Neutron stars, containing typically 1.4 solar masses within a diameter of about 15km, are among the smallest astronomical sources and the densest known form of directly observable matter in the Universe. Many aspects of the neutron star remain poorly understood. Most theoretical models for neutron stars cannot, so far, account for many of the observations, which have been largely made at radio wavelengths. This paper discusses the importance of multiwavelength studies, using large telescopes, to better understand the properties and behaviour of these objects.

A Mid-IR instrumentation study for OWL has been performed by the Max-Planck-Institut für Astronomie in Heidelberg (Germany), and a Dutch consortium led by the Leiden Observatory (The Netherlands). MIR imaging and spectroscopic observational capabilities are compared to contemporary IR to sub-millimeter facilities, especially concentrating on the MIR-capabilities of JWST(MIRI). Our best effort calculation of the sensitivity for both MIR imager and spectrograph indicate a huge discovery potential in numerous areas from our planetary system to the high redshift Universe. Here we focus on the field of exo-planets and nearby star formation. Starting with the science cases, top level requirements are deduced and summarized including MIR instrumental constrains for the telescope itself.

The extreme contrast in mass and luminosity between the extra-solar planets and their host stars make detailed studies of these planets very challenging. In particular, direct observations of extra-solar planets is still beyond the capabilities of the currently available instrumentation, save for perhaps a few extreme cases of very young and massive planets at large distances from the central star. While progress in instrumentation might allow significant progress in detection capabilities either with the 8 and 10-m ground-based telescopes (Planet Finder instruments on the VLT and Gemini) or with the next generation space telescope (JWST), imaging of extra-solar planets over a wide range of parameters, and possibly down to terrestrial planets, will require extremely large ground-based telescopes like OWL or dedicated space instrumentation (TPF or Darwin for instance). We outline here the scientific objectives of EPICS, the OWL Earth-like Planet Imager and Spectrograph, summarize the corresponding high level requirements, present the foreseen observing modes and give a first estimate of its performance.

In this contribution, we show how a future ELT ($>$25 m diameter) helps to understand the formation and early dynamical evolution of massive stars embedded in dust-enshrouded very compact HII regions. We describe how to exploit the ELT's near- and mid-IR enhanced sensitivity and high angular resolution to peer through huge amounts of dust extinction, taking direct nearly diffraction-limited images and doing IFU spectroscopy. Together with ALMA, an ELT will be a powerful observing platform to reveal one of the most hidden secrets of stellar astrophysics: the origin of massive stars.

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.

Brown dwarfs and very low-mass stars are likely to harbour planetary systems with rocky planets. We discuss the possibility of detecting them using accurate radial velocity measurements with a cross-dispersed high-resolution spectrograph coupled to a ground-based extremely large telescope.

Modelling and simulation, based on observational data, were used to examine the potential of ELT photometry for studies of the evolution of distant galaxies. An open cluster, a globular cluster and two mixed field populations were employed. Colour-magnitude and metallicity diagrams were examined. For younger populations, excellent turn-off-point age data and abundance data can be obtained even beyond 20Mpc. Higher population age weakens data if not improved with longer exposures. Still, the great potential of ELT photometry for studies of the evolution of galaxies is confirmed. Comments are given on adaptive optics and photometry.

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.

We will review the metallicity of the CIV and OVI phases of the IGM in the redshift range $z\sim 0.1$-5 together with their contributions to the cosmic baryon density. The current studies are limited to regions with overdensities of 3-10; the heavy element enrichment in regions with gas densities close to the mean cosmic density will be probed with future ELTs. We will also investigate the possibilities of detecting metals at $z\sim 10$ and underline the requirements on ELTs due to the anticipated faintness of the early generations of galaxies, QSOs and transient objects.

The project of the Russian 6m BTA telescope reconstruction was cancelled in 2004 because of financial problems. In the beginning, it was planned to replace the existing 42 ton parabolic main mirror by a new one of 8m diameter using the Zerodur blank provided by the Germain company Schott.

Russia still possesses a high potential in the development of large-scale optics and mechanics. A new 25m telescope with the segmented mirror was the subject of wide speculations. Strange as it may seem, the country with the world largest territory has no good site for a large astronomical observatory. Any project of the national observatory is therefore faced with the necessity of looking for a foreign partner. We are also studying the possibility of joining one of the large international projects, including the ESO projects OWL, ALMA and others. At the same time, work on a few Russian space astronomical missions is going on well.

The detection of telluric extra-solar planets implies an extremely high contrast long exposure imaging capability at near-infrared and probably visible wavelengths. We present here the core of any Planet Finder instrument which is the extreme AO sub-system. The level of AO correction directly impacts on the exposure time required for planet detection. The extreme adaptive optics system has to correct for the perturbation induced by the atmospheric turbulence as well as for the internal aberrations of the instrument itself. An example of application is proposed in the frame of the EPICS project (XAO system for the ESO OWL telescope).

We describe how the Virtual Observatory (VO) projects in Europe, the USA, Japan, and elsewhere are meeting the challenge of providing simple and efficient access to the data from the world's observational facilities, together with applications and computational resources required to support the analysis of this data. We note the pan-European Euro-VO project and its technological development VO-TECH project which are now in the process of designing the framework for comprehensive access to emerging high data volume facilities such as ESO's VISTA infrared survey telescope.

