A few decades ago, there appeared to be two distinct evolutionary paths for astronomical imaging instruments: toward larger telescopes and longer-baseline interferometers. A telescope would be based on a single massive monolithic mirror, for which the limiting size appeared to be on the order of 8 m. An interferometer could involve two or more telescopes spaced tens or hundreds of meters apart, possibly up to ten kilometers. More recently, the emphasis in interferometers has turned to larger numbers of subapertures rather than larger sizes or longer baselines, in order to improve (u, v) coverage (e.g. the Magdalena Ridge Observatory Interferometer), and considerable effort is being devoted to planning space interferometers, which will be discussed later in this chapter (section 12.2). On the other hand, following the success of the mosaic mirror Keck telescopes of 10-m diameter, the elements of which are carried by a common pointing mount, projects for larger versions up to 100 m, are now being studied under the generic name “Extremely Large Telescopes” (ELT).

The idea of using measurements of the correlation between temporal fluctuations in light intensity at different field points was proposed by R. Hanbury Brown as an alternative to interferometry for measuring the spatial coherence function and therefore obtaining stellar data with high resolution. He called it intensity interferometry. Basically, in terms which should by now be familiar to readers of this book, an extended body of angular diameter α, consisting of many incoherently emitting sources, produces a speckled wavefront at the observer in which the speckles have typical size λ/α and typical lifetime τc. A pair of observers separated by a distance considerably less than λ/α are in the same speckle and therefore see the same intensity fluctuations. Observers separated by larger distances are likely to be in different speckles and see fluctuations with lesser correlation. The method was originally used for radio astronomy, in order to overcome the problem of providing identical phase references at two receivers separated by a very long distance (Hanbury Brown et al. 1952). It was then noticed that the measured correlations were immune to severe fluctuations produced by ionospheric instabilities, since these were in a frequency range very different from those of the intensity fluctuations being correlated. This provided the incentive to extend the method to the optical region. One should remember that at that time, the Michelson stellar interferometer was the only interferometric instrument which had provided resolution exceeding the atmospherically limited seeing, having successfully measured the diameters of six stars, and Pease's attempts to extend the baseline from 6 to 15 meters had proved impractical because of problems of atmospheric turbulence and mechanical stability.

Aperture synthesis Aperture synthesis is the way that the Van Cittert–Zernike theorem (section 3.3) is used in practice to get higher resolution images than a single large aperture will allow. Although Michelson's stellar interferometer was the first implementation of the concept, and occurred before the formalization of coherence theory, its real application began in the late 1940s in radio astronomy, where today it is responsible for almost all high-resolution images. At radio frequencies the problem of getting high angular resolution is very acute because of the long wavelengths involved, and Michelson's stellar interferometer inspired Martin Ryle (1952) to use the same idea in radio astronomy. And so, while optical aperture synthesis languished for 60 years for lack of suitable electronics, the radio astronomical applications of the technique blossomed. There are several excellent texts on the theory and practice of aperture synthesis, mainly directed to the radio regime, such as Thompson (2001), Perley and Schwab (1989) and Rohlfs (1996), which will give the reader insight not only into the principles but also the techniques involved.

Interferometry has already provided many scientific results that could be obtained in no other way. In this chapter, we briefly discuss some of the more prominent results to date. This is not in any way meant to provide a comprehensive review, but is rather only a sampling of the kinds of science that has been done. As interferometry becomes more mature and the new interferometric arrays become operational, the scientific output is expected to increase dramatically, and one hopes that this review will rapidly become out of date!

One of two approaches to implementing NASA's Terrestrial Planet Finder is to build a space telescope that utilizes the techniques of coronagraphy and apodization to suppress diffraction and image exo-planets. We present a method for calculation of a telescope's apodizer which suppresses the side lobes of the image of a star so as to optimally detect an Earth-like planet. Given the shape of a telescope's aperture and given a search region for a detector, we solve an integral equation to determine an amplitude modulation (an apodizer) which suppresses the star's energy in the focal plane search region. The method is quite general and yields as special cases the product apodizer reported by Nisenson and Papaliolios (2001) and the Prolate spheroidal apodizer of Kasdin et al (2002), and Aime et al (2002). We show computer simulations of the apodizers and the corresponding point spread functions for various aperture-detector configurations.

ESPI has been proposed for direct imaging and spectral analysis of giant planets orbiting solar-type stars. ESPI extends the concept suggested by Nisenson and Papaliolios (2001) for a square aperture apodized telescope that has sufficient dynamic range to directly detect exo-planets. With a 1.5 M square mirror, ESPI can deliver high dynamic range imagery as close as 0.3 arcseconds to bright sources, permitting a sensitive search for exoplanets around nearby stars and a study of their characteristics in reflected light.

