What is the IRS and how does it work with Spitzer?

The IRS instrument is a device which records a fingerprint of the frequencies (akin to colors) of infrared radiation of an object.

The Spitzer telescope is turned to point at an object under study, and concentrates its light onto one of the three scientific instruments aboard. Two of these instruments consist of assortments of cameras; the IRS instrument consists of four spectrographs which each record a fingerprint of radiation at a different level of detail and covering a different range of color/frequency.

Spitzer dedicates several days of observations (called a campaign) to each instrument in turn, so it can be powered up, calibrated, and stabilized. Some instruments function better when the telescope as a whole is maintained at a lower temperature. By consuming the limited vital refrigerant onboard only as needed and grouping these especially demanding observations together, the spacecraft lifetime is also extended.

Spectroscopy -- the recording of these fingerprints -- is much more difficult in the infrared than it is in standard optical light. At room temperature, all objects glow brightly in the infrared. Imagine, if you will, the difficulty of looking through a pair of binoculars that were heated to a thousand degrees and glowing, lenses and all, at a bright cherry red -- at some faint distant object.

As a result, both the Spitzer telescope and IRS instrument are chilled by liquid helium from room temperature to only a few degrees above absolute zero, which is the temperature at which all atomic motion is minimized. Absolute zero is -451 degrees on the Fahrenheit scale!

At this frigid temperature, both the telescope and IRS instrument appear dark in infrared light and do not contaminate the faint light from some distant object. Since this is only the second infrared telescope cooled with liquid helium -- and the first infrared telescope with a spectrograph cooled this way, the Spitzer/IRS combination has unprecedented sensitivity. Not only can the Spitzer/IRS combination detect on its own objects fainter than any preceeding infrared telescope, it can actually divide their radiation into dozens of different colors, recording the spectrum. At some colors, it is a thousand times more sensitive than any preceeding astronomical instrument.

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-- DonBarry - 27 Sep 2004

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