Showing papers on "Sky published in 1999"

The Sloan Digital Sky Survey

Abstract: The Sloan Digital Sky Survey is an ambitious, multi–institutional project to create a huge digital imaging and spectroscopic data bank of 25% of the celestial sphere, approximately 10,000 degrees square centred on the north galactic polar cap. The photometric atlas will be in five specially chosen colours, covering the π sr of the Survey area to a limiting magnitude of r ′ ∼ 23.1, on 0″.4 pixels, resulting in a 1Tpixel map. This database will be automatically analysed to catalogue the photometric and astrometric properties of 108 stellar images, 108 galaxies, and 106 colour–selected QSO candidates; the galaxy data will, in addition, include detailed morphological data. The photometric data are used to autonomously and homogeneously select objects for the spectroscopic survey, which will include spectra of 106 galaxies, 105 QSOs, and 105 unusual stars. Although the project was originally motivated by the desire to study ‘large scale structure’, we anticipate that these data will impact on virtually every field of astronomy, from Earth–crossing asteroids to QSOs at z > 6. In particular, the ca . 12 TByte multi–colour precision–calibrated imaging archive should be a world resource for many decades of the next century.

Daylight simulation : validation, sky models and daylight coefficients.

Abstract: The application of lighting simulation techniques for daylight illuminance modelling in architectural spaces is described in this thesis. The prediction tool used for all the work described here is the Radiance lighting simulation system. An overview of the features and capabilities of the Radiance system is presented. Daylight simulation using the Radiance system is described in some detail. The relation between physical quantities and the lighting simulation parameters is made clear in a series of progressively more complex examples. Effective use of the inter-reflection calculation is described. The illuminance calculation is validated under real sky conditions for a full-size office space. The simulation model used sky luminance patterns that were based directly on measurements. Internal illuminance predictions are compared with measurements for 754 skies that cover a wide range of naturally occurring conditions. The processing of the sky luminance measurements for the lighting simulation is described. The accuracy of the illuminance predictions is shown to be, in the main, comparable with the accuracy of the model input data. There were a number of predictions with low accuracy. Evidence is presented to show that these result from imprecision in the model specification - such as, uncertainty of the circumsolar luminance - rather than the prediction algorithms themselves. Procedures to visualise and reduce illuminance and lighting-related data are presented. The ability of sky models to reproduce measured sky luminance patterns for the purpose of predicting internal illuminance is investigated. Four sky models and two sky models blends are assessed. Predictions of internal illuminance using sky models/blends are compared against those using measured sky luminance patterns. The sky model blends and the Perez All-weather model are shown to perform comparably well. Illuminance predictions using measured skies however were invariably better than those using sky models/blends. Several formulations of the daylight coefficient approach for predicting time varying illuminances are presented. Radiance is used to predict the daylight coefficients from which internal illuminances are derived. The form and magnitude of the daylight coefficients are related to the scene geometry and the discretisation scheme. Internal illuminances are derived for four daylight coefficient formulations based on the measured luminance patterns for the 754 skies. For the best of the formulations, the accuracy of the daylight coefficient derived illuminances is shown to be comparable to that using the standard Radiance calculation method. The use of the daylight coefficient approach to both accurately and efficiently predict hourly internal daylight illuminance levels for an entire year is described. Daylight coefficients are invariant to building orientation for a fixed building configuration. This property of daylight coefficients is exploited to yield hourly internal illuminances for a full year as a function of building orientation. Visual data analysis techniques are used to display and process the massive number of derived illuminances.

LA Palma Night-Sky Brightness

Abstract: The brightness of the moonless night sky above La Palma was measured on 427 CCD images taken with the Isaac Newton and Jacobus Kapteyn Telescopes on 63 nights during 1987 - 1996. The median sky brightness at high elevation, high galactic latitude and high ecliptic latitude, at sunspot minimum, is B = 22.7, V = 21.9, R = 21.0, similar to that at other dark sites. The main contributions to sky brightness are airglow and zodiacal light. The sky is brighter at low ecliptic latitude (by 0.4 mag); at solar maximum (by 0.4 mag); and at high airmass (0.25 mag brighter at airmass 1.5). Light pollution (line + continuum) contributes < 0.03 mag in U, approximately 0.02 mag in B, approximately 0.10 mag in V, approximately and 0.10 mag in R at the zenith. This paper is a summary of results which are presented in full elsewhere (Benn & Ellison 1998, La Palma Technical Note 115).

