Earth-based Telescopes Research Articles

Earth-based optical imaging of Mercury

In recent years, considerable progress has been achieved in producing resolved images of Mercury electronically with short exposures at Earth-based telescopes. For the purpose of obtaining images of the unknown portion of Mercury, the previously started series of observations of this planet by the short exposure method was continued. About 20,000 electronic images of Mercury have been acquired on 1–2 May 2002 under good meteorological conditions during the evening elongation. The phase angle of Mercury was 95–99° and the observed range of longitudes was 210–285°W. Observations were carried out using Ritchy–Chrétien telescope ( D = 1.29 m, F = 9.86 m) with the KS 19 filter cutting wavelengths shorter than about 700 nm. The planet’s disk was seen, on average, at an angle of 7.7″. A CCD with a pixel size of 7.4 × 7.4 ncm in the regime of short exposures was used. By processing a great number of electronic images, a sufficiently distinct synthesized image of the unknown portion of Mercury’s surface was obtained. The most prominent formation in this region is a giant basin (or cratered “mare”) centered at about 8°N, 280°W, which was given a working name “Skinakas basin” (after the name of the observatory where observations were made). By its size, the interior part of this basin exceeds the largest lunar Mare Imbrium. As opposed to Mare Imbrium, the Skinakas basin is presumably of impact origin. Its relief resembles that of Caloris Planitia but the size is much larger. A series of smaller formations are also seen on synthesized images. The resolution obtained on the surface of Mercury is about 100 km, which is close to the telescope diffraction limit. Also considered is the synthesized image obtained at the Mount Bigelow Observatory, on December 4, 2003 (Ritchy–Chrétien telescope, D = 1.54 m, F = 20.79 m, using the same CCD camera).

The VSOP 5 GHz Active Galactic Nucleus Survey. II. Data Calibration and Imaging

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with sub-milliarcsecond angular resolution using an orbiting 8 m telescope, HALCA, and global arrays of Earth-based telescopes. Approximately 25% of the observing time has been devoted to a survey of compact active galactic nuclei (AGNs) at 5 GHz that are stronger than 1 Jy—the VSOP AGN Survey. This paper, the second in a series, describes the data calibration, source detection, self-calibration, imaging, and modeling and gives examples illustrating the problems specific to space VLBI. The VSOP Survey Web site, which contains all results and calibrated data, is described.

The VSOP 5 GHz Active Galactic Nucleus Survey. III. Imaging Results for the First 102 Sources

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with sub-milliarcsec resolution using an orbiting 8 m telescope, HALCA, along with global arrays of Earth-based telescopes. Approximately 25% of the observing time is devoted to a survey of compact active galactic nuclei (AGNs) that are stronger than 1 Jy at 5 GHz-the VSOP AGN Survey. This paper, the third in the series, presents the results from the analysis of the first 102 Survey sources. We present high-resolution images and plots of visibility amplitude versus projected baseline length. In addition, model-fit parameters to the primary radio components are listed, and from these the angular size and brightness temperature for the radio cores are calculated. For those sources for which we were able to determine the source frame core brightness temperature, a significant fraction (53 out of 98) have a source frame core brightness temperature in excess of 10(12) K. The maximum source frame core brightness temperature we observed was 1.2x10(13) K. Explaining a brightness temperature this high requires an extreme amount of relativistic Doppler beaming. Since the maximum brightness temperature one is able to determine using only ground-based arrays is of the order of 10(12) K, our results confirm the necessity of using space VLBI to explore the extremely high brightness temperature regime.

Back to basics: naked-eye astronomical observation

For pupils of both sexes and all ages from about six upwards, the subject of Astronomy holds many fascinations - the rapid changes in knowledge, the large resource of available IT packages and above all the beautiful pictures from Hubble and the large Earth-based telescopes. This article, however, stresses the excitement and importance of naked-eye (unaided) first-hand observation, where light pollution allows, and suggests some techniques that may be used to enthuse and introduce youngsters to the glory of the night sky without recourse to computer screens.

