GeoSTAR prototype instrument, built under NASA's IIP program to demonstrate synthetic aperture radiometry at 50-60 GHz for atmospheric temperature sounding.

Spectrometer Photometer Imaging Receiver (SPIRE), with spider-web bolometers, is on the ESA Herschel Space Observatory, due to launch in 2009.

SPIRE bolometer array wafer.

Imagers for ground-based, suborbital, and space flight observations will play important roles in future observations. Imaging in the far-IR is much less advanced than at shorter wavelengths due to the relative immaturity of the technology. The implementation of direct-detection imagers is largely dependent on recent and future advances in detector large-format array technology. The implementation of heterodyne detection imagers is largely dependent on local oscillator development as well as mixer/detector array technology.

SPIRE

An example of an imager that has been built and delivered is the Herschel SPIRE. 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 Principle Investigator Matt Griffin of Queen Mary and Westfield College in England. The Herschel Spectral and Photometric Imaging REceiver (SPIRE) is expected to launch with the Herschel Space observatory in 2009.

Spider Web Bolometers

In contrast to HIFI's heterodyne technique, SPIRE will detect 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.

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.

Advanced Technology Imagers

Imagers in various stages of development include the following:

• The Q/U Imaging ExperimenT (QUIET), which is being developed for ground-based observations of the Cosmic Microwave Background. A future lower power version of QUIET will be appropriate for a space-based observation. QUIET is a low-noise polarimetric array that can function in the 50-100 GHz region. Microwave circuit techniques are used to amplify and detect the very weak signals and also measure the polarization properties of the CMB. QUIET will produce unprecedented maps of the CMB polarization and its spatial power spectrum.
• GeoSTAR, a Synthetic Thinned-Aperture Radiometer that would have the capability of atmospheric temperature and humidity sounding from geosynchronous orbit with unprecedented spatial resolution. The STAR approach requires a large number of individual radiometers spaced along the arms of the thinned aperture structure, along with correlators that provide the necessary phase relationships as the structure rotates. New MMIC fabrication materials and technologies that result in lower-power radiometers are key to the GeoSTAR future.
• Submillimeter-wave imaging is being developed. Laboratory prototype imaging in the 500-700 GHz region, using heterodyne receiver configurations, has demonstrated very high dynamic range, which enables the capability to image through layers of strongly absorbing materials.