From: william edmondson (w.h.edmondson_at_cs.bham.ac.uk)
Date: Wed Sep 29 2004 - 04:55:57 PDT
Hi again Pete
Re the use of the Hydrogen line. All is not as it seems.
I put that in the paper, sure, and as I understand it much SETI work
assumes the same thing - look for a signal at 1.42GHz......
But the technique is not limited to that, or even to RF. Optical SETI
would work fine using the pulsar/beacon paradigm.
However, think carefully about the 1.42. And this may really annoy
those involved in Project Argus... ;-(
Eric Gerard, the ex-director of the Nancay radio telescope in France,
has pointed out to me that searching at 1.42 does not actually make
sense if we assume ETI is interested in radio astronomy.
He wrote to me as follows:
"My first comment is that I am worried about the RFI that the ETI will
face and that we should also face if we start transmitting ourselves.
We of course content ourselves with passive listening to Habstars like
you
suggest in your Arecibo proposal but if all civilizations keep listening
and no one ever transmits....
I see that you plan to listen on 3 frequencies namely the HI line at
1420.405 MHz
and the 2 main line OH lines at 1665.40 and 1667.359 MHz.
As you know, those lines are fortunately protected from earth's
transmitters
since all transmissions are prohibited world wide in the band
1400.0-1427.0 MHz
(this is for HI) while the radioastronomy service has primary status in
the
band 1660.0-1670.0 MHz (this for the 2 OH main lines).
I must tell you about some experience that I have from the time
(1982-1989)
when I was IHW (Intenational Halley Watch) Discipline Scientist for
Europe.
We received a quite interesting proposal from John Ponsonby of Jodrell
Bank
(Univ.of Manchester), John had a very smart concept which consisted in
radiating some 50 kWatts at the main line of OH at 1667.359 MHz such as
to
quench the natural maser emission of comet Halley. Once the radar was
stopped,
the natural maser emission was to recover in about a half-hour and this
could
yield precious information as to how cometary masers work.
I defended John's concept in Washington DC during a meeting preparatory
to the
Halley worldwide campaign. ...and we lost. The IUCAF (International
Committee
for Radioastronomy Freqeuncy Allocations and Protections) refused to
grant the
right to transmit in a major protected band even for a limited amount
of time.
Since then, as you certainly are aware, The RFI situation has gotten
much
worse and radio freqeuncy protection is now a major concern in all radio
observatories worldwide.
If ETI know about pulsars, they certainly know about RFI and I am not
sure
that they don't reinforce themselves the protection of radio frequency
bands.
This is not to say that the concept of "utilizing pulsars as SETI
beacons" is
not valid but if they use 1420.405 MHz, 1665.40 and 1667.359 MHz, it
should be with utmost care or perhaps in a way such that they do not
cause
pollution or limited pollution.
And there may be several ways to do so: for instance on a time sharing
basis,
or with very clean transmitter sidelobes, or with special coding or
whatever."
So - we may be looking at the wrong frequencies. We'd have to think
about where to look instead, and one "obvious" (??) place would be the
'mid-points' of the H/OH pairings. 1542.9025 and 1543.882 MHz These
are nicely near each other, and bang in the middle of the water hole.
But maybe there are a million different notions of 'obvious' here.
Which is one reason why optical may be much easier (and pulsars are not
bright objects in the visible spectrum, so Schneider mode is
unproblematic).
Cheers
William
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