Isotopic Fractionation by Gravitational Escape

Present natural data bases for abundances of the isotopic compositions of noble gases, carbon and nitrogen inventories can be found in the Sun, the solar wind, meteorites and the planetary atmospheres and crustal reservoirs. Mass distributions in the various volatile reservoirs provide boundary conditions which must be satisfied in modelling the history of the present atmospheres. Such boundary conditions are constraints posed by comparison of isotopic ratios in primordial volatile sources with the isotopic pattern which was found on the planets and their satellites. Observations from space missions and Earth-based spectroscopic telescope observations of Venus, Mars and Saturn’s major satellite Titan show that the atmospheric evolution of these planetary bodies to their present states was affected by processes capable of fractionating their elements and isotopes. The isotope ratios of D/H in the atmospheres of Venus and Mars indicate evidence for their planetary water inventories. Venus’ H2O content may have been at least 0.3% of a terrestrial ocean. Analysis of the D/H ratio on Mars imply that a global H2O ocean with a depth of ≤ 30 m was lost since the end of hydrodynamic escape. Calculations of the time evolution of the 15N/14N isotope anomalies in the atmospheres of Mars and Titan show that the Martian atmosphere was at least ≥ 20 times denser than at present and that the mass of Titan’s early atmosphere was about 30 times greater than its present value. A detailed study of gravitational fractionation of isotopes in planetary atmospheres furthermore indicates a much higher solar wind mass flux of the early Sun during the first half billion years.

惑星探査における鉱物分光学の重要性

Planetary geology provides information on processes on the surface and interiorl of terrestria planets, satellites, and asteroids in the solar system. Surface composition and mineralogy is one of clues to understand planetary geology and, in particular, it is essential to determine the composition of planetary surface in order to discuss the origin and geologic/geochemical evolution. The mineralogical information is investigated using spectroscopy in the visible and near-infrared regions of the electromagnetic spectrum, because most of rock-forming minerals exhibit characteristic absorption features at these wavelengths. Especially, silicate minerals such as olivines and pyroxenes show diagnostic absorption bands due to the presence of Fe2+. The compositional interpretation of reflectance spectra using the remote sensing techniques is based on the principle of the crystal field theory. Recently, with the advances in computer and detector technology, the new field of imaging spectroscopy is developing in remote sensing. Imaging spectroscopy is an instrument of combination of imaging, technique to obtain planetary images, and spectroscopic technique. While the instrument is in use in the laboratory, in an aircraft, and in Earth-based telescope, only a few spaceborne instruments for planetary observation were applied because of difficulty in technical development. This instrument can obtain a three-dimensional cube data with two spatial and third spectral dimensions and may provide the determination of composition and mapping of compositions across the planetary surface at high resolution. To develop more general views on planetary geology, it will be required to develop the imaging spectroscopy for spacecrafts.

Remote sensing and geological studies of the Hadley‐Apennine region of the Moon

Abstract— We have used data from the Clementine and Lunar Prospector spacecraft in conjunction with reflectance spectra collected with Earth‐based telescopes to study the geology of the Hadley‐Apennine portion of the lunar Imbrium basin. The Apennine Mountains and the Imbrium backslope are composed of Imbrium basin ejecta with a noritic or anorthositic norite composition. We find that the two major facies of Imbrium ejecta, the Apenninus material and the Alpes Formation, differ in iron and titanium content. “Pure” anorthosite has tentatively been identified in the ejecta of the crater Conon, based on low‐iron content. A difference in Th and rare earth element (REE) abundance between the northeast Apennine Mountains (lower) and the southwest Apennines (higher) is noted. Pyroclastic deposits are common in the region and are dominated by mare basalt material, probably plug rock ejected in vulcanian eruptions. The Apennine Bench Formation, which is likely to be a deposit of non‐mare volcanic material, has an Fe, Ti and Th composition consistent with that of Apollo 15 KREEP basalt samples thought to be fragments of the Bench. Aristillus crater is a Th and REE hot spot, and the stratigraphy of the impact target site has been reconstructed from knowledge of the composition of the crater interior and exterior deposits. We infer that the target consisted of highland basement, KREEP plutonics and volcanics, and both high‐ and low‐Ti mare basalt.

Subaru Sees an Unruly Pair

A pair of embryonic stars, swaddled in gas and dust, emit parallel jets in this image from Japan9s new 8.3-meter Subaru Telescope on Mauna Kea in Hawaii. The Hubble Space Telescope discovered the jets, but Subaru is the first Earth-based telescope to see them.

Modeling and analysis of the Large Earth-based Solar Telescope dynamics

A model for numerical simulations of the dynamical behavior of the Large Earth-based Solar Telescope is described. The model includes the various subsystems of the telescope optics and mechanics, as well as its live optics system. The live optics system consists of subsystems for active optics, for adaptive optics, and for automatic alignment and guiding. The model also includes various kinds of disturbances, such as atmospheric turbulence and wind gusts. The modeling of the disturbances is also described. The dynamics of the telescope is analyzed using the numerical model. The simulation results confirm the fundamental importance of the live optics system of the telescope. The amount of atmospheric turbulence is found to be critical to telescope performance, although during favorable seeing conditions other sources of image degradation limit the performance. Wind gusts are of little concern during normal weather conditions, especially when the live optics system is used. In addition to the numerical time-domain model, a statespace model is used for frequency-domain analysis and for verification of the time-domain model. The results from the state-space model show that the image motion in the final focus is significantly reduced at frequencies below the eigenfrequency of the fast tip-tilt mirror.

