Calibrating the Moon

In the early 1600’s, 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.

Arecibo Observatory
The Arecibo radio telescope is currently the largest single-dish telescope in the world used in radio astronomy. In 1974, Arecibo was used to broadcast a message from Earth to the globular star cluster M13.
Credit: NAIC – Arecibo Observatory, David Parker / Science Photo Library

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.

Messier’s Mess

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 theyre 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.

Moon-bounced signal

Once we’ve done our electronic reconnaissance, its time for an end-to-end test of the whole ball of wax.

Earth and Moon
Image of the Earth and Moon taken by Galileo spacecraft.
Credit: NASA

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 vendors novelty has grown up.


Related Web Pages

SETI Institute
Arecibo