Estimating Cloud Type from Pyranometer Observations

Abstract: In this paper the authors evaluate an inexpensive and automatable method to estimate cloud type at a given location during daylight hours using the time series of irradiance from a pyranometer. The motivation for this investigation is to provide ground-based estimates of cloud type at locations where there are no human observations of sky condition. A pyranometer naturally measures the effect of intervening clouds along the solar beam path to the sensor. Because a daily time series of irradiance is nonstationary, it is appropriately scaled to yield a stationary time series. From the latter, the standard deviation and ratio of observed irradiance to clear-sky irradiance derived from a 21-min moving window are related to one of the following cloud types or conditions: cirrus, cumulus, cirrus and cumulus, stratus, precipitation or fog, no clouds, and other clouds. Comparisons with human observations at the Department of Energy Atmospheric Radiation Measurement Calibration and Radiation Testbed site in northern Oklahoma show that the pyranometer method and human observations are in agreement about 45% of the time. Many of the differences can be attributed to two factors: 1) the pyranometer method is weighted toward clouds crossing the sun’s path, while the human observer can view clouds over the entire sky, and 2) the presence of aerosols causes the pyranometer to overestimate the occurrence of cirrus and cirrus plus cumulus. When attenuation of the solar beam by aerosols is negligible or can be accounted for, the pyranometer method should be especially useful for cloud-type assessment where no other sky observations are available.

The toes of the ultra high energy cosmic ray spectrum

Abstract: We study the effects of the galactic magnetic field on the ultra high energy cosmic ray propagation. We show that the deflections of the cosmic ray trajectories can have many important implications such as (de)magnification of the cosmic ray fluxes by lensing effects (which can modify the spectrum of individual sources), the formation of multiple images of a source or the existence of regions of the sky to which the Earth is almost blind. The appearance of image pairs is related to the existence of critical curves in the magnification maps, which divide regions in the sky where the images have opposite parities. The results are pictorially illustrated as the stretching and folding of a `sheet' describing the sky seen on Earth. Making use of the most energetic AGASA events we emphasize the need to know the cosmic ray composition and the structure of the magnetic field when attempting to do detailed cosmic ray astronomy.

On the Non-Gaussianity Observed in the COBE-DMR Sky Maps

Abstract: In this paper we pursue the origin of the non-Gaussianity determined by a bispectrum analysis of the COBE-DMR 4-year sky maps. The robustness of the statistic is demonstrated by the rebinning of the data into 12 coordinate systems. By computing the bispectrum statistic as a function of various data partitions - by channel, frequency, and time interval, we show that the observed non-Gaussian signal is driven by the 53 GHz data. This frequency dependence strongly rejects the hypothesis that the signal is cosmological in origin. A jack-knife analysis of the coadded 53 and 90 GHz sky maps reveals those sky pixels to which the bispectrum statistic is particularly sensitive. We find that by removing data from the 53 GHz sky maps for periods of time during which a known systematic effect perturbs the 31 GHz channels, the amplitudes of the bispectrum coefficients become completely consistent with that expected for a Gaussian sky. We conclude that the non-Gaussian signal detected by the normalised bispectrum statistic in the publicly available DMR sky maps is due to a systematic artifact. The impact of removing the affected data on estimates of the normalisation of simple models of cosmological anisotropy is negligible.