Simulations of the dynamics of the lest

Abstract A model of the dynamics of the Large Earth-based Solar Telescope (LEST) is briefly described. The model includes the various subsystems of the telescope, with emphasis on its live optics system, which is used to correct for various kinds of disturbances, in particular the atmospheric turbulence, in real time. The disturbances are also modelled, and used as input to the telescope model. The modelling of the atmospheric turbulence is described, and results from simulations show the influence of the atmosphere during different seeing conditions.

Spectral Irradiance Measurements of Simulated Lightning in Planetary Atmospheres

Measurements of the spectral irradiance from approximately 380 to 820 nm are reported for laboratory simulations of lightning in the atmospheres of Venus, Jupiter, and Titan. The observations were made at 1 and 5 bars of pressure for Venus and Jupiter and at 1 bar for the Titan mixture. The spectra were obtained by observing laser-induced plasmas with a scanning spectrometer and an optical multichannel analyzer. Simulations of lightning show that atomic line and continuum radiation dominate the spectra. Weak molecular band radiation from CN was also observed for Venus and Titan. As the ambient pressure was increased from 1 to 5 bars, the prominence of the line radiation diminishes compared to the continuum radiation, some lines disappear, and the intensity of the molecular band radiation increases. Laboratory results for the venusian lightning spectrum are consistent with those found by the Venera 9 spectrometer when it viewed a storm on the nightside of Venus. For both Jupiter and Venus, narrow spectral features are present that are ideal for detecting lightning from Earth-based telescopes.

Edwin P. Hubble and the Transformation of Cosmology

In the first three decades of this century the modern science of cosmology was forged from general relativity theory and new observing methods and instruments, particularly large optical telescopes perched thousands of feet above sea level in California and Arizona. These radical changes in the theoretical and observational tools used by astronomers, physicists and mathematicians accompanied revolutionary changes in cosmology itself. The new cosmology of the early 1930s included two key cognitive features absent from the cosmology of the turn of the century: first, the existence of galaxies outside our own stellar system that are visible in Earth-based telescopes, and second, that these galaxies evince the expansion of the universe.

Spectral mineralogy of terrestrial planets: scanning their surfaces remotely

AbstractSpectral measurements of sunlight reflected from planetary surfaces, when correlated with experimental visible-near-infrared spectra of rock-forming minerals, are being used to detect transition metal cations, to identify constituent minerals, and to determine modal mineralogies of regoliths on terrestrial planets. Such remote-sensed reflectance spectra measured through earth-based telescopes may have absorption bands in the one micron and two micron wavelength regions which originate from crystal field transitions within Fe2+ ions. Pyroxenes with Fe2+ in M2 positions dominate the spectra, and the resulting 1 μm versus 2 µm spectral determinative curve is used to identify compositions and structure-types of pyroxenes on surfaces of the Moon, Mercury, and asteroids, after correcting for experimentally-determined temperature-shifts of peak positions. Olivines and Fe2+-bearing plagioclase feldspars also give diagnostic peaks in the 1 µm region, while tetrahedral Fe2+ in glasses absorb in the 2 µm region as well. Opaque ilmenite, spinel and metallic iron phases mask all of these Fe2+ spectral features. Laboratory studies of mixed-mineral assemblages enable coexisting Fe2+ phases to be identified in remote-sensed reflectance spectra of regoliths. Thus, noritic rocks in the lunar highlands, troctolites in central peaks of impact craters such as Copernicus, and high-Ti and low-Ti mare basalts have been mapped on the Moon's surface by telescopic reflectance spectroscopy. The Venusian atmosphere prevents remote-sensed spectral measurements of its surface mineralogy, while atmospheric CO2 and ferric-bearing materials in the regolith on Mars interfere with pyroxene characterization in bright- and dark-region spectra. Reflectance spectral measurements of several meteorite types, including specimens from Antarctica, are consistent with a lunar highland origin for achondrite ALHA 81005 and a martian origin for shergottite EETA 79001, although source regions may not be outermost surfaces of the Moon and Mars. Correlations with asteroid reflectance spectra suggest that Vesta is the source of basaltic achondrites, while wide ranges of olivine/pyroxene ratios are inconsistent with an ordinary-chondrite surface composition of many asteroids. Visible-near-infrared spectrometers are destined for instrument payloads in future spacecraft missions to neighbouring solar system bodies.

