The recently completed Southern African Large Telescope (SALT) is a low cost, innovative, 10 m class optical telescope, which began limited scientific operations in August 2005, just 5 years after ground-breaking. This paper describes the design and construction of SALT, including the first-light instruments, SALTICAM and the Robert Stobie Spectrograph (RSS). A rigorous systems engineering approach has ensured that SALT was built to specification, on budget, close to the original schedule and using a relatively small project team. The design trade-offs, which include an active spherical primary mirror array and a fixed altitude telescope with a prime focus tracker, although restrictive in comparison to conventional telescopes, have resulted in an affordable 10 m class telescope for South Africa and its ten partners. Coupled with an initial set of two seeing-limited instruments that concentrate on the UV-visible region (320 – 900 nm) and featuring some niche observational capabilities, SALT will have an ability to conduct some unique science. This includes high time resolution studies, for which some initial results have already been obtained. Many of the versatile modes available with the RSS - which is currently being commissioned - are unique and provide unparallelled opportunities for imaging polarimetry and spectropolarimetry. Likewise, Multi-Object Spectroscopy (with slit masks) and imaging spectroscopy with the RSS, the latter using Fabry-Perot étalons and interference filters, will extend the multiplex advantage over resolutions from 300 to 9000 and fields of view of 2 to 8 arcminutes. Future instrumentation plans include an extremely stable, fibre-fed, high resolution échelle spectrograph and a near-IR (to between 1.5 to 1.7 $\mu$m) extension to the RSS. Future development possibilities include phasing the primary mirror and AO. Finally, extrapolations of the SALT/HET designs to ELT proportions remain viable and are surely more affordable than conventional, fully steerable, designs.

Some of the achievements and capabilities of the current generation of 8–10 metre optical-infrared telescopes are reviewed. The challenges of building yet larger optical-infrared telescopes in the diameter range 30 to 100 metres are discussed.

IAU Symposium 232 allows a snapshot of ELTs at a stage when design work in several critical mass projects has been seriously underway for two to three years. The status and some of the main initial design choices are reviewed for the North American Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT) projects and the European Euro-50 and the Overwhelmingly Large (OWL) projects. All the projects are drawing from the same “basket” of science requirements, although each project has somewhat different ambitions. The role of the project offices in creating the balance between project scope, timeline and cost, the “iron triangle” of project management, is emphasized with the OWL project providing a striking demonstration at this meeting. There is a reasonable case that the very broad range of science would be most efficiently undertaken on several complementary telescopes.

The ESO strategy for short listing ELT candidate sites is reviewed: it uses a specially designed tool to allow the simultaneous use of existing databases with various relevant parameters.

Both optical CCD detectors and infrared detector arrays have steadily evolved over the past 5 years, and have reached very satisfactory levels of quantum efficiency and dark current. Optical CMOS detector arrays have rapidly improved and are beginning to compete with CCD detectors. A new development of great relevance for future instrumentation is the integration of the readout electronics into an integrated circuit in close proximity to the detector, or, in the future, its vertical integration with the detector array and multiplexer. This paper reviews the present status of these technologies and identifies opportunities and risks for the next decade.

The 8-instrument suite studied by the ESO community in the frame of the conceptual study of the 100 m OWL telescope is briefly presented. Potential capability for unique science and the main technical challenges are identified.

Adaptive Optics (AO) will be essential for accomplishing many, if not most, of the science objectives currently planned for Extremely Large Telescopes including GMT, OWL, and TMT. AO will be needed to support a range of instrumentation, including near infrared (IR) imagers and spectrometers, mid IR imagers and spectrometers, “planet finding” instrumentation and wide-field optical spectrographs. Multiple advanced AO systems, utilizing the full range of concepts currently under development, will need to be combined into an integrated architecture to meet a broad range of requirements for field-of-view, spatial resolution and spectral bandpass.

In this paper, we describe several of the possible options for these systems and outline the range of issues, trade studies and component development activities which must be addressed. Some of these challenges include very high-order, large-stroke wavefront correction, tip-tilt sensing with faint natural guide stars to maximize sky coverage, laser guide star wavefront sensing on a very large aperture and achieving extremely high contrast ratios for the detection of extra-solar planet and other faint companions of nearby bright stars.

Over the coming two decades astronomers will plan, construct and exploit extremely large telescopes operating at both optical (ELTs) and radio (SKA) wavelengths. In studies of galaxy evolution and cosmology the combination of the two types of facility are likely to be extremely powerful. In this paper I will review the key questions that the ELT/SKA combination seem set to solve, focussing on the unique advantages afforded by optical and radio observations against the backdrop of the other theoretical and observational information to be available by 2020.

