Annu. Rev. Astron. Astrophys. 1991. 29: 499-541 Copyright © 1991 by Annual Reviews. All rights reserved

4.3 Future Developments

The technical advances of the past decade have enormously increased the efficiency of conducting spectroscopic observations of galaxies. While detector and spectrometer technology will likely maintain its progressive pace, the construction of numerous large-aperture optical telescopes equipped with multiple-object spectrographs promises enormous growth. The emphasis in radio astronomy will be on large centimeter-wavelength apertures with good interference rejection over wide bandwidths. Here we discuss a few specific major projects foreseen for the coming decade.

OPTICAL SPECTROSCOPY SURVEY TELESCOPES Two important projects that have the potential for revolutionizing redshift survey work have been defined in great detail. Gunn (1990b) proposes the construction of a Ritchey-Chretien 2.5-m telescope with a large (25% blockage) secondary, a 3°-wide field, and an f-ratio optimized for multifiber spectroscopy and CCD imaging surveys. Because up to b_J = 19, the sky density is approximately 85 galaxies per square degree, the telescope should be able to acquire about 500 galaxies per field, which would be accessed by two spectrographs, each fed from fibers from half the field. To that magnitude limit and at a resolving power of approximately 1000, an adequate signal-to-noise ratio could be obtained with an exposure of about one hour. A survey of about 1800 fields - 10,300 square degrees - should yield 870,000 spectra within four years. A comparably ambitious photometric CCD transit survey would be even faster than the Spectroscopic survey, and both survey tasks could be completed in just over five years.

The Spectroscopic Survey Telescope, proposed by a consortium that includes Pennsylvania State University and the University of Texas adopts a spherical primary, as in the Arecibo antenna, except that the optical reflector will be mounted at a 30° angle to the vertical, on a platform that can rotate in azimuth. Located at the McDonald site in West Texas, the telescope will cover declinations between -5° and +67°. At a given configuration, the field of view of the primary will be 12°, which will be accessed by two independent Gregorian subreflectors, each producing a 2' field of view that can be independently pointed. The main mirror will consist of 85 one-meter spherically figured segments, arranged in a circular frame 10 m in diameter. The light-gathering power of the telescope will be equivalent to that of an 8.5-m diameter single mirror. At low to moderate resolution (R ~ 500-100) a spectrum of a twentieth magnitude quasar could be acquired in about 10 minutes (Ramsey et al 1988).

RADIO TELESCOPES Two new projects will have an important impact on 21-cm spectroscopy: the upgrade of the Arecibo antenna and the construction of the new Green Bank telescope (GBT).

In the Arecibo upgrade, which will start in early 1991, spherical aberration correction by the present lossy line feeds will be replaced with a Gregorian subreflector assembly, and a noise-Suppressing ground screen will be added around the rim of the primary (von Hoerner 1989). These improvements will increase the absolute gain of the antenna, nearly eliminate vignetting at high zenith angles, allow substantially reduced system temperatures, greatly increase instantaneous observing bandwidths, and add interference-rejection capacity. For applications to 21-cm spectroscopic survey work, these improvements will increase the speed of operation by factors of between 8 and 60 (Giovanelli 1987), albeit with no change of the declination horizon, which restrict the telescope to about one third of the whole sky. Completion of this upgrade is scheduled for 1993.

The GBT, which has a similar development schedule as the Arecibo upgrade, will have a paraboloidal aperture of modern design capable of reaching any declination north of about -45°. The unblocked character of its 100-m aperture will permit an extremely high gain-to-system temperature ratio for its size: about 0.13-0.15 Jy^-1, which is only a factor of two or less times smaller than that of the current much larger aperture Arecibo antenna (0.23 Jy^-1). Its wide sky coverage (especially South of the Equator), extremely high sensitivity to fairly extended, low H I surface brightness objects, and high interference rejection characteristics underscore its promise.