the Space Infrared Telescope Facility (SIRTF), consisting of a 0.9-m telescope cooled by a five-year supply of liquid helium and mounted on a free-flying spacecraft. As proposed, SIRTF will be launched by a Titan IV-Centaur into a high earth orbit with an altitude of 100,000 km. It will operate at high efficiency, with tens of hours of uninterrupted coverage possible on a single area of sky. This initiative will unite two proven technologies to make a national observatory of unprecedented power. First, the technology for cooled telescopes has been demonstrated by two Explorer satellites, IRAS and the Cosmic Background Explorer (COBE). The thermal background for SIRTF will be a million times less than that for a terrestrial telescope. Second, SIRTF will take full advantage of the U.S.-led revolution in infrared detector arrays. IRAS had only 62 detectors; SIRTF will have over 100,000. Figure 4.1b shows SIRTF's expected sensitivity compared to the brightness expected for representative extragalactic objects. Figure 4.2 compares SIRTF with the European Infrared Space Observatory (ISO) mission, to be launched in 1994. Because of its larger aperture and its use of larger and more sensitive detector arrays, SIRTF will be thousands of times more capable than ISO. SIRTF will follow up on ISO's discoveries in addition to breaking new ground with its own deep surveys. All key technologies have been successfully demonstrated, either on the ground or with precursor space missions, during a decade of study by NASA. SIRTF could be initiated in 1994 and launched around 2000 to provide valuable overlap with the Hubble Space Telescope (HST) and the Advanced X-ray Astrophysics Facility (AXAF).

The committee's highest priority for ground-based astronomy is an 8-m-diameter telescope for the summit of Mauna Kea, Hawaii, optimized for low-background, diffraction-limited operation in the infrared between 2 and 10 µm but also useful in the optical regions of the spectrum. Mauna Kea is recognized as the best terrestrial site for an infrared telescope because of its low level of water vapor, the primary absorber and emitter of infrared radiation in the earth's atmosphere. Further, the remarkable stability of the atmosphere above this mountain results in minimal distortion of astronomical images. From this dry and stable site, an 8-m telescope would achieve diffraction-limited resolution of 0.1 arcsecond at 2 µm using modest “adaptive optics” techniques to correct for residual atmospheric distortion. The infrared-optimized 8-m telescope will gain its marked increases in sensitivity relative to the sensitivity of other large telescopes owing to the 0.1-arcsecond images possible in the infrared with adaptive optics, and to the low emissivity expected for a silver-coated monolithic mirror. The telescope would provide large amounts of data from new infrared detectors on a large telescope at high spatial resolution. Among the many projects the infrared-optimized 8-m telescope would carry out, two for which it would be particularly well suited are deep searches for and spectroscopic studies of primeval galaxies (Plate 4.2).

The proposed national infrared-optimized 8-m telescope will differ from



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