New Instruments Shed Light On Astronomy's Future

KONA, HAWAII-The snowy 14,000-foot summit of Mauna Kea, with its bevy of powerful telescopes including the just-completed Keck, provided an appropriate backdrop for a recent ocean-side gathering of astronomers and engineers to discuss "Astronomical Telescopes and Instrumentation for the 21st Century." Last month's meeting was organized by the Society of Photo-Optical Instrumentation Engineers, and included a status report on liquid mirror telescopes, discussion of a space observatory that may make optical and x-ray astronomers best friends, and a modest proposal to cover the globe with a network of small, automated telescopes.     

TELESCOPES: Astronomers Overcome 'Aperture Envy'

Many users of small telescopes are disturbed by the trend of shutting down smaller instruments in order to help fund bigger and bolder ground-based telescopes. Small telescopes can thrive in the shadow of giant new observatories, they say--but only if they are adapted to specialized projects. Telescopes with apertures of 2 meters or less have unique abilities to monitor broad swaths of the sky and stare at the same objects night after night, sometimes for years; various teams are turning small telescopes into robots, creating networks that span the globe and devoting them to survey projects that big telescopes don't have a prayer of tackling.     

Interferometric coupling of the Keck telescopes with single-mode fibers

Here we report successful interferometric coupling of two large telescopes with single-mode fibers. Interference fringes were obtained in the 2- to 2.3-micrometer wavelength range on the star 107 Herculis by using the two Keck 10-meter telescopes, each feeding their common interferometric focus with 300 meters of single-mode fibers. This experiment demonstrates the potential of fibers for future kilometric arrays of telescopes and is the first step toward the 'OHANA (Optical Hawaiian Array for Nanoradian Astronomy) interferometer at the Mauna Kea observatory in Hawaii. It opens the way to sensitive optical imagers with resolutions below 1 milli-arc second. Our experimental setup can be directly extended to large telescopes separated by many hundreds of meters.     

Telescopes and automation

In stellar astronomy, the future of automation techniques lies in their application to the largest telescopes. In such a case, the cost of automation is small compared to that of the telescope project as a whole, and is well repaid in the savings in observing time. A "stand-alone" capability for manual operation in event of computer breakdown is usually provided for systems like this; an example is the 107-inch (272-cm) telescope under construction at the McDonald Observatory of the University of Texas, which will be associated with an IBM 1800 computer.     

Active optics, adaptive optics, and laser guide stars

Optical astronomy is crucial to our understanding of the universe, but the capabilities of ground-based telescopes are severely limited by the effects of telescope errors and of the atmosphere on the passage of light. Recently, it has become possible to construct inbuilt corrective devices that can compensate for both types of degradations as observations are conducted. For full use of the newly emerged class of 8-meter telescopes, such active corrective capabilities, known as active and adaptive optics, are essential. Some physical limitations in the adaptive optics field can be overcome by artificially created reference stars, called laser guide stars. These new technologies have lately been applied with success to some medium and very large telescopes.     

Radio astronomy with very large arrray

The construction of the Very Large Array of radio telescopes has been completed, and this new research instrument is now being used to make radio images of astronomical objects with a resolution comparable to or better than that of ground-based optical telescopes. The role of the Very Large Array in current and future research is discussed both in principle and in terms of a sample of observing projects.     

Infrared Astronomy After IRAS

The 250,000 sources in the recently issued Infrared Astronomy Satellite (IRAS) all-sky infrared catalog are a challenge to astronomy. Many of these sources will be studied with existing and planned ground-based and airborne telescopes, but many others can no longer even be detected now that IRAS has ceased to operate. As anticipated by advisory panels of the National Academy of Sciences for a decade, study of the IRAS sources will require the Space Infrared Telescope Facility (SIRTF), a cooled, pointed telescope in space. This instrument may be the key to our understanding of cosmic birth-the formation of planets, stars, galaxies, active galactic nuclei, and quasars. Compared with IRAS and existing telescopes, SIRTF's power derives from a thousandfold gain in sensitivity over five octaves of the spectrum.     

