THE AIRES FAR INFRARED DETECTOR ARRAY

Edwin Erickson, Jessie Dotson, Jam Farhoomand, Christopher Mason

A unique, state-of-the-art array of detectors is being developed as part of AIRES, the Airborne Infrared Echelle Spectrometer for SOFIA (the Stratospheric Observatory for Infrared Astronomy). SOFIA is a Boeing 747 which will carry a 2.7-meter telescope to operating altitudes up to 45,000 feet. It is under development by NASA and the DLR (German aerospace center). SOFIA is scheduled to begin operations at Ames in late 2004. AIRES -- which is being built at Ames -- is the facility spectrometer for SOFIA. AIRES will measure atomic and molecular spectral lines at far-infrared wavelengths, roughly 30 to 400 times the wavelengths of visible light, to probe physical characteristics of astronomical sources such as star forming regions and our Galactic center.

Here we describe highlights of the AIRES detector development. Infrared light collected by the SOFIA telescope will be distributed by the optical system of AIRES to its semiconductor detectors so as to permit simultaneous separation of different wavelengths in each of 24 imaging picture elements (pixels) viewing the sky. The detectors will be arranged in a 16x24 rectangular grid with pixels spaced 8 hundredths of an inch apart. Each detector is a chip of antimony-doped germanium mounted in an integrating cavity and fed with light from the spectrometer by a conical light collector, shown in Figure 1.

The AIRES optical system and detector assembly will be cooled in a cryostat to a few degrees Kelvin, as required to achieve the highest possible sensitivity to the infrared radiation collected by the SOFIA telescope. The detectors convert the light from the spectrometer into electrical signals. These are amplified and multiplexed by adjacent integrated circuits that route the signals to the data system outside the cryostat.

These unique multiplexing amplifiers were designed specifically for AIRES by industrial specialists collaborating with AIRES team members. The device technology was originally developed for SIRTF (Space Infrared Telescope Facility), but the new circuits are tailored for the wider range of infrared backgrounds expected on SOFIA. They feature externally programmable gains to accommodate different observing conditions, and so will be suitable for a variety of SOFIA instruments as well as some spaceborne instruments. The AIRES team pioneered this development, and has tested several devices, confirming that their cryogenic noise and gain performance meets AIRES needs.

The entire detector package -- detectors, amplifiers, and array assembly -- is custom designed and built, with much of the work done at Ames by the AIRES team. In previous tests the detector configuration sketched in Figure 1 was shown to work well. During fiscal year 2000 , testing of the amplifiers and design and fabrication of a 2x24 protoflight detector module have made great progress. This unique detector system, essential for AIRES’ success, is well on its way to achieving its design performance.

Caption Figure 1: Detail of the AIRES far infrared detector geometry.