This PDF file contains the front matter associated with SPIE Proceedings Volume 6986, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

I describe the design of the Thirty Meter Telescope, a project to build a ground-based thirty-meter telescope. The partners include the University of California, Caltech, the Association of Canadian Universities for Research in Astronomy (ACURA), and NSF. The Project is currently in the design and development phase and will be ready for a 2009 construction start.

The conceptual design phase for the Giant Magellan Telescope (GMT) has been completed and the project is continuing the development of the telescope structure and instrumentation. The upper truss of the telescope has been revised to reduce the tilt of the secondary mirror assembly, which was the major contributor to image blur caused by windshake of the structure. A factor of 5-10 reduction is obtained in the static analysis. The generation of the first 8.4 m off-axis mirror blank for GMT is nearing completion. The metrology for grinding and polishing the mirror to its final figure has been designed and is being constructed. Multiple, independent tests are provided to verify the final mirror figure and insure mirror-to-mirror repeatability. Loose abrasive grinding and polishing of the mirror is ready to start with mirror completion expected in early 2009. GMT instruments mount in the telescope below the primary mirror. The candidate list of first generation instruments is provided. Las Campanas Observatory has been selected as the site for GMT.

The EELT project is in Phase B (detailed design), a 3-year, 57.2 Mσ activity that will result in a Proposal for Construction by end 2009 or early 2010. The requirements for the basic reference design, starting point for the current phase, were defined through a community process that led to the convergence of earlier concepts into a single European project. That process owed much to Arne's wisdom and vision. This paper reports on the status of the Phase B and on the development of the EELT science case and Design Reference Mission, and examines the issue of the impact of the telescope size on science and how much this impact depends on Adaptive Optics technology. The design of the telescope is described in a separate paper in these proceedings.

The European Extremely Large telescope project is in phase B and scheduled to present a construction proposal to the ESO Council by the end of 2009 or early 2010. The telescope baseline is for a fully steerable 42-m, segmented primary, 5 mirror design, fully adaptive system with Nasmyth and coude foci. This paper describes the current state of affairs with the European ELT and, in view of this conference celebrating Arne Ardeberg's contributions to astronomy, contains the occasional, totally incomplete, retrospective to earlier work in the field focusing on a single paper by Ardeberg et al in 1996.

In the last ten years, Chinese astronomers proposed two optical and near-infrared Extremely Large Telescope (ELT) projects, one is 30m Chinese Future Giant Telescope (CFGT) the other is a 30m Ring Interferometric Telescope (RIT). The current status in the development of these two projects and their key technology research and progress are described in this paper. Both of the two plans are the preliminary projects of the National Astronomical Observatories (NAOC) of the Chinese Academy of Sciences (CAS), and finance-supported from the NAOC. The National Science Foundation of China (NSFC) also gives supports to key technology researches of the projects (such as, adaptive optics and high-resolution imaging techniques). In the final part of this paper a brief description is given on the possibility of international cooperation in the development of Chinese ELT. A review is also presented of other two ground-based Extremely Large Projects (ELP), one is Five-hundred-Meter Aperture Spherical Radio Telescope (FAST), and the other is a challenging PANDA (The Prydz bay, Amery ice shelf and Dome A Observatories) project in Antarctica.

Extremely expensive new telescopes involve a compromise between the extreme ambitions of the scientific community, whose support justifies the financial costs, and the need to have a telescope design which can actually be built today at appropriate cost. In this article I provide a brief history of the process which built community support in Europe for what has become the European Extremely Large Telescope project (E-ELT). I then review remaining tensions between the community science case and day-one technical performance. While the range of very strong scientific cases which support the E-ELT project will largely be delivered, and lead to a quite outstanding scientific return, there are - as always! - demands for even more impressive performance. In addition to what the E-ELT will deliver, much of the community wants high spatial resolution at wavelengths shorter than one micron. Affordable adaptive optics systems will work best, initially at somewhat longer wavelengths. Planned performance enhancement during its operational life is very desirable in the E-ELT.

A number of Extremely Large Telescopes for visual-infrared and adjacent wavelengths are in various degrees of progress. All have primary mirrors with equivalent diameters larger than 20 m and are intended for operation with adaptive optics systems. We discuss several ELT observing parameters as functions of wavelength. Stellar energy distributions and atomic line spectra are inspected as are the transmission of the Earth's atmosphere, the emissivity of the sky and telescope and instruments as well as detector sensitivity, resolution and signal-to-noise ratio. The spatial resolution depending on the size of the diffraction limited adaptive optics point spread function is discussed. We have evaluated the ELT efficiency in terms of Johnson V to N band photometry, simulating diffraction-limited ELT images of a stellar field at 4 Mpc and 4 kpc, respectively. We conclude that the information content at shorter wavelengths is of dominant nature and that there is every reason to do the utmost to include shorter wavelengths in the AO regime. We propose to adopt a short-wavelength goal of 1 000 nm for first light AO with later updates reaching down to visual wavelengths.

