radar). Our observing is very efficient
and we are continuing with the next 600 stars.
Contributed by: Dr. Jill Tarter and Jim Miller
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