Scoping out the new telescopes
POSTED JUN 5, 2003

1. Terrific telescopes

2. Great gratings

3. A sharper image





An artist's idea of a lucky view of a gamma-ray burst. While the RHESSI spacecraft (foreground) was watching giant explosions near the Sun, a gamma-ray burst (white sphere and ring), flared far away in the universe. The highly polarized light from the burst means it probably came from a region with intense magnetic fields. Would that explain these awesome pops, the most powerful in the universe? From GSFC.





The whole point of telescope design is to gather more particles of light and analyze them more precisely.





The dumbbell is a planetary nebula that was formed when a red giant star ejected its outer envelope near the end of its life. The expanding cloud of gas became visible when the hot core of the star (center) was exposed and the high-energy, ultraviolet light from the core ionized the cloud. This image combines three pictures, taken through narrow spectrographic filters that showed particular species of atoms. Photo: George Jacoby, WIYN/NSF, courtesy National Optical Astronomy Observatories.







In astronomy, little things add up. A better lens coating can increase  the value of a telescope with a dozen lenses.

  Nailing 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).

yellow and grey satellite in foreground, with sun in middleground, and white ringed white light in background

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...

The recent observation was a lucky break for a satellite that was ogling flares on the surface of the sun.

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.

yellow cloudy ball, ringed in magenta in starry black space

Whassup with telescopes downstairs?
Here on Earth, bigger telescopes are being built at better sites. Huge scopes are being built for a few percent of what a big space telescope would cost. Instruments are being used to analyze photons trapped by the new scopes to a higher accuracy. Better mirrors, better lenses, and better computers are spitting out data by the pallet.

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:

Diffraction gratings
- better filters to deconstruct light into its elementary wavelengths.

Innovative telescope design
- a stationary mirror that cuts costs by 80 percent.

Active optics
- cheaper telescope mirrors that retain their shape.

Adaptive optics
- telescopes that compensate for atmospheric turbulence.

Oriented to the goal
The whole point of telescope design is to gather lots of photons -- particles of light -- from an interesting part of the universe, and then to precisely analyze them. That may sound easy, but it's tough, especially if you are on a budget. And as the scientific requirements intensify -- the simple astronomical riddles have already been solved -- the demands on telescopes grow all the more intense.

A boxy dome telescope sits high over a mountain sunset.The WIYN telescope, perched on an Arizona mountaintop. Photo: University of Wisconsin-Madison

"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?


The Why Files

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