The Near-Infrared Sky Emission at the South Pole in Winter

Abstract: The Antarctic plateau provides superb sites for infrared astronomy, a result of the combination of low temperatures, low levels of precipitable water vapor, high altitude, and atmospheric stability. We have undertaken measurements of the sky background from 1 to 5 μm at the South Pole, using a single channel InSb spectrometer, the Infrared Photometer Spectrometer (IRPS), during the winter (dark) period of 1995. The IRPS records the DC level of the sky flux through a 4° beam and a variety of broadband and narrowband (1%) filters. It can be scanned in elevation from horizon to horizon through the zenith. We find a 20-100 times reduction in the background of thermal emission compared to that from mid-latitude sites such as Siding Spring and Mauna Kea, with typical background levels of 80-200 μJy arcsec-2 at 2.43 μm, 100-300 mJy arcsec-2 at 3.6 μm and ~0.5 Jy arcsec-2 at 4.8 μm. Airglow emission contributes significantly to the sky flux shortward of ~2.4 μm, which is why the Kdark (2.27-2.45 μm) band emission does not drop to the 10-20 μJy arcsec-2 levels originally predicted. The darkest window for IR observations from the South Pole is from 2.35 to 2.45 μm, where the fluxes from the atmosphere may drop to as low as ~50 μJy arcsec-2 at times. Airglow dominates the emission at J (1.25 μm) and H (1.65 μm), but the flux levels of 300-600 μJy arcsec-2 and 800-2000 μJy arcsec-2, respectively, are also one-third to one-half those at temperate sites. We find no evidence for any significant contribution from auroral emission to the Kdark band. During twilight, when the Sun is <10° below the horizon, scattered sunlight contributes to the sky background with a Rayleigh-type spectrum. Scattered moonlight is also evident in the sky emission at the J band when the Moon is up.

The FIRST Unbiased Survey for Radio Stars

Abstract: Comparison of the VLA FIRST survey with various catalogs of bright stars allows an examination of the prevalence of stellar radio emission independent of optical selection criteria. This FIRST unbiased survey for radio stars covers nearly 5000 deg2 of the northern sky to a flux density limit of 0.7 mJy at 20 cm. Using astrometric catalogs that include proper-motion information, we have detected 26 stellar radio sources, doubling the number of such objects previously known in this region of high-latitude sky. We also show that, in the absence of good proper motions, even the 1'' precision of the FIRST positions is insufficient to avoid crippling chance coincidence rates. We calculate the fraction of radio detections as a function of stellar magnitude and show that, when proper motions from the Guide Star Catalog II become available, the number of stellar radio source detections should increase fourfold.

The color of the Martian sky and its influence on the illumination of the Martian surface

Abstract: The dust in the atmosphere above the Mars Pathfinder landing site produced a bright, red sky that increases in redness toward the horizon at midday. There is also evidence for an absorption band in the scattered light from the sky at 860 nm. A model of the sky brightness has been developed [Markiewicz et al., this issue] and tested against Imager for Mars Pathfinder (IMP) observations of calibration targets on the lander. The resulting model has been used to quantify the total diffuse flux onto a surface parallel to the local level for several solar elevation angles and optical depths. The model shows that the diffuse illumination in shadowed areas is strongly reddened while areas illuminated directly by the Sun (and the blue forward scattering peak) see a more solar-type spectrum, in agreement with Viking and IMP observations. Quantitative corrections for the reddening in shadowed areas are demonstrated. It is shown quantitatively that the unusual appearance of the rock Yogi (the east face of which appeared relatively blue in images taken during the morning but relatively red during the afternoon) can be explained purely by the changing illumination geometry. We conclude that any spectrophotometric analysis of surfaces on Mars must take into account the diffuse flux. Specifically, the reflectances of surfaces viewed under different illumination geometries cannot be investigated for spectral diversity unless a correction has been applied which removes the influence of the reddened diffuse flux.

First geometrical path length probability density function derivation of the skylight from high-resolution oxygen A-band spectroscopy: 2. Derivation of the Lévy index for the skylight transmitted by midlatitude clouds

