DIRECTIONS FOR SPACE-BASED LOW-FREQUENCY RADIO ASTRONOMY .2. TELESCOPES

Astronomical studies of celestial sources at low radio frequencies (0. 3 to 30 MHz) lag far behind the investigations of celestial sources at high radio frequencies. In a companion paper [Basart et al., this iss ue] we discussed the need for low-frequency investigations, and in thi s paper we discuss the telescopes required to make the observations. R adio telescopes for use in the low-frequency range can be built princi pally from ''off-the-shelf'' components. For relatively little cost fo r a space mission, great strides can be made in deploying arrays of an tennas and receivers in space that would produce data contributing sig nificantly to our understanding of galaxies and galactic nebulae. In t his paper we discuss an evolutionary sequence of telescopes, antenna s ystems, receivers, and (u, v) plane coverage. The telescopes are space -based because of the disruptive aspects of the Earth's ionosphere on low-frequency celestial signals traveling to the Earth's surface. Orbi ting antennas consisting of array elements deposited on a Kevlar ballo on have strong advantages of nearly identical multiple beams over 4 pi steradians and few mechanical aspects in deployment and operation. Th e relatively narrow beam width of these antennas can significantly hel p reduce the ''confusion'' problem. The evolutionary sequence of teles copes starts with an Earth-orbiting spectrometer to measure the low-fr equency radio environment in space, proceeds to a two-element interfer ometer, then to an orbiting array, and ends with a telescope on the lu nar farside. The sequence is in the order of increasing capability whi ch is also the order of increasing complexity and cost. All the missio ns can be accomplished with current technology.