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| Above, a photo of the SIRTF instrument module with the four IRS instruments mounted. Below, a schematic diagram of the IRS instruments. All of the spacecraft's instruments are mounted near the lower end of the cylindrical telescope, directly above a liquid-helium tank used for cooling. Light from the telescope travels down the center axis of the cylinder and is deflected at a riight angle into each instrument. Each instrument is positioned at a slightly different point in the focal plane so that operators can choose which instrument is active by adjusting the pointing of the telescope. |
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The Space Infrared Telescope Facility (SIRTF), scheduled for launch Aug. 23, picks up where the hardware store leaves off. SIRTF's science payload, which consists of a Cornell-designed infrared spectrograph (IRS) and two imaging cameras, is sensitive to the redder-than-red shades of the infrared (IR) band. IR light, made up of waves just longer than those that our brains interpret as red, can't be detected by the human eye or with traditional optical telescopes.
While SIRTF's cameras will take sophisticated IR snapshots of distant galaxies, dust clouds and objects too cool to emit visible light, the IRS's job is to determine their precise infrared "colors."
It is a scientific, not aesthetic, pursuit and it isn't as simple as tracking down a matching paint chip.
A spectrograph breaks up incoming light into its component colors, or wavelengths.(its spectrum), by passing it through a very narrow slit and recording the intensity in each wavelength. Astronomers read peaks and valleys in the spectrum, called emission and absorption lines, like a chemical barcode to determine the unique chemical mix of the object being observed.
It is the sensitivity of the IRS, though, that sets it apart from its predecessors, says James Houck, the spectrograph's principal investigator and Cornell's Kenneth A. Wallace professor of astronomy. "Most of it comes out of having better detectors," notes Houck.
Indeed, the IRS's detectors are hundreds of times more sensitive than those aboard the first IR space telescope, the Infrared Astronomical Satellite (IRAS), launched in 1983.
Houck's involvement with the IRS spans two decades and multiple budgets and redesigns. The original IRS proposal, one of three selected by NASA for SIRTF in 1984, was for a portable spectrograph intended to make periodic trips into space aboard the space shuttle. "SIRTF' used to be the Shuttle Infrared Telescope Facility," Houck recalls.
Expected to have a five-year life, the current SIRTF spacecraft is no commuter. And because the observatory's Earth-trailing orbit puts it out of reach for repairs, reliability is critical. To this end, the IRS has no moving parts, and observers can only select from among its four shoebox-sized detectors by repointing the entire telescope. For versatility, two of the detectors record spectra in a finely drawn "palette" of 500 colors; the two other detectors operate in a coarser 60-color mode optimized for observing very faint objects.
The detectors are housed in a light-tight aluminum capsule, called the multiple instrument chamber, about the size of a bus tire. The capsule protects the detectors and parts of SIRTF's two cameras from their supporting electronics, which otherwise would leak enough IR noise to swamp the sensitive chips.
Despite its limited presence in orbit -- SIRTF is only the third civilian IR space mission -- IR astronomy is a vital complement to observation in the optical, radio and high-energy bands. That's because IR radiation breezes through the interstellar dust that can obscure optical light from astronomers' view.
From Earth, however, IR has an Achilles' heel: in all but a few frequency windows, it is absorbed by water vapor in Earth's atmosphere, meaning that many of the images and spectra that SIRTF will take from space would be unattainable if sought in terrestrial comfort.
Among the objects the IRS will turn its silicon eye toward will be the glow of the debris disks that surround young stars. Astronomers believe that planets form from these scraps of gas and rock, the leftovers of star formation.
The IR observatory also will study nascent stars. Hidden from the prying eyes of astronomers by clouds of dust, stars enjoy a sort of cosmic privacy for their births and, in some cases, deaths. Because IR light can penetrate this dust, the IRS will allow astronomers to witness, for the first time, events like the deaths of Wolf-Rayet stars, which spend their death throes spouting plumes of dust and gas.
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