Much of the progress in astronomy follows imaging with improved resolution. In observing stars, current capabilities are only marginal in beginning to image the disks of a few, although many stars will appear as surface objects for baselines of hundreds of meters. Since atmospheric turbulence makes ground-based phase interferometry challenging for such long baselines, kilometric space telescope clusters have been proposed for imaging stellar surface details. The realization of such projects remains uncertain, but comparable imaging could be realized by ground-based intensity interferometry. While insensitive to atmospheric turbulence and imperfections in telescope optics, the method requires large flux collectors, such as being set up as arrays of atmospheric Cherenkov telescopes for studying energetic gamma rays. High-speed detectors and digital signal handling enable very many baselines to be synthesized between pairs of telescopes, while stars may be tracked across the sky by electronic time delays. First observations with digitally combined optical instruments have now been made with pairs of 12-meter telescopes of the VERITAS array in Arizona. Observing at short wavelengths adds no problems, and similar techniques on an extremely large telescope could achieve diffraction-limited imaging down to the atmospheric cutoff, achieving a spatial resolution significantly superior by that feasible by adaptive optics operating in the red or near-infrared.

Observational High Time Resolution Astrophysics differs from conventional astrophysics in regard to the detectors employed which have a time resolution less than that obtainable through CCD with a normal readout τ < a few minutes. This paper looks at the implications for HTRA from extremely large telescopes and specifically, as an exemplar its possible impact on pulsar astrophysics. We demonstrate, by using the derived point-spread-function from models of the Euro50 telescope, the possible effects active and adaptive mirrors have on observing rapidly varying astronomical objects.

The physical basis of the detection and characterisation of extrasolar Earth-like planets and of biosignatures in the reflected light and thermal emission regimes is reviewed. Both approaches have their advantages and disadvantages, including artefacts, in the determination of planet physical parameters (mass, size, albedo, surface and atmospheric conditions etc) and biosignatures.

In the way of major new instruments for ground-based optical astronomy, maximizing the science favors a large hypertelescope. If equipped with adaptive optics and a laser guide star, it can provide direct high-resolution images of faint extra-galactic and cosmological sources. The signal/(photon noise) ratio is theoretically higher than with interferometer schemes relying upon aperture synthesis, using a few large apertures to reconstruct images. The crowding limit on complex objects, the direct-imaging field, and the dynamic range are also improved with many small apertures. The adaptive phasing of hypertelescopes, achievable on bright stars with modified wave sensing techniques such as "dispersed speckle" analysis, is also achievable on very faint sources with a modified version of a laser guide star. This makes large hypertelescopes capable of observing cosmological deep fields of faint galaxies. Pending space versions, the size of which can in principle reach hundreds and thousands of kilometers, terrestrial hypertelescopes limited in size to one or two kilometers can be built at suitable sites and used efficiently from ultra-violet to millimeter wavelengths. Some sites can allow the coupling of a hypertelescope with an ELT, an alternate option which can also be efficient for imaging deep fields with a high-resolution.

Astrobiology is a new discipline that promises to answer one of mankind's so-called 'great questions', that is whether we are alone in the Universe. In order to do so, new technologies are required since the key element is to detects signs of life - biomarkers - at interstellar distances. If we are going to be able to do so from the ground or if we will have to deploy instruments in space is still somewhat un-clear. The issue is complex, and at the heart of the matter is the question of which wavelengths will be suitable.

Here we describe the general guidelines for astronomical site testing in the visible range. We recall the main parameters which drives and orientates a site characterization among other parameters of less importance. One needs to select a site from which the stars are visible as much time as possible, with the best transparency and allowing the best angular resolution. We recall that, in order to appear, the optical turbulence requires a medium where simultaneously a vertical gradient of the refractive index and a vertical gradient of the horizontal wind speed are present. If one of these conditions is not fulfilled, no optical turbulence will disturb the light propagation. Few exemples are detailed, and few clues are given to orient the choice of a site throughout the infinite possibilities in the world. A special thank is given to A. Ardeberg for his major contribution in telescope environment leading to minimize dome and mirror seeing, which has been implemented in the Nordic Optical Telescope and then inspired the next generation of very large telescopes.

This paper discusses the concept of employing artificial guide stars to correct atmospheric turbulence induced wavefront distortion for Extremely Large Telescopes (ELTs) conducting science observations at near IR and visible wavelengths. A set of generic laser guide star requirements for ELTs is established with the caveat that a community wide consensus on laser parameters would benefit the advancement of laser technology. Currently available laser guide star systems are examined in light of the requirements and their limitations referenced to the requirements are discussed. A short summary is provided of current research and development efforts to advance laser performance. Finally, a new concept is presented for generating very bright optical guidestars using high power microwaves to create, sustain, and move about the sky a high altitude, compact plasma.

