Instruments on SOFIA will be able to operate with the spectral resolution of 1 km s−1 needed for dynamical studies of galactic sources in wavelengths inaccessible from the ground. SOFIA will make spectroscopic observations in the large variety of molecular and atomic transitions that characterize far-infrared and submillimeter wavelengths. For bright sources, SOFIA 's maps will contain nine times the spatial information of maps made by SIRTF, although SIRTF will be much more sensitive for broadband observations. This information will be extremely useful in probing the physical conditions in protostars, planetary debris disks, and galaxies. Finally, SOFIA will provide excellent training for young experimentalists and a valuable opportunity to develop and test new instruments.

Other projects operating in the infrared and submillimeter region are discussed in the Working Papers (NRC, 1991) and include an Explorer mission to make a spectral survey of important classes of objects at submillimeter wavelengths; a ground-based survey of the entire sky at 1 to 2 µm with a threshold 50,000 times fainter than that of the only other survey, which is now 20 years old; allocation of funds to equip ground-based telescopes with revolutionary new infrared arrays; and a radio telescope using arrays of receivers to look for anisotropies in the cosmic background radiation that might provide clues to when and how galaxies formed.

HIGH SPATIAL RESOLUTION

Basic physical principles limit the smallest angle that a telescope can discern to a value approximately equal to the wavelength of the radiation observed divided by the characteristic size of the telescope. At visible wavelengths this limit is a few hundredths of an arcsecond for a 5-m telescope, although this limit has not been achieved until very recently, and then only under special circumstances, the limiting factor in practice being the degradation by turbulence in the earth 's atmosphere. The highest quality of astronomical images at the best ground-based sites on the rarest, most stable nights is about 0.3 arcseconds. At radio wavelengths, the limiting resolution of the largest single antennas varies with wavelength from about five to a few hundred arcseconds. Radio astronomers have developed the technique of interferometry in which small telescopes spread over large distances are linked together to simulate a single telescope with an aperture equal to the largest separation between the component telescopes. The Very Large Array (VLA) consists of 27 telescopes separated by up to 35 km and is capable of resolving structures about 0.3 arcsecond in size. Very long baseline interferometry uses telescopes distributed over the entire earth to resolve objects smaller than a thousandth of an arcsecond.

Six of the programs recommended in Chapter 1 stress 10-fold or greater improvements in spatial resolution compared with that possible with existing facilities. At visible and infrared wavelengths new technologies may reduce the



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