Showing papers on "Sky published in 1994"

Sloan Digital Sky Survey

Abstract: The Sloan Digital Sky Survey is a project which will produce a detailed digital phometric map of half the northern sky to about 23 magnitude using a special purpose wide field telescope of 2.5 meter aperture. This map will be used to select about a million galaxies and 100,000 quasars, for which high resolution spectra will be obtained using the same telescope. A catalog will be produced of all the detected objects, about 100 million galaxies and a similar number of stars, and a million quasar candidates.

On determining the spectrum of primordial inhomogeneity from the COBE DMR sky maps: Method

Abstract: The natural approach to a spectral analysis of data distributed on the sky employs spherical harmonic decomposition. A common problem encountered in practical astronomy is the lack of full sky coverage in the available data. For example, the removal of the Galactic plane data from the Cosmic Background Explorer (COBE) Differential Microwave Radiometer (DMR) sky maps compromises Fourier analysis of the cosmic microwave background (CMB) temperature distribution due to the loss of orthogonality of the spherical harmonics. An explicit method for constructing orthonormal functions on an incomplete (e.g., Galaxy-cut) sphere is presented. These functions should be used in the proper Fourier analysis of the COBE DMR sky maps to provide the correct input for the determination of the spectrum of primordial inhomogeneity. The results of such an analysis are presented in an accompanying Letter. A similar algebraic construction of appropriate functions can be devised for other astronomical applications.

On Determining the Spectrum of Primordial Inhomogeneity from the COBE DMR Sky Maps: I. Method

Abstract: The natural approach to a spectral analysis of data distributed on the sky employs spherical harmonic decomposition. A common problem encountered in practical astronomy is the lack of full sky coverage in the available data. For example, the removal of the galactic plane data from the COBE DMR sky maps compromises Fourier analysis of the cosmic microwave background (CMB) temperature distribution due to the loss of orthogonality of the spherical harmonics. An explicit method for constructing orthonormal functions on an incomplete (e.g. Galaxy-cut) sphere is presented. These functions should be used in the proper Fourier analysis of the COBE DMR sky maps to provide the correct input for the determination of the spectrum of primordial inhomogeneity. The results of such an analysis are presented in an accompanying Letter. A similar algebraic construction of appropriate functions can be devised for other astronomical applications.

Powerful model for the point source sky: Far-ultraviolet and enhanced midinfrared performance

Abstract: I report further developments of the Wainscoat et al (1992) model originally created for the point source infrared sky The already detailed and realistic representation of the Galaxy (disk, spiral arms and local spur, molecular ring, bulge, spheroid) has been improved, guided by CO surveys of local molecular clouds, and by the inclusion of a component to represent Gould's Belt The newest version of the model is very well validated by Infrared Astronomy Satellite (IRAS) source counts A major new aspect is the extension of the same model down to the far ultraviolet I compare predicted and observed far-utraviolet source counts from the Apollo 16 'S201' experiment (1400 A) and the TD1 satellite (for the 1565 A band)

The Two Micron All Sky Survey

Abstract: The Two Micron All Sky Survey (2MASS) will provide a uniform survey of the entire sky at three near-infrared wavebands: J(lambda(sub eff) = 1.25 micrometers), H(lambda(sub eff) = 1.65 micrometers), and K(sub s)(lambda(sub eff) = 2.16 micrometers). A major goal of the survey is to probe large scale structures in the Milky Way and in the Local Universe, exploiting the relatively high transparency of the interstellar medium in the near-infrared, and the high near-infrared luminosities of evolved low- and intermediate-mass stars. A sensitive overview of the near-infrared sky is also an essential next step to maximize the gains achievable with infrared array technology. Our assessment of the astrophysical questions that might be addressed with these new arrays is currently limited by the very bright flux limit of the only preceding large scale near-infrared sky survey, the Two Micron Sky Survey carried out at Caltech in the late 1960's. Near-infrared instruments based on the new array technology have already obtained spectra of objects 1 million times fainter than the limit of the TMSS! This paper summarizes the essential parameters of the 2MASS project and the rationale behind those choices, and gives an overview of results obtained with a prototype camera that has been in operation since May 1992. We conclude with a list of expected data products and a statement of the data release policy.

