The AIRS instrument required advancements in five technology areas in order to meet its scientific objectives. Advances in technology developed for the AIRS instrument included:
- low-noise infrared detectors
- low-vibration and long-lifetime cryocoolers
- optical filters
- Detector read-out electronics
- Aspheric diffraction gratings
Low-Noise Infrared Detectors
Each detector module had a unique set of requirements and constraints in terms of wavelength coverage, signal and background flux, and sensitivity. The optimal detector material for the AIRS wavelength range is Mercury-Cadmium-Telluride (HgCdTe or MCT). Lockheed Martin Infrared Imaging Systems (LMIRIS, now part of BAE Systems) developed the infrared detectors. Meeting the sensitivity requirements at the longer wavelengths was particularly difficult, and so two different kinds of detectors were made from this material, each optimized for the given wavelength range.
Infrared detectors are noisy when warm, so cryocoolers are used to lower the temperature of detectors to 58 K. The challenging cryocooler requirements which were all met included:
- long lifetime (now ~20 years and going strong)
- excellent temperature control (±0.01 C°; detectors are very temperature sensitive)
- good power efficiency (wasted power makes even more heat)
- low vibration output (shaking makes blurry images).
TRW in Redondo Beach (now part of Northrop-Grumman Space Technologies). The AIRS coolers were the first pulse tube coolers used in space, and the technology is still state-of-the-art.
Detector Read-Out Electronics
The AIRS focal surface has about 4,500 detector pixels, each of which must be ‘read out’ by the electronics. The electronics need to amplify the signal, compare the signal to a zero signal to remove drift, turn the signal into a digital number, and combine the pixel signals together for sending to the spacecraft for transmission. Several of these tasks need to be done closely to the detectors, where there is no space. LMIRIS undertook considerable development in the area of radiation tolerant read-out integrated circuit (ROIC) design and fabrication, and detector-to-readout interconnect in order to make high-performance, miniaturized ROICs.
Optical filters are located at the aperture slits and over the detectors. These filters are needed so that each detector pixel only receives single wavelength, rather than a confusing mixture. Since some wavelengths are very bright while others are not, the out-of-band rejection for the filters needed to be very good. The filters were made by Optical Coating Laboratory Inc (OCLI, now part of VIAVI/OSP).
Aspheric Diffraction Gratings
The AIRS optical design required a diffraction grating which is aspheric (slightly non-flat), in order to correct aberrations from other optics on the system. The AIRS grating was made by Diffraction Products Inc. The grating was made by depositing a thick layer of gold onto a substrate, and then grooves into the soft gold coating using a diamond. Very precise computer-controlled metrology allowed the groove-cutting machine to also make the surface aspheric by adjusting the height of the diamond.