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.

Wide-field spectroscopy, in its various forms, has much to contribute to ELT science, so care is needed in trade-offs between telescope size and field of view. Integral field spectroscopy over large areas at high spatial resolution, and especially multiple integral fields, will be essential tools. For wide-field surveys, next-generation multi-object spectrographs (MOS) on 8m-class telescopes will likely out-perform similar instruments on ELTs, due to the smaller fields of view of the current ELT designs. However, there may be $D^4$ gains for medium-resolution MOS if adaptive optics can provide enhanced ‘seeing’ of $\sim0.1''$ over wide fields. New technologies such as OH-suppression fibres offer revolutionary gains, so there is a difficult balance to be achieved in applying the latest technology and having instruments ready for ELT first light.

Modern optics focuses on photonics and quantum optics, studying individual photons and statistics of photon streams. Those can be complex and carry information beyond that recorded by imaging, spectroscopy, polarimetry or interferometry. Since [almost] all astronomy is based upon the interpretation of subtleties in the light from astronomical sources, quantum optics has the potential of becoming another information channel from the Universe. The observability of quantum statistics increases rapidly with telescope size making photonic astronomy very timely in an era of very large telescopes.

We illustrate the need for planning surveys and coordinated observations, using representative science cases developed for ELTs by the GSMT Science Working Group. We conclude that enabling surveys and coordinated observations is critical to ELTs and that ground-based optical and radio and space-based facilities are needed. A world-wide systems approach is needed to support ELT campaigns. We discuss early planning for ELT operations, based on discussions in progress within the TMT partnership. We conclude that developing an architecture to accommodate end-to-end observation and data management will be essential to achieving near-term and legacy goals.

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.

The stellar Initial Mass Function (IMF) is a quantity which accounts for the distribution of the masses of stars, when they are formed. All the information available on the IMF in the low-mass regime comes from studies of our galaxy alone. Investigations on the content of low-mass stars in other neighbouring galaxies are limited by observational constraints, which do not allow the detection of the fainter stars with statistical significance. Only recently results from observations with the Hubble Space Telescope (HST) of stellar populations in the Large Magellanic Cloud (LMC) down to $\sim$ 0.7 M$_\odot$ confirm systematic variations in the low-mass IMF expected from theoretical considerations (Gouliermis et al. 2005). Direct imaging of resolved stellar populations in massive young clusters throughout the Local Group would be possible with Extremely Large Telescopes (ELTs). Hence, a sizeable sample of young clusters for which IMF variations can be detected would become available. We present our method for testing the efficiency of observations with ELTs in detecting low-mass stars in compact clusters of the Local Group galaxies. We plan to simulate imaging with ELTs and use the results of their photometry in order to investigate the effect on the derived low-mass IMF. This method demonstrates the advantages that will be introduced to crowded field photometry in close-by galaxies with ELTs.

This paper describes a novel concept for the direct detection of exoplanets involving two spacecraft. One spacecraft is an occulter, designed to provide adequate starlight suppression within its shadow. The second spacecraft, a conventional telescope of standard quality, is flown into the shadow of the first and used to collect the light from the target planet. The design of the occulter and its expected performance are discussed. It is shown that is possible to simultaneously achieve the necessary contrast ratios and inner working angles necessary to have a scientifically meaningful mission. A brief discussion of the estimated tolerances indicate that such a mission is feasible in the near term.