Origins

The farthest of the galaxies that can be seen through the large ground-based telescopes of modern astronomy, such as those on La Palma in the Canary Islands, are so far away that they appear as they did close to the time of the origin of the universe, perhaps some 10 billion years ago. Much has been learned, and much has still to be learned, about the young universe from optical and radio telescopes, but these instruments cannot be used to look directly at the universe in its first few hundred thousand years. Instead, they are used to search the relatively recent past for relics of much earlier times. Together with experiments planned for the next generation of elementary particle accelerators, astronomical observations should continue to extend what is known about the universe backward in time to the Big Bang and may eventually help to reveal the origins of the physical laws that govern the universe.     

Neutrino astrophysics: a new tool for exploring the universe

In the past four decades a new type of astronomy has emerged, where instead of looking up into the sky, "telescopes" are buried miles underground or deep under water or ice and search not for photons (that is, light), but rather for particles called neutrinos. Neutrinos are nearly massless particles that interact very weakly with matter. The detection of neutrinos emitted by the Sun and by a nearby supernova provided direct tests of the theory of stellar evolution and led to modifications of the standard model describing the properties of elementary particles. At present, several very large neutrino detectors are being constructed, aiming at the detection of the most powerful sources of energy and particles in the universe. The hope is that the detection of neutrinos from these sources, which are extra-Galactic and are most likely powered by mass accretion onto black holes, will not only allow study of the sources, but, much like solar neutrinos, will also provide new information about fundamental properties of matter.     

Soft X-ray Images of the Solar Corona with a Normal-Incidence Cassegrain Multilayer Telescope

High-resolution images of the sun in the soft x-ray to extreme ultraviolet(EUV) regime have been obtained with normal-incidence Cassegrain multilayer telescopes operated from a sounding rocket in space. The inherent energy-selective property of multilayer-coated optics allowed distinct groups of emission lines to be isolated in the solar corona and the transition region. The Cassegrain telescopes provided images in bands centered at 173 and 256 angstroms. The bandpass centered at 173 angstroms is dominated by emission from the ions Fe IX Fe X. This emission is from coronal plasma in the temperature range 0.8 x 10(6) to 1.4 x 10(6)K. The images have angular resolution of about 1.0 to 1.5 arc seconds, and show no degradation because of x-ray scattering. Many features of coronal structure, including magnetically confined loops of hot plasma, coronal plumes, polar coronal holes, faint structures on the size scale of supergranulation and smaller, and features due to overlying cool prominences are visible in the images. The density structure of polar plumes, which are thought to contribute to the solar wind, has been derived from the observations out to 1.7 solar radii.     

Long base line interferometry: a new technique

The technique of using magnetic-tape recorders and atomic frequency standards to operate two widely separated radio telescopes as a phase-coherent interferometer when the stations have no radio-frequency connecting link has been successfully tested at the National Research Council of Canada's Algonquin Radio Observatory.     

Spectral line interferometry with independent time standards at stations separated by 845 kilometers

An upper limit of 0.02 second of arc has been determined for a hydroxyl radical (OH) emission region associated with the radio source W3, with the use of a Michelson interferometer consisting of two radio telescopes 845 kilometers apart. Timing was provided at the stations by independent atomic frequency standards. The 1665-megahertz radiation was translated to video frequency and recorded digitally on magnetic tapes which were later processed by computer, yielding fringe phase and amplitude as a function of frequency over the received bandwidth.     

Planetary science. The glitter of distant seas

It has long been known that Saturn's largest moon, Titan, has a thick nitrogen atmosphere, which obscures the underlying surface. In his Perspective, Lorenz highlights the report by Campbell et al., who have used the giant Arecibo and Green Bank radio telescopes as a radar to probe Titan's hidden surface. The surface appears to be distinct from those of the icy satellites of Jupiter, in both brightness and polarization. The new data show sharp spikes in the reflected microwave spectrum, indicating large, smooth areas of radar-dark material. These features suggest the widespread existence of lakes or seas of liquid hydrocarbons on Titan.     