The scientific capabilities of the James Webb Space Telescope (JWST) fall into four themes. The End of the Dark Ages: First Light and Reionization theme seeks to identify the first luminous sources to form and to determine the ionization history of the universe. The Assembly of Galaxies theme seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and active nuclei within them evolved from the epoch of reionization to the present. The Birth of Stars and Protoplanetary Systems theme seeks to unravel the birth and early evolution of stars, from infall onto dust-enshrouded protostars, to the genesis of planetary systems. The Planetary Systems and the Origins of Life theme seeks to determine the physical and chemical properties of planetary systems around nearby stars and of our own, and investigate the potential for life in those systems. To enable these four science themes, JWST will be a large (6.5m) cold (50K) telescope launched to the second Earth-Sun Lagrange point early in the next decade. It is the successor to the Hubble Space Telescope, and is a partnership of NASA, ESA and CSA. JWST will have four instruments: The Near-Infrared Camera, the Near-Infrared multi-object Spectrograph, and the Tunable Filter Imager will cover the wavelength range 0.6 to 5 microns, while the Mid-Infrared Instrument will do both imaging and spectroscopy from 5 to 27 microns. The scientific investigations described here define the measurement capabilities of the telescope, but they do not imply that those particular observations will be made. JWST is a facility-class mission, so most of the observing time will be allocated to investigators from the international astronomical community through competitively-selected proposals.

A description of the Herschel Space Observatory and the Atacama Large Millimeter Array is presented. Their scientific potential and possible synergistic effects with ELTs is discussed. Herschel is a space-based far-infrared and submillimeter telescope to be launched in 2008 and will offer 3 years of routine science operations. ALMA will become fully operational around 2012 and will co-exist with ELTs. The synergy of being able to observe the same objects with similar angular resolution and sensitivity at long and short wavelengths will contribute to our understanding of astrophysical objects and processes in general.

The direct detection of light emitted or reflected by extrasolar planets will soon allow studies of the physical properties of their surfaces and atmospheres. This article gives an overview of the scientific perspectives and the techniques that are currently being developed for observations of gas giants and terrestrial planets from the ground and in space.

Understanding the energy and matter content of the Universe and its evolution are key goals for the international astronomical community and all extremely large telescope projects. ELTs will bring a wide range of tools to bear on questions relating to the nature of dark energy and the formation and early evolution of galaxies. Natural seeing and ground-layer corrected instruments on ELTs will quantify the assembly of galaxies at modest redshifts, while near-IR diffraction-limited instruments will probe the internal structure of early galaxies and the epoch of first light and reionization.

The absorption lines seen in the spectra of high redshift QSOs are important tools for studying the early evolution of galaxies and intergalactic medium. In this presentation I briefly review various available observations, our understanding of different types of absorption systems, and highlight some of the issues that can be addressed with ELTs.

The star formation, mass assembly and chemical enrichment histories of galaxies, and their present distributions of dark matter, remain encoded in their stellar populations. Distinguishing the actual distribution functions of stellar age, metallicity and kinematics at several locations in a range of galaxies, sampling across Hubble types and representative environments, is the information required for a robust description of galaxy histories. Achieving this requires large aperture, to provide the sensitivity to reach a range of environs and Hubble types beyond the Local Group, to provide high spatial resolution, since the fields are crowded, and preferably with optical performance since age-sensitivity is greatest near the main-sequence turn-off, and metallicity-sensitivity for these warm stars is greatest in the optical.

Future extremely large telescopes will be the engines of major progress in the fields of star and planet formation, brown dwarfs, and extrasolar planets. Their throughput will enable spectroscopic studies of the structure of brown dwarf atmospheres; reveal the composition and kinematics of protoplanetary disks; extend radial velocity searches for extrasolar planets to fainter stars and lower masses; and characterize the surfaces of the most distant Kuiper Belt objects. Their resolution will allow us to resolve and track the orbits of close binary young stars and brown dwarfs, establishing their dynamical masses and calibrating their evolutionary tracks; probe the inner region of young stellar object disks, resolving the jet collimation region in accreting systems and disk inner holes in more evolved ones; and move the horizon for stellar companion searches inward to 40 mas, enabling the direct detection of hot young planets orbiting 10 AU from young T Tauri stars. The high contrast imaging capability of future ELTs is uncertain, and depends on future developments in extreme adaptive optics. Wavefront control considerations suggest a fundamental atmospheric contrast limit of 10$^{-8}$ for companion searches to nearby solar-type stars, below which detections are unlikely to be possible. To reach this performance level, continuing investments will be needed in extreme adaptive optics work, in addition to careful attention to the specialized requirements of ultra-high contrast imaging in ELT design. Above the 10$^{-8}$ contrast limit, a 30m telescope has the potential to directly image about a dozen of the currently known radial velocity planets.