Exploration of crustal/mantle material for the earth and moon using reflectance spectroscopy

Near-infrared reflectance spectra have been acquired a) for Moses Rock diatreme in SE Utah using an airborne imaging spectrometer (AIS) and b) for small areas in and around the large lunar impact crater Copernicus using a spectrometer on earth-based telescopes. The high spectral resolution and precision of these data allow several mineral components of surface material to be identified and analyzed in a spatial context. The derived mineralogical information is used to address specific geological problems. For the terrestrial study the distribution of the measured abundance of mantle derived ultramafic microbreccia across Moses Rock dike indicates that flow stabilized into a few channels during the violent eruption. For the lunar study the variety of rock types identified in crustal material of deep-seated origin is distinct from the dominant rock types sampled from the upper lunar crust and suggests a complex crustal evolution.

Image-Slicers

The diameter of stellar images produced by Earth-based telescopes often exceeds the width of the entrance slit of the associated spectrograph. It is not uncommon for more starlight to be wasted on the jaws of the slit than is transmitted by the slit. The purpose of an image-slicer is to redirect the light from the slit jaws into the slit; to do so, the length of the slit illuminated by starlight must be increased, either by stacking slices of the image along the slit, or by elongating the image and superimposing full-length slices of it. Thus, in order to work, an image-slicer of whatever type must lose spatial resolution of the sky along the slit.

Space 25

Not so long ago space was pristine. Even in the two decades following the orbiting of Sputnik, the two superpowers — the United States and the Soviet Union — were still the only truly significant actors there. And though they had previously launched sonne innpressive unnnanned satellites, their first astronauts were little nnore than tourists in space. Even the nnagnificent moon landing was no more a precursor of human colonization of a new “continent” than was the first landfall by the Vikings in North America. And until recently most discoveries about the far universe were being made from earth-based telescopes.

Tones of the Oscillating Sun

Tones of the Oscillating Sun

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.

Space telescope leads a new generation

The Space Telescope, approved for a space shuttle launch in 1985, will resolve visible and ultraviolet light orders of magnitude better than earth‐based telescopes. Even though its aperture of 2.4 m will be less than half of the aperture of the reflector at Mt. Palomar, it is considered relatively large and immensely superior to existing orbiting telescopes. It is expected that the Space Telescope, with its freedom from thermal jitter and the hindrance of atmospheric absorption, will increase the user load on larger aperture earth‐based telescopes, and construction of a new series of large telescopes is now in the planning stages. The National Academy of Sciences is expected to release shortly a report by its Astronomy Survey Committee, headed by George Field of the Harvard‐Smithsonian Observatory, that will recommend building a 15‐m optical‐infrared ‘National Telescope’ [Phys. Today, August 1981, p. 17]. In addition, a 7.6‐m single reflector is planned at the University of Texas, a 10‐m segmented reflector is planned at the University of California, and the University of Arizona is experimenting with designs of a Multiple‐Mirror Telescope (MMT) larger than their new 4.5 m (equivalent) MMT. All of these are larger than the Russian 6‐m telescope, whose Pyrex reflector has not performed well, apparently because of gravitational sag and thermal inertial stresses.

Altered basaltic glass: A terrestrial analog to the soil of Mars

Analyses of Martian surface soil by Viking and Earth-based telescopes have been interpreted as indicating a regolith dominated by the weathering products of mafic or ultramafic rocks. Basaltic glass has previously been proposed as a more likely precursor than crystalline rock, given the low efficiency of surface weathering under present Martian conditions. On Earth large volumes of basaltic glass formed by quenching of magma by water. A similar interaction, between magma and ground ice, may have been a common occurrence on Mars. On the basis of this scenario palagonite, the alteration product of basaltic sideromelane glass, was studied as a possible analog to Martian soil. Samples from Iceland, Alaska, Antarctica, Hawaii, and the desert of New Mexico and Mexico were examined by optical and scanning electron microscopy, electron microprobe analysis, X-ray diffraction, spectrophotometry, and magnetic and thermogravimetric analysis. We suggest that palagonite is a good analog to the surface soil of Mars in chemical composition, particle size, spectral signature, and magnetic properties. Our model for the formation of fine-grained Martian surface soil begins with eruptions of basaltic magma through ground ice, forming deposits of glassy tuff. Individual glass shards are then altered by low-temperature hydrothermal systems to palagonitic material. Dehydration and aeolian abrasion strip the alteration rinds from the glass, and wind storms distribute the silt-sized palagonitic fragments in a planet-wide deposit.