The very-high-energy gamma-ray sky

Over the past few years, very-high-energy gamma-ray astronomy has emerged as a truly observational discipline, with many detected sources representing different galactic and extragalactic source populations-supernova remnants, pulsar wind nebulae, giant molecular clouds, star formation regions, compact binary systems, and active galactic nuclei. It is expected that observations with the next generation of stereoscopic arrays of imaging atmospheric Cherenkov telescopes over a very broad energy range from 10(10) to 10(15) electron volts will dramatically increase the number of very-high-energy gamma-ray sources, thus having a huge impact on the development of astrophysics, cosmology, and particle astrophysics.     

Astrophysics from the moon

The surface of the moon would be an excellent location for astronomical telescopes, and, if a lunar base were to be established, the construction and maintenance of instruments would become feasible. The prospects are reviewed, with particular attention given to large optical aperturesynthesis instruments analogous to the Very Large Array of the National Radio Astronomy Observatory. Typical parameters for a particular system are presented.     

Real-time, very-long-baseline interferometry based on the use of a communications satellite

The Hermes satellite, a joint Canadian-American program, has been used to provide a communication channel between radio telescopes in West Virginia and Ontario, for very-long-baseline interferometry (VLBI). This system makes possible instantaneous correlation of the data as well as a sensitivity substantially better than that of earlier VLBI systems, by virtue of a broader observational bandwidth. With the use of a geostationary communications satellite it is possible to eliminate the tape recorders and the most troublesome part of the postobservational data processing. A further possibility is the development of a phase-coherent interferometer.     

Frequencies of occultations of stars by planets, satellites, and asteroids

Calculations show that several occultations of stars by the large satellites of the outer planets, Pluto, and the large asteroids could be observed each decade with existing equipment at Earth-based telescopes. A systematic program of occultation predictions and observations is urged in order to improve our knowledge about the atmospheres, sizes, shapes, topography, and positions of these poorly understood bodies, in support of forthcoming spacecraft missions to the outer solar system.     

Copernicus crater central peak: lunar mountain of unique composition

Olivine is identified as the major mafic mineral in a central peak of Copernicus crater. Information on the mineral assemblages of such unsampled lunar surface material is provided by near infrared reflectance spectra (0.7 to 2.5 micrometers) obtained with Earth-based telescopes. The composition of the deep-seated material comprising the Copernicus central peak is unique among measured areas. Other lunar terra areas and the wall of Copernicus exhibit spectral characteristics of mineral assemblages comparable to the feldspathic breccias returned by the Apollo missions, with low-calcium orthopyroxene being the major mafic mineral.     

Possible production of high-energy gamma rays from proton acceleration in the extragalactic radio source markarian 501

The active galaxy Markarian 501 was discovered with air-Cerenkov telescopes at photon energies of 10 tera-electron volts. Such high energies may indicate that the gamma rays from Markarian 501 are due to the acceleration of protons rather than electrons. Furthermore, the observed absence of gamma ray attenuation due to electron-positron pair production in collisions with cosmic infrared photons implies a limit of 2 to 4 nanowatts per square meter per steradian for the energy flux of an extragalactic infrared radiation background at a wavelength of 25 micrometers. This limit provides important clues about the epoch of galaxy formation.     

Transcontinental baselines and the rotation of the Earth measured by radio interferometry

Nine separate very-long-baseline interferometry (VLBI) experiments, carried out in 1972 and 1973 with radio telescopes 3900 kilometers apart, yielded values for the baseline length with a root-mean-square deviation about the mean of less than 20 centitneters. The corresponding fractional spread is about five parts in 10(8). Changes in universal time and in polar motion were also detertnined accurately from these data; the root-mean-square scatter of these results with respect to those based on optical methods were 2.9 milliseconds and 1.3 meters, respectively. Solid-earth tides were apparently detected, but no useful estimate of their amplituide was extracted.