We review the current status of searches for high redshift galaxies and quasars, together with their derived luminosity function, star formation history and relative contribution to the UV ionizing background. These searches have been carried out in the optical and near-infrared with the HST for very deep pencil beam surveys and ground-based telescopes for deep large area surveys. The peak of galaxy and black hole formation activity occurs at $z\sim 2$-3, with a strong decrease in UV luminosity density in the redshift interval $z\sim 3$ to 6. This decrease seems to extend beyond $z\sim 6$ for luminous galaxies, as there are only a few $z\sim 7$-8 secure galaxy candidates. The star formation rate at $z\sim 6$ is dominated by 0.1 L$^{\star}$ galaxies. The $z \gtrsim 5$ samples for both types of sources are too small to constrain the faint end of their luminosity function or even its knee. Consequently, predictions of the number density of $z \gtrsim 7$ galaxies is highly uncertain; their search with JWST and ELTs should cover areas of at least $\sim$1 deg$^2$ and will require long campaigns (${\gtrsim} 1$ month). Faint quasars of small black hole mass should also be detected in these surveys. Searches for $z\sim10$ rare, bright quasars and higher $z$ transient sources, gamma-ray bursts and population III SNe, should be undertaken with dedicated facilities. The physical properties of high $z$ galaxies and the metal-enrichment of the intergalactic medium at early epochs will be determined by high-resolution spectroscopic observations with ELTs requiring substantial exposure times (${\sim} 100$ hr).

We report an on-going blank-field multi-wavelength deep and wide survey of the Subaru/XMM-Newton Deep Survey Field (SXDF). The SXDF has been the focus of a wide range of multi-wavelength observing programs spanning the X-ray to the radio. These observations cover a large enough area (the initial optical imaging covers $\sim$1.3 deg$^{2}$) and depth ($B=28.2$) that they are not affected by large-scale structures (which exist on tens of Mpc scales) and allow us to study the distribution and evolution of high-$z$ galaxies and AGNs, and thus constrain theories for their formation. Our early results include: i) an indication of the primeval Large Scale Structure (LSS) at z $\sim$ 5.7, ii) an indication for the down-sizing of galaxy formation at z $\sim$ 1, iii) identifications of passively evolving systems, and evidence for early formation and the passive evolution of present-day early-type galaxies, and iv) discovery of a large number of optically obscured QSOs. As for the next step forward - we express our hope to use the next generation optical/IR Extremely Large Telescopes (ELTs) to obtain larger and deeper spectroscopy samples of the high-$z$ objects.

A highlight science case for the European ELT is: First light - The First Galaxies and the Ionization State of the Early Universe. It aims at understanding the formation and evolution of the first sources of light at the end of the Dark Ages and of the re-ionization of the Universe. The corresponding instrument requirements are: a few tens of integral field units with spatial sampling $\sim$20mas and individual fields of ${\sim}1''$ over a wide field of view of $5' \times 5'$ or larger. Multi-Object Adaptive Optics is required to locally provide significant image quality enhancement. Spectroscopic observations are required in the near IR domain with a spectral resolution of a few 1000. MOMFIS is a preliminary instrument concept designed for OWL around this science case. The instrument concept and preliminary design are presented. Development efforts are estimated, as well as development risks and required R&D activities.

In this paper, we present a first comparison of different Adaptive Optics (AO) concepts to reach a given scientific specification for 3D spectroscopy on Extremely Large Telescope (ELT). We consider that a range of 30%–50% of Ensquarred Energy (EE) in H band (1.65$\mu$m) and in an aperture size from 25 to 100mas is representative of the scientific requirements. From these preliminary choices, different kinds of AO concepts are investigated: Ground Layer Adaptive Optics (GLAO), Multi-Object AO (MOAO) and Laser Guide Stars AO (LGS). Using Fourier based simulations we study the performance of these AO systems depending on the telescope diameter.

The amount and spatial distribution of dark matter in elliptical galaxies are poorly known, despite extensive observations with multi-object spectrographs on 4-10m-class telescopes. We examine the prospects for measuring the structure of dark matter halos in elliptical galaxies — and the orbital properties of their planetary nebula and globular cluster systems — from radial velocity measurements made with WFOS, a wide-field optical spectrograph operating on the Thirty Meter Telescope (TMT).