APEX, the Atacama Pathfinder EXperiment, has been successfully commissioned and is operational. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the world’s outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of only 17–18 µm makes APEX suitable for observations up to 200 µm, through all atmospheric submm windows accessible from the ground. First scientific results will be presented in the accompanying papers of this special issue.

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The Atacama Pathfinder EXperiment (APEX) : a new submillimeter facility for southern skies

1 .1 The Function of Astronomical Telescopes Astronomical telescopes are devices which collect as much radiation from astronomical (stellar) objects and put it in as sharp (small) an image as possible. Both collecting area and angular resolution play a role. The relative merit of these two functions has changed over the years in optical astronomy, with the angular resolution initially dominating and then, as the atmospheric seeing limit was reached, the collecting area becoming the most important factor. Therefore it is the habit these days to express the quality of a telescope by its (collecting) diameter rather than by its angular resolution. With the introduction of techniques that overcome the limits set by atmospheric seeing, the emphasis is changing back to angular res­ olution. This time, however, the constraint is set by the diffraction limit of the telescope so that both angular resolution and collecting power of a telescope will be determined by its diameter. Both telescope functions will therefore go hand-in-hand. Although speckle image reconstruction techniques have been successful in giving diffraction-limited images, the most powerful and promising technique for all astronomical applications is the one using adaptive optics. For an unresolved image, this technique puts most of the collected photons in as small an image as possible, thereby allowing better discrimination against the sky background, improving high spectral and spatial resolution spectroscopy, and enhancing inter-

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ADAPTIVE OPTICS FOR ASTRONOMY: Principles, Performance, and Applications

We describe a new code for the deep analysis of stellar elds, designed for Adaptive Optics (AO) Nyquist- sampled images with high and low Strehl ratio. The Point Spread Function (PSF) is extracted directly from the im- age frame, to take into account the actual structure of the instrumental response and the atmospheric eects. The code is written in IDL language and organized in the form of a self-contained widget-based application, pro- vided with a series of tools for data visualization and anal- ysis. A description of the method and some applications to AO data are presented.

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Analysis of isoplanatic high resolution stellar fields by the StarFinder code

This article reviews different deconvolution methods. The all-pervasive presence of noise is what makes deconvolution particularly difficult. The diversity of resulting algorithms reflects different ways of estimating the true signal under various idealizations of its properties. Different ways of approaching signal recovery are based on different instrumental noise models, whether the astronomical objects are pointlike or extended, and indeed on the computational resources available to the analyst. We present a number of recent results in this survey of signal restoration, including in the areas of superresolution and dithering. In particular, we show that most recent published work has consisted of incorporating some form of multiresolution in the deconvolution process.

https://www.astro.rug.nl/~peletier/signal/starck.pdf

Deconvolution in astronomy: A review

Context: Optical long-baseline interferometry is moving a crucial step forward with the advent of general-user scientific instruments that equip large aperture and hectometric baseline facilities, such as the Very Large Telescope Interferometer (VLTI). Aims: AMBER is one of the VLTI instruments that combines up to three beams with low, moderate and high spectral resolutions in order to provide milli-arcsecond spatial resolution for compact astrophysical sources in the near-infrared wavelength domain. Its main specifications are based on three key programs on young stellar objects, active galactic nuclei central regions, masses, and spectra of hot extra-solar planets. Methods: These key science goals led to scientific specifications, which were used to propose and then validate the instrument concept. AMBER uses single-mode fibers to filter the entrance signal and to reach highly accurate, multiaxial three-beam combination, yielding three baselines and a closure phase, three spectral dispersive elements, and specific self-calibration procedures. Results: The AMBER measurements yield spectrally dispersed calibrated visibilities, color-differential complex visibilities, and a closure phase allows astronomers to contemplate rudimentary imaging and highly accurate visibility and phase differential measurements. AMBER was installed in 2004 at the Paranal Observatory. We describe here the present implementation of the instrument in the configuration with which the astronomical community can access it. Conclusions: .After two years of commissioning tests and preliminary observations, AMBER has produced its first refereed publications, allowing assessment of its scientific potential.

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AMBER, the near-infrared spectro-interferometric three-telescope VLTI instrument

I: Basic principles.- to Fourier Optics and Coherence.- Optical Propagation and Image Formation Through the Turbulent Atmosphere.- Radio Telescopes: Basic Concepts.- Continuously Movable Telescopes for Optical Interferometry.- II: Module and phase estimation, image reconstruction.- Amplitude Estimation from Speckle Interferometry.- Notes on Noise Calibration of Speckle Imagery.- Amplitude Estimation from Diluted Array Interferometry.- First Order Imaging Methods: An Introduction.- Speckle Imaging with the Papa Detector and the Knox-Thompson Algorithm.- Phase Closure Imaging - Theory & Practice.- Speckle Masking, Speckle Spectroscopy, and Optical Aperture Synthesis.- Merits of Phase Restoration Methods.- Pupil-Plane Interferometry.- Real-Time Wavefront Sensing and Adaptive Optics.- Differential Interferometry.- Image Restoration.- Deconvolution Ancient and (Very) Modern.- III: Scientific and technical prospects.- Astrophysical Programs for High Angular Resolution Optical Interferometry.- Frontiers of Interferometry.- Perspectives in Optical Interferometry.- IV: Contributed seminars and posters.- Cophasing Telescope Arrays.- Some Thoughts on the Combination of Beams in Interferometers Using Telescopes of Unequal Size.- Real-Time Image Sharpening.- Scasis, Diffraction-Limited Imaging Using a Redundant Pupil Plane-Interferometer.- Aperture Synthesis in Space: Overview and Results from the Esa Study Group.- Estimation and Reconstruction from Aberrated Amplitude Interferometer Measurements.- Lunar Occultations or Milliarcsecond Resolution and Their Relation to Interferometric Techniques at Present and in Future.- List of posters.

D. M. Alloin

Papers

Diffraction-limited imaging with very large telescopes