Science drivers from major new astronomical missions are helping to define the development of the VO. Scientifically this is in terms of developing systems able to meet the demands of the main science programmes shaping the ELTs. VOs must be able to handle the large data streams from the complex multiplexed instruments on the ELTs, and provide access to applications required to analyse/interpret the data. VOs must enable the effective distribution of ELT data to the global community.

Conversely, the rapid development of the Virtual Observatory, offers opportunities for major new projects such at the ELTs. This could be: in the design of their down stream data-flow systems; in terms of opening up access to ‘real-time’ availability of ancillary data flows; in multi-wavelength observational programmes.

We highlight these areas, and give some specific current examples of early VO usage in delivering science from, e.g. the mining of deep multi-wavelength surveys to study the high redshift universe.

For a European ELT, we discuss science requirements and technical options. Science includes from the solar system to the first stars and galaxies. ELT and instrument needs are commented. Design options are discussed for the primary mirror, its segmentation and shape, and an adaptive secondary mirror. Adaptive optics and enclosure are commented.

The capability of extremely large telescopes (ELTs) to resolve stellar populations in distant galaxies has been investigated through simulation. With real stellar clusters as templates, images were created using a realistic point spread function model. Through variation of parameters such as aperture, Strehl ratio and exposure, as well as use of various stellar populations and densities, we study ELT capability to probe galaxy formation and evolution history. We conclude that a 50m ELT should allow study of these processes in the Virgo galaxy cluster.

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).

Supernova remnants are the remains of the outer envelope of the supernova progenitor star. Neutron stars are one of the possible remnants of the region where the explosion detonated. Our knowledge, from galactic studies, of SNRs is well established as their morphology can be understood in terms of the initial blast and out-gassing during the last stages of the progenitor. However, observationally the link between neutron stars and SNRs is only poorly established. During a supernova there are a number of possibilities to produce a condensed remnant - no remnant; a neutron star (pulsar?); a magnetar; a black hole or something more exotic. We do not know what fraction of supernovae go down these possible paths. In the ELT era we will have the first real opportunity to sample the pulsar population in external galaxies and get a more comprehensive survey of optical emission from local pulsars. Such a survey would have significantly reduced biases compared to the current state of radio surveys particularly in the area of pulsar-SNR statistics. Furthermore, a 50m telescope will be able to survey galaxies out to at least 20 Mpc for young SNRs using H$\alpha$:[OIII] and H$\alpha$:[SII] ratios. Currently there are over 1500 radio pulsars detected, 14 of which have been observed at optical wavelengths. Although small in number the family of optical pulsars yield much useful information in bridging the gap between the long wavelength radio emission and the high energy gamma-ray emission from pulsars. Specifically optical techniques are currently the only way of detecting polarisation in the high-energy regime. The advent of ELTs will increase the detection rate of local galactic pulsars and provides the possibility of detecting a significant number of extragalactic optical pulsars. Phenomenologically, the Pacini scaling law predicts $\sim$150 galactic pulsars to have pulsed optical emission with an m$_{V} <$32. Using SKA, Crab-like giant radio pulses should be detectable out to 7 Mpc. In contrast Crab-like pulsars would have a normal peak pulsed m$_{V}\,\sim$31 at 10 Mpc making ELT optical observations more sensitive than radio observations and the best method for extragalactic pulsar discovery. To date only five AXPs have observed IR emission and two optical emission. ELTs will be able to sample the AXP population within the Galaxy as well as the local group again providing better statistics for the birth rate of AXPs compared to ‘normal’ pulsars. A combination of an AXP and ‘normal’ pulsar survey will make a significant contribution to the birthrate question - what fraction of supernovae produce pulsars compared to AXPs and other condensed objects?

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).

Verification of theoretical predictions of an oscillating behavior of the fine-structure constant, $\alpha$, with cosmic time requires high precision measurements at individual redshifts, while in earlier studies the mean $\Delta\alpha/\alpha$ values averaged over wide redshift intervals were usually reported. This requirement can be met via the Single Ion Differential $\alpha$ Measurement (SIDAM) procedure. We apply SIDAM to the FeII lines associated with the damped Ly$\alpha$ system observed at $z = 1.15$ in the spectrum of HE0515–4414. The weighted mean calculated on the base of carefully selected 34 FeII pairs is $\langle \Delta\alpha/\alpha \rangle = (-0.07\,{\pm}\,0.84)\times10^{-6}$. The precision of this estimate represents a large improvement over previous measurements of $\Delta\alpha/\alpha$.

A brief description of the QuantEYE instrument proposed as a focal plane instrument for OWL is given. This instrument is dedicated to the very high speed observation of many active phenomena with a photon counting capability of up to 1GHz. The system samples the beam in 10$\times$10 subpupils, each focused on a fast photon counting detector.

A software for simulations of observations with an Integral Field Spectrograph attached to an extreme adaptive optics system is presented here. This code, written in IDL, has been conceived within the CHEOPS project, a second generation “Planet Finder” instrument for ESO's VLT, and it has been successfully extended to the case of various ELTs in order to test the capabilities in detecting planets of an instrument based on Integral Field Spectroscopy (IFS). We present here, beside a general description of the procedure adopted to simulate IFS observations, a set of significant simulations of detections using OWL for different kinds of planets, in order to understand which objects are the best targets for an IFS-Planet Finder mounted on a 100m-class telescope.