In connection with the planning for Extremely Large Telescopes, I revisit a 2001 paper in which Cacciani and I describe the use of Sodium Laser Guide Stars (LGSs) for diffraction limited daytime astronomical observations. The enabling technology for seeing LGSs in broad daylight is the availability of very narrow band magneto-optical filters. Considering the dominance of the atmospheric scattering of sunlight at wavelengths below 3.5 μm, daytime use is only indicated for mid- and thermal IR observations. The launch of the 6.5 meter aperture James Web Space Telescope (JWST) appears to be assured and planned for 2013, preceding the most optimistic projections for the completion date of the first ELT. The projected thermal background of the JWST is very much less than that of ground-based telescopes so that any competing ground-based observations are limited to those parameters not covered by the JWST: angular resolution (requiring apertures > 6.5 meter) and spectral resolution (R>3000). I compare the benefits of daytime observations with Na-LGS equipped telescopes and interferometers at moderate latitudes and in the Antarctic (specifically Dome C). In both cases daytime observations extend the amount of observing time available for TIR observations. Antarctic observations have the advantage of having very good seeing during the daytime, significantly better than nighttime seeing. In contrast the seeing at moderate latitude sites significantly deteriorates during daytime resulting in lower quality observations than during nighttime. In addition Antarctic sites are less hostile to maintenance and operations during daytime (summer) observations as compared to nighttime (winter) observations.

This paper summarises the problems related to atmospheric dispersion (AD), both active (angular dispersion due to change in the telescope zenith angles) and adaptive (the atmospheric turbulence depends on wavelength since the refractive index and r₀ are also wavelength dependent). We compare the AO effects and its compensation by deformable mirrors (achromatic by definition and incapable of compensating the atmospheric dispersion) using GLAO and LTAO with those of the atmospheric dispersion. We present preliminary optical designs for ADCs in Euro50 and E-ELT to compensate the zenith dependence dispersion of the atmosphere.

When performing adaptive optics (AO) correction for the effect of atmospheric fluctuations, different effects, related to measuring the wavefront at one wavelength and observing at another one, may show up. Some of these effects are related to the dispersive properties of the atmosphere. Others are related to diffraction, both regarding light propagation through the turbulent atmosphere to the entrance pupil of the telescope and regarding the inevitable Fraunhofer diffraction taking place for light propagating from the exit pupil to the image plane of the telescope. In this paper some of these effects are revised and discussed, in particular the way in which uncorrected wavefront errors in the point spread function (PSF) will scale with color, and the way in which dispersion affects the Strehl ratio and the background level of the AO corrected PSF. Also discussed is the trade-off between AO spectral bandwidth and Poisson noise.

The next generation of telescopes, the Extremely Large Telescopes (ELTs), will have a multitude of control loops to maintain nearly diffraction-limited performance in the presence of atmospheric turbulence and external disturbances, for instance from wind. Integrated simulation models combining structural and optical modeling together with control system modeling are efficient tools for prediction of performance of ELTs. Such models include submodels of structures, adaptive optics, atmosphere, wind load, deformable mirrors and a segmented primary mirror. So far the models applied have been applicable to observations in the K-band. However, there is a desire to also operate the ELTs with adaptive optics at wavelengths in the visible range. We here give estimates of the feasibility of performing such simulations. We set up scaling laws for the design parameters as a function of wavelength of operation and we show that the execution time for an integrated model of an ELT depends dramatically on the operation wavelength. We also discuss the consequences of different choices of model refinement. Finally we present estimates of the execution time for integrated models of ELTs. We show that accurate modeling in the K-band calls for long execution times, even with parallel computers. For wavelengths in the visible range, only the very simplest models are feasible due to execution time limitations, thereby precluding many interesting studies related to noise sensitivity and limiting magnitude for guide stars.

The next generation of large ground-based optical and infrared telescopes will provide new challenges for designers of astronomical instrumentation. The varied science cases for these extremely large telescopes (ELTs) require a large range of angular resolutions, from near diffraction-limited performance via correction of atmospheric turbulence using adaptive optics (AO), to seeing-limited observations. Moreover, the scientific output of the telescopes must also be optimized with the consideration that, with current technology, AO is relatively ineffective at visible wavelengths, and that atmospheric conditions will often preclude high-performance AO. This paper explores some of the issues that arise when designing ELT instrumentation that operates across a range of angular-resolutions and wavelengths. We show that instruments designed for seeing-limited or seeing-enhanced observations have particular challenges in terms of size and mass, while diffraction-limited instruments are not as straightforward as might be imagined.

Adaptive Optics for ELT are compared with the ones for existing 8m class facilities, expecial with respect to the covered fields and wavelenght range. Suggestions for wavefront sinergy from both natural and aritifical references are briefly outlined. Some final remarks for possible AO developements on ELTs are given.

Discussions at the Symposium ELTs: Which wavelengths? in Lund in December 2007 are summarized and in particular comments are made on the relation between the optimization of the presently planned ELTs, and their corresponding background science cases. The division of labour between the ELTs and the JWST is commented on. The need for an ELT (and/or a future Space Telescope) for the optical wavelength region is stressed. Possible strategies for pursuing the ELT projects are commented on.