The detection of extra-solar planets made by direct imaging is an extremely challenging goal for astronomers due to the possibility to access the physical properties of planets and not only their existence. Using 8 m class telescopes joint with dedicated techniques (such as Simultaneous Differential Imaging aiming to suppress the speckle noise) it is possible, at present, to attain detection limit of 9–11 mag at 0.5 arcsec i.e. to access 3–10 M$_{J}$ planets orbiting around young (100–200 Myr) nearby and late type stars. Searches for extra-solar planets carried out with the present technology are quite fundamental and critical not only for discovery of planets but also because it permits us to put constraints on theories of planets formation and migration. Besides, our understandings of the performances of sophisticated techniques such as the SDI is fundamental to plan new observational strategies, new generation instruments and telescopes. Speckles noise is, indeed, the main source of noise for observations in the NIR and visible and our ability in suppressing it is not so easily scaled at different parameters space. In this contribution I will present the main results that we obtained in on-going searches for planets carried out with NACO and NACO/SDI in the last years. A particular attention will be dedicated in comparing different observational strategies and in the employment of image processing techniques for recognizing, in an automatic way, planet features in deep images obtained with ground-based telescopes and AO facilities.

We present configurations of shaped pupil coronagraphs optimized for a realistic telescope to directly detect extrasolar giant planets in the mid-IR wavelengths. This study is linked to the development of the coronagraphic instrument aboard SPICA (SPace Infrared telescope for Cosmology and Astrophysics). We made a systematic assessment of the performance of the “checkerboard” and “concentric ring” masks, introducing a large central obstruction (C.O.) due to a secondary mirror and its related support spiders. With a small secondary mirror, we also propose a modification to the original symmetrical checkerboard apodization, which enables us to achieve a $10^{-7}$ contrast level at $4.0\ \lambda/D$. The transmission through the optimal binary masks exhibits abrupt increases and plateaus as the inner working angle (IWA) is increased. We attribute these properties of binary apodization function to the existence of threshold IWAs that allow large openings in the pupil.

ELT's, interferometers and their multi-aperture direct-imaging form called hypertelescopes are expected to provide images of exo-Earths, and even resolved images in the latter case. Both terrestrial and space versions are potentially usable, although high-performance adaptive optics will be needed on Earth. Current advances in the way of hypertelescopes on Earth and in space are briefly described.

Nulling interferometers such as Darwin or TPF will require a rather sophisticated data processing in order to perform a reliable planet detection and characterization. We propose a Bayesian method, which follows the maximum a posteriori (MAP) approach, to solve this problem. Our method accounts for the noise statistics and optimally combines the data from a nulling interferometer at all observed wavelengths to perform reliable planet detection. The problem to be solved is however multi-modal. We show how, in practice, the global optimum of the MAP criterion can be found by our method; the latter also provides the most likely spectral energy distributions of all planets. Additionally, we show that a proper regularization allows us to achieve an improved robustness of the detection and could lead to shorter observation times.

The standard AIC (Achromatic Interfero Coronagraph) has a “coudé” geometry (the output beam leaves at right angle from the input beam). Thus, some extra optical parts are required to fit such a device within the optical train between a telescope and its IR camera. To avoid this drawback, we present two mono-axial variants of the AIC.

The high contrast (typically $10^{10}$) and small angular separation between a planet and its parent star are the main challenges that need to be overcome to detect and characterize Earth-like planets around the nearest stars. Therefore, exoplanet imaging requires the use of a coronagraph, that ideally efficiently cancels the light from the star and has minimal influence on the planet image. The Phase Induced Amplitude Apodization Coronagraph relies on pupil apodization by geometrical remapping of the flux in the pupil plane. This method combines the advantages of classical pupil apodization with high throughput ($\approx$100%) and high angular resolution (${\approx}\lambda/D$), and has some unique advantages over most coronagraphs, such as low chromaticity, low sensitivity to stellar angular size and to small pointing errors. As a result, planet detection time is about 50–100 times shorter in comparison with classical coronagraphic techniques (Martinache et al. 2005).

Both the advantages of the PIAAC and the main factors affecting the performance of the coronograph will be examined in our laboratory experiment in which high quality PIAA optics wilkl be combined with wavefront control to demonstrate achromatic high contrast imaging ($10^6$ or more) at small angular separation (less than $2\lambda/D$). We present here a description and current status of this experiment together with a short analyses of the main factors affecting the performance of the coronograph.

