POSTED JUN 5, 2003
down galactic mysteries
Last month, scientists reported that gamma ray bursts -- ancient explosions that are the most powerful in the universe -- originate from regions with "highly structured magnetic fields." That's Greek to us Why filers, but it does suggest the source of the curious detonations that are detected on Earth about once a day.
Maybe the new data, courtesy of the RHESSI satellite, will help solve the big mystery: How was so much energy -- temporarily equal to a million trillion suns-- generated? (Hint: the collapse of giant stars probably plays a role. See "Big Blast..." in the bibliography).
To astronomers, the bursts follow an annoyingly random schedule. Within a few seconds, like any decent explosion, only cinders remain. And if you don't catch a burst bursting, it's hard to fathom what could cause such an explosion. But since they arrive at random, there's no way to point a telescope at them in advance...
In a press release, Steven Boggs, assistant professor of physics at the University of California at Berkeley, who helped write a May 22 paper in Nature on the discovery, said the strong polarization of the light measured by RHESSI offers a clue toward the origin of the bursts. Those structured magnetic fields are apparently stronger than those at the surface of neutron stars, which used to hold the title of Mr. Magnetic Universe. "The polarization is telling us that the magnetic fields themselves are acting as the dynamite, driving the explosive fireball we see as a gamma-ray burst," Boggs said.
It's an old routine. Chance may favor the prepared mind, but it definitely favors an astronomer equipped with a nifty new observing tool (RHESSI was launched in Feb. 2002). Galileo and the other inventors of the telescope almost 400 years ago started the routine: You build a new astronomy instrument, and you learn new things about the universe. Galileo, of course, first saw the moons of Jupiter using his Mark I scope.
And while space telescopes like RHESSI, Chandra and Hubble, have gotten their share of headlines, the technology of ground-based telescopes has been advancing rapidly. It's not just giant scopes like Keck, either.
These days, a category of telescope that was once considered the poor relation of the sexy, spacy telescopes is proving that the obituaries for Earth-based astronomy were, well, somewhat premature. Sure, the view from an orbital telescope is not obscured by pesky air. But Hubble and Co. are vastly more expensive than terrestrial instruments.
Whassup with telescopes downstairs?
In astronomy, little things add up. Even "simply" putting a better coating on a lens can measurably increase the scientific value of a telescope that has a dozen lenses.
In this Why File, we'll look at four advances in telescope technology, and show how they are improving our harvest of cosmic understanding:
Oriented to the goal
"There are thresholds in astronomy that you want to cross," says Matthew Bershady, a professor of astronomy at University of Wisconsin-Madison who takes an active interest in telescope design. "You always want to be above the detector noise." By that he means that you want errors to reflect the actual photons being collected, not noise due to balky or erroneous machinery.
As we'll see, much of the cutting edge of astronomy concerns spectrography -- the analysis of light waves to see (among other things) which chemicals are present in the light's source. Spectrography has been around for almost 100 years, and as it improves, scientists want to look more precisely at specific wavelengths.
That involves dividing the incoming light more finely. But just as a gambler who divvies his winnings among 10 backers gives less to each one than a poker player who has only two backers, high-class spectrography makes fewer photons available at each wavelength. "When you're at very high spectral resolution, you get very few photons per wavelengths from each position in the sky," says Bershady.
That, in turn, raises the chance that the results will be bogused by detector noise.
With an inefficient telescope or instrument, he says, "Eventually you start losing to the detector; the observation becomes less efficient." In such conditions, he says, taking more pictures ironically only makes matters worse. "If the dominant source of noise is the detector, you are worse off than if you only take a picture once. But if you are photon-limited, no matter how often you take snapshots, you will get the same result."
So how can we trim telescope noise?
©2003, University of Wisconsin, Board of Regents.