Many areas of extragalactic research will greatly benefit from observations with a very large telescope. We concentrate, here, on four of them as illustrative examples:

First we briefly discuss the study of the absorption lines in quasar spectra.

In the second section we suggest the possibility of doing serendipity large field imaging during high spectral resolution exposures on point sources.

In sections 3 and 4 we discuss two topics which have long been recognized as important drivers for the construction of very large telescopes, the determination of the nature of the missing mass, and the study of elliptical galaxies.

The previous speaker, Marie-Helene Ulrich, discussed several problems in extragalactic research which require the use of a very large ground-based telescope. She emphasized studies of relatively nearby objects. In my talk I will discuss problems which have to do with observations of objects at very large distances.

Most studies of very distant objects involve studies of distant clusters of galaxies since this is one of the very few ways to pick a galaxy which is almost certainly very distant rather than a low-luminosity nearby object. Two exceptions are studies of radio and X-ray selected galaxies which very often are distant objects. Another exeption is the study of quasars which are almost all at very large distances. Even in these exceptions there is an interest in knowing whether these objects are associated with clusters or not.

It is now evident that major advances in observational sensitivity in other regions of the electromagnetic spectrum invariably lead to heavy demand for complementary observations with existing large (3- to 5-meter) optical/infrared (O/IR) telescopes, and that such observations are often essential to the interpretation and understanding of phenomena revealed by the former. The Einstein X-ray mission, the International Ultraviolet Explorer (IUE), the Very Large Array (VLA) and the recent Infrared Astronomical Satellite (IRAS) are all clear demonstrations of this effect.

The 15-Meter National New Technology Telescope (NNTT) Program, growing out of the earlier "Next Generation Telescope" studies at Kitt Peak National Observatory, is now responding to the recently published recommendation of the Astronomy Survey Committee of the American National Academy of Sciences:

This report offers a short description of observational facilities used at the 6-meter optical telescope BTA of the Special Astrophysical Observatory of the Academy of Sciences of the USSR.

The purposes of the report:

1. - to give an idea on the general strategics for the development of the observational facilities of the BTA;

2. - to present the principle information on the achieved observational possibilities. The first point may prove to be useful for the projects of Very Large Telescopes (VLT), that are discussed at this meeting.

The University of California has been actively designing a ten meter telescope (TMT) for optical and infrared groundbased astronomical observations since 1977. The project, now developed in considerable detail, is described in a series of Ten Meter Telescope Reports, technical notes, and publications (see, for example, Nelson 1980,1981,1982). In order to achieve an acceptable cost for the project, the design departs substantially from conventional telescope designs. Recognizing that the cost is roughly proportional to the weight of the structure and to the enclosed volume, we have made the reduction of weight and size a high priority goal of the design. To achieve these objectives a light-weight segmented primary mirror was designed. The parabolic primary is a mosaic of 36 hexagonal segments. In addition, the primary focal ratio is f/1.75, thus resulting in a short telescope tube; this allows a very compact dome.

VLT preliminary studies were initiated at ESO as early as 1978 (1). After ESO's move from Geneva to Munich (1980) a study group chaired first by R. Wilson, then by J.P. Swings was set up and among a number of conceptual ideas that were analysed, the concept of a “limited array” emerged as the most attractive (2). A significant driver was then, interferometry; however after a theoretical analysis performed by F. Roddier and P. Lena (3), it became clear that interferometry with large telescopes would not be cost-effective in the visible range. In the IR, the situation would be more favorable but the overall efficiency would depend on factors that are not reliably known at present. The conclusion was that it might be difficult to justify a huge investment too exclusively oriented towards interferometry, such as an array of movable large telescopes, but on the other hand the possibility of a coherent coupling in the IR should be maintained as a prime requirement since new developments in detectors and in adaptive optics could make it effective and scientifically very rewarding.

In late 1979 a plan to build a very large telescope was presented to University of Texas President Peter Flawn. A small startup budget was subsequently granted by the University administration, and we asked Aden and Marjorie Meinel to carry out a design concept study, which they completed in early 1980. Following their report, a study contract was awarded to the Western Development Laboratories Division of Ford Aerospace & Communications Corp., for a preliminary design and cost estimate.

It is generally agreed that construction of monolithic mirrors up to 8-10 meters aperture is within current technology. The major concern that has prevented construction of telescopes larger than the Palomar 5-m telescope outside the Soviet Union has been cost; it has been shown (Meinel and Meinel, 1980a) that the single most important item in determining the cost of a large telescope is the weight of its primary mirror. We chose a monolithic, lightweight 7.6-m (300-inch) mirror as representing a significant advance from presently existing telescope apertures while also being well within the current state-of-the-art. Because a lightweight mirror cannot support its figure against gravity and other disturbances as well as can a conventional thick mirror, we have investigated methods of active control of the mirror's figure. The now maturing technology of adaptive optics (Hardy 1980, 1981, 1982) has been drawn upon extensively in planning this telescope. Results of finite element analyses of an ultra-lightweight monolithic 7.6-m mirror blank have been published (Ray et.al., 1982, 1983). A description of the proposed mirror figure monitoring system has been given (Tull and Young, 1983).

At the present time one of the major activities within the international astronomical community is the preliminary design of large optical/infrared telescopes. During the last few years workshops, conferences, and many discussion groups have been assembled to evaluate alternative telescope concepts. Segmented mirrors, multi-mirrors with dilute or filled apertures and arrays of smaller telescopes have been discussed and all give the increase in the collecting area required for the next generation of astronomical instrumentation.

This paper is concerned with the process of creating a major observatory at minimum cost. Its premises are that cost optimization starts with an appropriately modestly priced building block, a 7.5m telescope. There are many possible configurations in which this building block can be used, ranging from a single telescope or a set of separate telescopes with at most electronic data addition, to an MMT in which the light of all telescopes can be phased when desired. In addition there are options for using elements in a non-redundant linear array or Michelson inteferometer. We provide the reader with a means of estimating costs of different facilities all on the same basis, and tied to the actual cost of producing the MMT.

It is also important to place the observatory on a site which maximizes the output per unit cost of the facility. We discuss the selection process which has led to Mt. Graham in Arizona as being likely to satisfy this criterion. We give brief details of our preliminary knowledge about the site.

A brief background and a description are given of the plan to build a large telescope of 5m to 7.5m class as the Japanese National Large Telescope.

To map the distribution of galaxies and quasars in the universe, we desire a telescope large enough to obtain spectra to about 20th magnitude in under 20 minutes integration and which is devoted entirely to such spectroscopic survey projects in dark time. We also desire extensive bright time observing to monitor large numbers of G-M stars with high-resolution spectroscopy for many years in search of solar-like activity. These projects are suited to a telescope with a large fixed primary, assembled from many spherically figured segments. We present a design for such a telescope that consists of 73 segments, each of 0.9-m diameter and 26-m radius of curvature. Mirror blanks of this size can be cut from standard Pyrex sheets. Effective aperture of the telescope exceeds 7-m; the focal plane system can track objects for 40 minutes, and sky coverage of 48 degrees is obtained by using a fixed tilt for the primary and making the entire telescope and dome rotatable. The focal plane system is lightweight and precisely pointable because spectrographs are coupled to the focus by fiber optic cables. Off-the-shelf components and existing technology are used to keep engineering and development costs low; we must remain within a budget feasible for a university. Because the telescope will be equipped with standard, minimal instrumentation and is intended for very routine observing programs, operating costs will also be low. Hardware components are now being assembled in a laboratory to develop the focal plane control system and the mirror support system.