In our search for clues as to the nature of the exosphere of HD209458 (Moutou et al., 2001 ; Moutou et al., 2003, Iro et al., 2004), we have acquired VLT/UVES data during an ambitious observational campaign performed in June-September 2002 and covering 6 transits of the exoplanet. The resolving power was R=100000 in the 0.475-0.68 micron range. We search for ions and neutral molecules (such as H2O+, CO+, CH+, etc) originating in the planets exosphere and located in the evaporated material around the planet, occulting its primary star. We present in this paper a tentative search in the spectral regions where features of sodium or H$_2$O$^+$ can be present.

We discuss the evolution of the habitable zone around low mass and intermediate mass stars as they evolve off the main sequence. This work shows that this new class of stars should be included in the search for life because if planets could be found in their habitable zones, and these planets showed evidence for life, it is possible to empirically determine a lower limit to the timescale for the formation of life. This time scale is not well determined from the study of the Earth (or planets around main sequence stars), as life formation initially occurred during a period of heavy bombardment from comets and asteroids during the formation of the solar system. Our initial research was recently published (Lopez, Schneider, & Danchi 2005). We will describe our work in progress, in which we perform calculations and simulations aiming to demonstrate the potential of TPF and Darwin for the search and characterization of planets around evolved stars.

The performance of high contrast imaging systems is very often limited by the presence of static speckles in the point-spread function of the central source. Several techniques have already been proposed to discriminate a faint companion from these residual speckles. These techniques used different criteria to separate a speckle from a companion: polarization, spectral information or coherence. Here, we propose a new imaging device, the Self-Coherent Camera (SCC), that is based on the lack of coherence between the stellar light and the planet that is searched for. This SCC is a simple instrument that allows us to reach the fundamental limitation of the photon noise by calibrating the speckles in the recorded images. After the description of the general problem of discriminating speckles from planets, we will explain the principle of the SCC. Then, we will analyze the different limitations of this technique as well as the performance that can be reached with current telescopes.

In this paper we first introduce Pupil Replication. We then show simulation results for a pupil replicated classical coronagraph. Furthermore, we show experimental results as a proof of concept as well as a preliminary design for the replication optics.

The Phase-Induced Amplitude Coronagraph (PIAAC) uses a lossless beam apodization, performed by aspheric mirrors, to produce a high contrast PSF. Thanks to the lossless apodization, this concept offers a unique combination of high theoretical throughput ($\approx$100%), high angular resolution ($\lambda/d$), small inner working angle (${\approx} 1.5 \lambda/d$), low chromaticity and low sensitivity to pointing errors or angular star diameter. Together, these characteristics make the PRC an ideal choice for direct imaging of extrasolar terrestrial planets (ETPs) from space. We show that a visible telescope smaller than 4m would then achieve the goals of the TPF mission, while other coronagraphs considered for TPF require telescope diameters typically 2 to 3 times larger. On a large size (8m) space telescope, ETPs can be searched for around a significantly larger sample of stars, thus enabling a much higher scientific return.

The ALADDIN concept is an integrated Antarctic-based L-band experiment whose purpose is to demonstrate nulling interferometry and to prepare the DARWIN mission. Because of their privileged location, the relatively modest collectors (1 m) and baseline (up to 40 m) are sufficient to achieve a sensitivity (in terms of detectable zodi levels) which is about twice better than that of a nulling instrument on a large interferometer (such as GENIE at the VLTI), and to reach the 20-zodi threshold value identified to carry out the DARWIN precursor science. These numbers are based on a preliminary design study by Alcatel Alenia Space and were obtained using the same simulation software as the one employed for GENIE. The integrated design enables top-level optimization and full access to the light collectors for the duration of the experiment, while reducing the complexity of the nulling breadboard.

We present experimental results of a new procedure of measurement and pre-compensation of the non-common path aberrations in Adaptive Optics. A significant Strehl ratio increase (from 70 to 95.5 % in R band) is demonstrated.