Large Ground-based Telescopes Research Articles

Implementation and Use of Wide Fields in Future Very Large Telescopes

AbstractThe full potential of the next generation of larger telescopes will be realized only if they have well instrumented large fields of view. Scientific problems for which very large ground-based optical telescopes will be of most value often will need surveys to very deep limits with imaging and slitless spectroscopy, followed by spectroscopy of faint objects taken many at once over the field. Improved instruments and detectors for this purpose are being developed. Remotely positioned fibers allow the coupling of light from many objects in the field to the spectrograph slit. CCD arrays, operated in the TDI or drift scan mode, will make large area detectors of high efficiency that may supercede photographic plates. An ideal telescope optical design should be based on a fast parabolic primary, have a field of at least 1° with achromatic images < 0.25 arcseconds and have provision for dispersive elements to be used for slitless spectroscopy and compensation of atmospheric dispersion over the full field. A good solution for a general purpose telescope that can satisfy these needs is given by a three element refractive corrector at a fast Cassegrain focus. A specialized telescope dedicated to sky surveys, with better image quality and higher throughput than presently available, might be built as a scaled up Schmidt with very large photographic plates. Better performance in most areas should be obtained with a large CCD mosaic detector operated in the drift scan mode at a telescope with a 2-mirror reflecting corrector.

Interferometry with Large Telescopes

1. Introduction. A popular - often textbook supported- view is that the diffraction limit of large telescopes is out of reach from ground-based observatories.We summarize on Fig.1 the scope of angular resolving power, in the range lOOnm to 10. nm, as it appears today.A quick look reveals, expressed in absolute angular terms, a number of stimulating points :(i) recent advances in speckle techniques allow to reach the diffraction limit of large ground-based telescopes, notwithstanding the seeing effects. Initially devised to measure double bright stars separation in the visible, these techniques have been extended to weak visible stars (mv ~ 14 to 16), to infrared sources (l-5µm), to full image reconstruction, and even to diffraction limited images obtained with optics suffering from residual aberrations (Faint Object Camera on the Space Telescope);

Texas symposium: Astronomy: relativity's relative?

Texas symposium Astronomy: relativity's relative? from Virginia Trimble The uneasy marriage between general relativity and astronomy, solemnized by the biennial series of Texas Symposia on Relativistic Astrophysics, has endured another two years*, although the parties concerned continue to regard many of each others' concerns with tolerant puzzlement. The relativists, who were rather badly outnumbered on the programme, have been worrying about such issues as the logical structure of general relativity, ways of improving numerical calculations within it and possible approaches to a quantum theory of gravity that might, someday, be unified with gauge theories of the other forces. S. Deser (Brandeis University) reported progress towards demonstrating that energy, properly defined, is always positive within general relativity, so that real systems are guaranteed to have ground states and stability. The proof is essentially complete both locally and globally for cases with zero cosmological constant, A , and for global-positive A solutions. The case with negative A is still not quite fully understood, but the speaker expressed confidence that it would all turn out all right. T. Nakamura (Kyoto University) dis- cussed results of numerical solutions for the collapse (in two dimensions) of a rapid- ly rotating star, in which the matter is pro- perly included but gravitational radiation neglected. Considering the small number of grid points used, the results come impressively close to what we think should be the 'right ' answer. Thus, for several different patterns of angular momentum distribution, most of the material collapses only when the ratio of angular momentum to mass is small enough for a stable Kerr black hole eventually to result. Larger ratios always lead to much of the matter be- ing ejected, although the detailed ejection pattern is probably an artefact of the grid pattern, numerical viscosity and so on. Finally, P. Van Nieuwenhuizen (SUNY, Stony Brook) pointed out some of the pros and cons of supergravity, the extension of general relativity which adds to its gravitational field carried by gravitons a second field, carred by fermions (spin = 3/2) called gravitinos. On the plus side, it has now been demonstrated that supergravity (unlike general relativity) is renormalizable at least for all Feynman diagrams with one or two loops, though there are still some uncertainties at higher levels. A negative aspect is that it is not clear that even the largest (N = 8) supersymmetry has enough freedom to include both non-zero cosmological constant (vacuum energy) and all the kinds of fermion field we know exist. The speaker expressed guarded optimism that further significant progress could be expected, perhaps along the lines of multi- dimensional space. The astronomers, meanwhile, remain enamoured of telescopes and the objects they reveal. A half-day session addressed large ground-based optical telescopes for the late 1980s and beyond. The University of Texas 300-inch (7.6 m) project, dis- cussed by H. Smith (University of Texas), is the furthest along in funding. It will probably be a thin

Very large ground-based telescopes for optical and IR astronomy

Optical and IR astronomers are taking a hard look at their ground-based facilities and devising new ways of making more economic, bigger and better telescopes. Features of instruments of the 15-m class are likely to include servo control to compensate for atmospheric wavefront errors as well as structural deformation, large honeycomb mirror blanks and mirror surfaces produced by economical techniques developed for aspherics.

On the possibility of H2-near-infrared detection in comets with large gas production

Comets with large gas production offer a unique chance to observe a H2-flux of about 105 photon cm−2 s−1 sr−1 (1 Rayleigh) at wavelengths 8497.4 A, 8560.2 A and 8747.9 A-i.e., where photon counting methods are still applicable. In the following it will be shown that population of the vibrational levels, giving rise to these quadrupole overtone transitions, is dominated by photodissociation of methane, and that the emission even of quadrupole lines is not attenuated by collisional quenching. Wavelength scanning by ±1 A is shown to be enough to discriminate between cometary and atmospheric emissions by phase-sensitive subtraction techniques. Solid angle of Ω<10−7 sr has to be used, whence follows that a large ground-based telescope combined with a tilting Fabry-Perotfilter is best suited for detection of the near-infrared H2-emissions at reasonable counting rates and sufficient rejection of the atmospheric background.

Prospects for development of ground-based optical astronomy

A number of refinements in ground-based optical telescopes and their detecting equipment, and also serious development of measures for neutralization of interference effects of the earth's atmosphere, permit the efficiency of observation of faint astronomical objects to be improved by at least an order of magnitude in the coming years. In the case of extragalactic astronomy and observational cosmology it is shown that progress in large ground-based optical telescopes has a decisive role in formation of contemporary concepts of the universe. Some of the problems of astronomy which will be solved by means of these instruments in the near future are discussed.

Measurement of averaged squared modulus of atmospheric-lens modulation transfer function

The theory of speckle interferometry shows that the averaged squared modulus of the atmospheric-lens modulation transfer function becomes proportional, for high frequencies, to the lens modulation transfer function. The experimental curve obtained by comparison of the power spectrum of a turbulence-degraded image and that of a nondegraded image of the solar granulation is consistent for high frequencies with the theoretical curve. We propose to use it to correct power spectra obtained with a large ground-based telescope, at frequencies up to the diffraction limit.