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SPIRE Instrument

As the name implies, the Spectral and Photometric Imaging Receiver (SPIRE) has two main components: a low-to-medium resolution spectrometer and a photometer. It is being developed by a consortium of European and American scientists and engineers, led by Principal Investigator Matt Griffin of Queen Mary and Westfield College in England.

Spider Web Bolometers Spider Web Bolometer

In contrast to HIFI's heterodyne technique, SPIRE detects photons directly, by means of five arrays of bolometers.

Bolometers can detect very small amounts of energy and convert them to electrical signals. They are currently the most sensitive direct detectors for light in the far-infrared to millimeter range.

The photometer uses three of those arrays, creating images via 326 bolometers that observe wavelengths centered at 250, 350, and 500 microns. Since each array has a bandwidth of 33%, SPIRE's photometer effectively covers the entire range from about 208 to 583 microns.

Spider Web Bolometer SPIRE's bolometers are of a "spider web" design developed by Co-Investigator Dr. James Bock of JPL. Each consists of a weblike mesh of silicon nitride, which absorbs light and conducts the energy to the tiny thermistor that sits at the center of the web. The thermistor is made of NTD (neutron transmutation doped) germanium, a substance manufactured in a nuclear reactor. It takes about 100 photons in the far-infrared/submillimeter range to heat it up enough to generate an electrical signal.

The bolometer's weblike structure, which gives it about 30 times less mass than previous designs, has several advantages.
  • It significantly reduces the bolometer's heat capacity (which means that it takes less energy to register a change in temperature) compared to bolometers with solid-sheet absorbers. This gives SPIRE a high mapping speed (that is, it can gather information quickly and move on to the next target instead of needing to stare at individual objects for long periods of time to allow more photons to strike it).

  • It reduces interactions with cosmic rays, since there is less area for the particles to hit.

  • It minimizes its "microphonic response," the tendency to convert vibrations from equipment movement into electronic noise.

SPIRE Bolomter Array

SPIRE's bolometers are very effectively isolated from the heat of surrounding equipment. This low thermal conductivity gives the device high sensitivity, with a minimum of confusing noise to interfere with detecting very weak signals.

This type of bolometer, which is also used by Planck's High Frequency Imager, has been used successfully in many experiments - most notably the BOOMERANG investigation of the Cosmic Microwave Background.

SPIRE's bolometers are stable to time scales of up to 30 seconds, which allows Herschel to pan the telescope across the sky instead of beam-switching like PACS.

Tiny "feedhorns" attached to the bolometers funnel light to the detectors.

Cold electronics

These bolometers operate at temperatures very close to 0.3 Kelvin (0.3 degrees above absolute zero, equivalent to -273 C), and their signals are very small and difficult to read. Conventional electronics don't operate well at such low temperatures, so cold electronics have been designed to boost the signal. These particular cold electronics have never before flown in space.

The system includes Junction Field Effect Transistor (JFET) amplifiers, which operate at a temperature of 130 K but sit on membranes which isolate them so effectively that the environment only 1/4 of an inch away remains at 10 K. In addition, the equipment at 10 K is itself thermally isolated from nearby components at 0.3 K.

Fourier Transform Spectrometer
SPIRE's imaging Fourier Transform Spectrometer (FTS) has greater speed, resolution, and sensitivity than a conventional absorption spectrometer. "Fourier transform" refers to a mathematical method for converting a signal value that is a function of time, space, or both into a function of frequency. This type of spectrometer produces an interferogram from the incoming broadband radiation (a function of signal intensity vs. time), which is then digitized and converted to an absorption spectrum by means of a Fourier transform.

The FTS observes the range of wavelengths from 200 to 670 microns. It can perform spectroscopy over the entire band simultaneously, providing a good measure of line ratios which are needed to determine physical conditions and abundances. And since it is imaging, it enables an observer to map the total intensities of many lines in a star-forming region at one time, and study how these lines vary with respect to the other physical conditions of the ISM.

SPIRE's main objectives are to investigate very distant galaxies and to study the earliest stages of star formation, when the protostar is still enveloped by an interstellar cloud of gas and dust.

In particular, SPIRE has been investigating the formation of spheroids and elliptical galaxies at high redshift, the details of whose evolution would have a significant implication for the star forming history of the universe.

The instrument has also been used to study the formation and early evolution of AGNs and quasars, to determine how massive black holes were formed in very distant galaxies, and to study large-scale structure in the early universe.

And, together with PACS, it has observed faint objects in our own solar system.

For more on SPIRE, visit the Herschel Space Observatory website at IPAC.
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