Twilight and daytime colors of the clear sky

Abstract: Digital image analysis of the cloudless sky's daytime and twilight chromaticities challenges some existing ideas about sky colors. First, although the observed colors of the clear daytime sky do lie near the blackbody locus, their meridional chromaticity curves may resemble it very little. Second, analyses of twilight colors show that their meridional chromaticity curves vary greatly, with some surprising consequences for their colorimetric gamuts.

A band-differenced angular signature technique for cirrus cloud detection

Abstract: A new approach to cloud detection is introduced that exploits the difference between two solar spectral reflectances as a function of view angle. The resulting band-differenced angular signature is sensitive to the contribution of Rayleigh scattering from above the tops of clouds and can be used to discriminate high clouds from lower level clouds and clear sky. The authors use model simulations to show that this technique could be applied to measurements from the Multiangle Imaging SpectroRadiometer, scheduled for launch on the first platform of the Earth Observing System. Results show the technique to work best over ocean and snow surfaces. Over such surfaces, the minimum detectable high cloud optical thickness (at 0.55 /spl mu/m) would typically be 0.5 without the use of any a priori scene information, and lower if such information is available. >

Horizon brightness revisited: measurements and a model of clear-sky radiances

Abstract: Clear daytime skies persistently display a subtle local maximum of radiance near the astronomical horizon. Spectroradiometry and digital image analysis confirm this maximum's reality, and they show that its angular width and elevation vary with solar elevation, azimuth relative to the Sun, and aerosol optical depth. Many existing models of atmospheric scattering do not generate this near-horizon radiance maximum, but a simple second-order scattering model does, and it reproduces many of the maximum's details.

A maskless gamma-ray all-sky imager: BATSE/CGRO

Abstract: The non-collimated detectors of BATSE (Burst And Transient Source Experiment) on the Compton Gamma Ray Observatory (CGRO) can be used as a high sensitivity hard X-ray and low energy gamma-ray all-sky imager in the energy range between 20 keV and 2 MeV. The fluxes from sources in the sky are modulated as the spacecraft orbits the Earth. The CGRO orbital precession further allows sampling of the sky in strips corresponding to the limb of the Earth at any given time. The modulation data are transformed into images by various reconstruction methods. High sensitivity images of location accuracy of about 0.1/spl deg/ and source separation of about 1/spl deg/ are obtained. >

Sky subtraction with fiber-spectrographs

Abstract: The sky subtraction performance of multi-fiber spectrographs are discussed, analyzing in detail the case of the OPTOPUS system at the 3.6 meter ESO telescope at La Silla. A standard technique, based on flat-fields obtained with a uniformly illuminated screen on the dome, provides poor results. A new method has been developed, using the [O I] emission line at 5577 A as a calibrator of the fiber transmittance, taking into account the diffuse light and the influence of each fiber on the adjacent ones, and correcting for the effects of the image distortions on the sky sampling. In this way the accuracy of the sky subtraction improves from 2-8% to 1.3-1.6%.

Decametric survey of discrete sources in the northern sky: XI. The Results of the UTR-2 Very Low-Frequency Sky Survey in the Declination Range 41to 52

Abstract: The results of the very-low frequency sky survey of discrete sources made with the UTR-2 radio telescope within the declination zone 41° to 52° are presented. The UTR-2 radio source catalogue contains the estimates of the coordinates and flux densities of 432 sources measured at a number of the lowest frequencies used in contemporary radio astronomy within the range from 10 at 25 MHz. The questions of establishing the UTR-2 sky survey sensitivity, completeness and reliability of the resulting catalogue are considered. The coordinates of the sources measured at very low frequencies have been compared with respective data obtained in the 4C survey at 178 MHz.