The Extra-Solar Planetary Imager (ESPI) is envisioned as a space based, high dynamic range, visible imager capable of detecting Jovian like planets. Initially proposed as a NASA Midex (NASA/Medium Class Explorer) mission (PI:Gary Melnick), as a space-based 1.5 x 1.5 m(2) Jacquinot apodized square aperture telescope. The combination of apodization and a square aperture telescope reduces the diffracted light from a bright central source increasing the planetary to stellar contrast over much of the telescope focal plane. As a result, observations of very faint astronomical objects next to bright sources with angular separations as small as 0.32 arcseconds become possible. This permits a sensitive search for exo-planets in reflected light. ESPI is capable of detecting a Jupiter-like planet in a relatively long-period orbit around as many as 160 to 175 stars with a signal-to-noise ratio > 5 in observations lasting maximally 100 hours per star out to similar to16 parsecs. We discuss the scientific ramifications, an overview of the system design including apodizing a square aperture, signal to noise issues and the effect of wavefront errors and the scalability of ESPI with respect to NASA s Terrestrial Planet Finder mission.

Magnetic elements on the quiet Sun are buffeted by convective flows that cause lateral motions on timescales of minutes. The magnetic elements can be observed as bright points (BPs) in the G band at 4305 Å. We present observations of BPs based on a long sequence of G-band images recorded with the Dutch Open Telescope and postprocessed using speckle-masking techniques. From these images we measured the proper motions of isolated BPs and derived the autocorrelation function of their velocity relative to the solar granulation pattern. The accuracy of BP position measurements is estimated to be less than 23 km on the Sun. The rms velocity of BPs (corrected for measurement errors) is about 0.89 km s-1, and the correlation time of BP motions is about 60 s. This rms velocity is about 3 times the velocity measured using cork tracking, almost certainly due to the fact that isolated BPs move more rapidly than clusters of BPs. We also searched for evidence of vorticity in the motions of G-band BPs.

The Extrasolar Planet Observatory (ExPO) is envisioned as a Discovery-class space telescope for the direct detection and characterization of extra-solar planets. ExPO would also demonstrate the feasibility of a number of technologies which could be critical to the ultimate success of the Terrestrial. Planet Finder (TPF) mission. ExPO would detect a wide range of planet types (gas planets, ice planets, super-Earths and Earth-like planets) in the visible and near infrared, and do spectrophotometry and spectroscopy on. many of the detected objects.). The apodized square aperture (ASA) coronagraphic space telescope is designed to resolve faint companions near much brighter point-like sources by achieving very high dynamic range (contrast >10(9)) imaging at separations as small as 0.1 arcsec.

We present radial-velocity measurements obtained with the ELODIE and AFOE spectrographs for GJ 777 A (HD 190360), a metal-rich ((Fe/H)= 0.25) nearby (d= 15.9 pc) star in a stellar binary system. A long-period low radial-velocity amplitude variation is detected revealing the presence of a Jovian planetary companion. Some of the orbital elements remain weakly constrained because of the smallness of the signal compared to our instrumental precision. The detailed orbital shape is therefore not well established. We present our best fitted orbital solution: an eccentric ( e= 0.48) 10.7-year orbit. The minimum mass of the companion is 1.33 MJup.

Received ; accepted – 2 – ABSTRACT One of two approaches to implementing NASA's Terrestrial Planet Finder is to build a space telescope that utilizes the techniques of coronagraphy and apodization to suppress diffraction and image exo-planets. We present a method for calculation of a telescope's apodizer which suppresses the side lobes of the image of a star so as to optimally detect an Earth-like planet. Given the shape of a telescope's aperture and given a search region for a detector, we solve an integral equation to determine an amplitude modulation (an apodizer) which suppresses the star's energy in the focal plane search region. The method is quite general and yields as special cases the product apodizer reported by Nisenson and Papaliolios (2001) and the Prolate spheroidal apodizer of Kasdin et al (2002), and Aime et al (2002). We show computer simulations of the apodizers and the corresponding point spread functions for various aperture-detector configurations.

The mission of NASA's Terrestrial Planet Finder (TPF) is to find Earth-like planets orbiting other stars and characterize the atmospheres of these planets using spectroscopy. Because of the enormous brightness ratio between the star and the reflected light from the planet, techniques must be found to reduce the brightness of the star. The current favorite approach to doing this is with interferometry: interfering the light from two or more separated telescopes with a π phase shift, nulling out the starlight. While this technique can, in principle, achieve the required dynamic range, building a space interferometer that has the necessary characteristics poses immense technical difficulties. In this paper, we suggest a much simpler approach to achieving the required dynamic range. By simply adjusting the transmissive shape of a telescope aperture, the intensity in large regions around the stellar image can be reduced nearly to zero. This approach could lead to construction of a TPF using conventional technologies, requiring space optics on a much smaller scale than the current TPF approach.