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
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
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 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'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
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.
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
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.