Astronomy and the Extreme Ultraviolet Explorer satellite

Abstract: The extreme ultraviolet wave band (100 to 912 angstroms) was thought until recently to be useless to astronomy, primarily because the opacity of the interstellar medium would prevent observations at these wavelengths. However, the interstellar medium has been found to be markedly inhomogeneous in both density and ionization state and the sun is fortunately located in a region of low extreme ultraviolet opacity. The Extreme Ultraviolet Explorer, launched in June 1992, has surveyed the sky in this wave band and has detected a wide variety of astronomical sources at considerable distances, including some extragalactic objects. Studies in the extreme ultraviolet band have already begun to increase our understanding of the contents of the universe.

Va-et-Vient' spectroscopy: a new mode for faint object CCD spectroscopy with very large telescopes

Abstract: After a review of the sources of noise in faint object low resolution spectroscopy, we discuss the limitations in the S/N ratio due to the flat-fielding errors and sky line sampling residuals after sky subtraction. These errors become rapidly dominant with high flux, and limit the S/N at very low resolution. In order to partly cancel these difficulties, we propose a new observing mode for faint object spectroscopy, called Va-et- Vient spectroscopy. Thanks to multiple back-and-forth shifts of charges on the CCD, synchronized with the telescope shifts, the object and a close reference sky are integrated in the same physical conditions on the CCD through the spectrograph. This method enables us to stay in a photon noise regime on long sky background noise limited exposures

The autocorrelation function and fluctuations of the soft X-ray sky on scales from 1 arcmin to 5°

Abstract: We investigate the spatial anisotropy and inhomogeneity of the soft X-ray sky, using one field of deep exposure and six collinear fields of medium exposure observed with the ROSAT Position Sensitive Proportional Counter. We first remove time-variable contaminations and excise bright sources from the data, followed by simultaneous spectral fitting to determine the diffuse extragalactic X-ray background count rate; we then reconstruct the strip composed of the six medium fields which span 2°(RA)×7°(Dec.) of the sky. Using the deep field and the merged strip, we calculate the weighted angular autocorrelation function and the rms fluctuation of the surface intensity of the X-ray sky in the energy band ∼0.4-2.4 keV from 1 arcmin to 5°

The need for standard skies and models for energy conservation measures

Abstract: Historically, several standard skies have been adopted which enabled the development of calculation/prediction methods used in daylight design. Only recently have the scanning sky radiance/luminance meters made possible automated quick and precise routine measurements of sky distributions. The large data records gathered for various locations and time intervals stimulate their evaluation by computer based statistics. Thus sky classification systems, sky types and models based on averaged data were evolved. Due to their concepts and data base these sometimes question the validity of the general and simple CIE standard skies defined in accordance with theoretical and practical needs and established on chosen critical or extreme instantaneous states of the atmosphere. This paper discusses current queries on the basis of gradation scattering indicatrix analysis, being the theoretical basis of the sky models, and provides a comparison and representation of physically sound evidence compiled from measurements. ...

Influence of a solar eclipse on twilight.

Abstract: The morning twilight of the presunrise sky was measured at the Hoher-List Observatory during the total eclipse of 22 July 1990. The location of observation was far away from the central eclipse zone. The luminance showed a deep minimum in twilight during the main phase of the solar eclipse compared with normal conditions. A first order scattering model explains the observations reasonably well and shows that the sky radiation during the first phase of twilight at a location far away from the central umbra depends primarily on the height profile of the air pressure between ~ 100 and 200 km.

First Byurakan Spectral Sky Survey. Blue stellar objects. VIII. zone +65° ≤ δ ≤, + 69°

Abstract: The eighth list of blue stellar objects in the second part of the First Byurakan Spectral Sky Survey (FBS) is presented. The objects are located in the zone +65° ≤ δ ≤ +69° and 5 h 15m ≤ α ≤ 18 h 05m.The list contains data on 98 blue stellar objects detected, of which 73 are previously unknown. A preliminary classification of the objects is given. The exact coordinates of the centers and the limiting magnitudes of the plates for this zone have been determined using the charts of the Palomar Sky Survey.