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ZL2VAL > SETI 01.01.05 12:50l 138 Lines 6735 Bytes #999 (0) @ WW
BID : C40596ZL2VAL
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Subj: New plans afoot for SETI
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From: ZL2VAL@ZL2AB.#46.NZL.OC
To : SETI@WW
*The New and Improved SETI*
*By Seth Shostak*
SETI Institute
posted: 30 December 2004
06:41 am ET
"So what's new with SETI?"
It's an easy question: a query that the media frequently pose, and for
obvious reasons. Of course, it would be nice to say, "well, we detected
three Type II civilizations last week, but they weren't especially
interesting," and sometimes I do this for effect. But of course it's not
true, and until it is, some people assume that there's "nothing" new
with SETI.
Wrong. Despite all the difficulties that beset it (mostly connected with
funding), SETI is currently experiencing a paroxysm of creative ferment.
The new year is sure to be memorable, as glossy new instruments come
on-line. Success in SETI depends on speed: how quickly can you check out
large expanses of celestial acreage? Well, SETI is about to seriously
crank up its speed, and metaphorically trade in chariots for jets.
At Harvard University, a survey telescope designed to sweep massive
swaths of the sky in a hunt for extraterrestrial laser flashes is
becoming a reality. In Puerto Rico, the famed Arecibo telescope is
getting a new feed that will speed up searches by seven times. And in
California, the SETI Institute and Berkeley's Radio Astronomy Lab will
soon be scanning the star-clotted realms of the inner Milky Way with the
first-stage implementation of the Allen Telescope Array (ATA).
The ATA, as readers who have props on their beanies know, will
eventually boast 350 antennas, each 20 feet in diameter. This impressive
antenna farm will be spread over about a half square-mile of terrain.
There are good reasons for building an array, as such a phalanx of
antennas is called, rather than a single, large dish. But those reasons
may not be obvious. Perhaps you think that, when it comes to hunting for
alien transmitters, all that counts is picking up enough radio photons
to find the signal. If so, then the sole index of merit for a SETI radio
telescope should be its size - the total acreage of its antennas. This
is akin to claiming that the only thing that counts in wrestling is
weight. As it happens, the ATA weighs in well: its collecting area will
eventually be comparable to the new Green Bank Telescope in West
Virginia, or the Very Large Array in New Mexico, both top-of-the-line
research instruments.
But while size matters, it's not the only item checked on the ATA's
feature list. Building an array offers at least three major advantages
over a single-dish antenna of the same collecting area:
1. Because the ATA's individual antennas are relatively small, the field
of view of the instrument is very wide (degrees across at some
wavelengths). Just about everyone has peered through cheap binoculars
having only a narrow field of view. They don't peer long.
2. As an array, the ATA can make maps of the sky. In other words, it's
like a radio camera, producing images. Now that's rather different than
a single-dish instrument, which can be likened to looking at the sky
through a three-foot long soda straw. With that kind of setup, you only
measure one small spot on the sky at a time. But if you really want a
picture of what you're looking at, you need more than a single spot
measurement.
OK, you discreetly protest, but making maps of the sky "isn't" what
SETI's about, is it? That's for the radio astronomers interested in
charting galaxies or nebulae. Well, as it turns out, the ability to
break up a large field of view into small (radio) pixels is also good
for the SETI crowd. Consider this: you're a radio astronomer, and your
day job is mapping stuff like the Andromeda galaxy. You want your radio
pixels to be in a regular, row-and-column matrix, like the members of a
marching band. It's a pixel arrangement similar to what your digital
camera's CCD has.
Fine. But for SETI purposes, you could spread the pixels around a bit,
like a few grains of sand thrown onto a black piece of paper. The idea
is to arrange those pixels to land on nearby stars - the very
neighborhoods you wish to search for alien-generated signals. So now
instead of having one soda straw to view the sky, you have a fist-full,
each carefully aimed at a likely stellar system.
Another benefit of this capability is that, since these radio pixels are
produced with computation, rather than being etched on silicon, you can
make "negative" pixels - small patches of sky where you "don't" pick up
any signals. That's useful not so much for blocking unsavory signals
from tasteless extraterrestrials, but rather for blocking the
interfering screech of our own, orbiting telecommunications satellites.
3. The mass of steel required to keep a big radio reflector from sagging
or collapsing (always a drag) increases rapidly with size. So consider
the following "gedanken" experiment: instead of building one large
antenna, you build two smaller ones, each with half the collecting area.
The acreage of each dish is now down by a factor of two, but the amount
of steel in each is down by a factor of nearly three. So the two dishes
together weigh less than the larger single one.
It's an exercise left to the student to extend this argument to yet
smaller dishes, but clearly the advantage of doing so accumulates. The
bottom line is the bottom line: it's less expensive to build lots of
small antennas than one big one, even when the total collecting area is
the same.
The ATA is enjoying its first growth spurt, and by spring will consist
of 33 antennas, equivalent in collecting area to a 100-foot antenna.
That's large enough to do serious science, and the array's first SETI
assignment will be to slowly scan the flat, inner sections of the Milky
Way. This is a test bed project certainly, but it's also a valuable SETI
experiment. And as the ATA continues to expand, so do its speed and
abilities.
No, it's not the discovery of yet another Type II civilization. But it's
a breathtaking step toward making such discoveries happen. So you see,
there "is" news.
-=###=-
73, Alan
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