Abstract: For the first time Levy indices (γ) of the solar light transmitted by cloudy skies at mid latitude (50°N, 8.2°E) are reported. The Levy index describes the dependence of the mean geometrical paths ( ) of photons transmitted from cloudy skies as a function of the vertical cloud extension (Hc) expressed by the scaling law ∼ Hcγ. For a set of 33 individual cloudy sky measurements, reported cloud types and heights, Levy indices are deduced. It is found that the inferred Levy indices cluster into the range of 1 ≤ γ ≤ 2 for “all sky” observations, and into a range 1.5 ≤ γ ≤ 2 for optically very thick clouds. The observations provide evidence that the cloudy sky geometrical path lengths are Levy distributed with the γ value depending on the cloud morphology (the shape of individual clouds and the spatial arrangement of the clouds) but also on the internal cloud inhomogeneities. Because of a particular sensitivity of our method to detect the radiative transfer (RT) caused by clouds (rather than by the clear sky parts of the atmosphere), the inferred type of the path statistics is expected to reflect mostly the path length distribution caused by cloud inhomogeneities rather than by the cloud morphology. Since the cloud inhomogeneities are caused by dynamic processes (besides other factors), the RT transfer is expected to be closely connected to atmospheric dynamics. From this it is concluded that the absorption of solar radiation in cloudy skies is connected to the dynamic state (whether it is stratified or convective) of the cloud cover. In particular, it is expected to be different (mostly larger) than calculated by naively assuming homogeneous horizontally infinite cloud covers in conventional non statistical RT models.

The Northern Sky Optical Cluster Survey I: Detection of Galaxy Clusters in DPOSS

Abstract: The Northern Sky Optical Cluster Survey is a project to create an objective catalog of galaxy clusters over the entire high-galactic-latitude Northern sky, with well understood selection criteria. We use the object catalogs generated from the Digitized Second Palomar Sky Survey (DPOSS, Djorgovski et al. 1999) as the basis for this survey. We apply a color criterion to select against field galaxies, and use a simple adaptive kernel technique to create galaxy density maps, combined with the bootstrap technique to make significance maps, from which density peaks are selected. This survey attempts to eliminate some of the subjective criteria and assumptions of past surveys, including detection by eye (Abell 1958, ACO 1989) and assumed luminosity functions and cluster profiles (PDCS, Postman et al. 1995). We also utilize more information (especially colors) than the most similar recent survey, the APM (Dalton et al. 1992). This paper presents the details of our cluster detection technique, as well as some initial results for two small areas totaling ~60 square degrees. We find a mean surface density of ~1.5 clusters per square degree, consistent with the detection of richness class 0 and higher clusters to z~0.3. In addition, we demonstrate an effective photometric redshift estimator for our clusters.

The Narrow-band Ultraviolet Imaging Experiment for Wide-field Surveys (NUVIEWS)-I: Dust scattered continuum

Abstract: We report on the first results of the Narrow-band Ultraviolet Imaging Experiment for Wide-field Surveys (NUVIEWS), a sounding rocket experiment designed to map the far-ultraviolet background in four narrow bands. This is the first imaging measurement of the UV background to cover a substantial fraction of the sky. The narrow band responses (145, 155, 161, and 174 nm, 7-10 nm wide) allow us to isolate background contributions from dust-scattered continuum, H2 fluorescence, and CIV 155 nm emission. In our first flight, we mapped one quarter of the sky with 5-10 arcminute imaging resolution. In this paper, we model the dominant contribution of the background, dust-scattered continuum. Our data base consists of a map of over 10,000 sq. degrees with 468 independent measurements in 6.25 by 6.25 sq. degree bins. Stars and instrumental stellar halos are removed from the data. We present a map of the continuum background obtained in the 174 nm telescope. We use a model that follows Witt, Friedman, and Sasseen (1997: WFS) to account for the inhomogeneous radiation field and multiple scattering effects in clouds. We find that the dust in the diffuse interstellar medium displays a moderate albedo (a=0.55+/-0.1) and highly forward scattering phase function parameter (g=0.75+/-0.1) over a large fraction of the sky, similar to dust in star forming regions. We also have discovered a significant variance from the model.

The color of Mars: Spectrophotometric measurements at the Pathfinder landing site

Abstract: We calculate the color of the Martian sky and surface directly using the absolute calibration of the Mars Pathfinder (MPF) lander camera, which was observed to be stable during the mission. The measured colors of the Martian sky and surface at the Pathfinder site are identical to the Viking sites, i.e., a predominantly yellowish brown color with only subtle variations. These colors are distributed continuously and fall into five overlapping groups with distinct average colors and unique spatial characteristics: shadowed soil, soil, soil/rock mixtures, rock, and sky. We report that the primary difference between the sky color and the color of the rocks is due to a difference in brightness. Measurements of the sky color show that the sky reddens away from the Sun and toward the horizon and that the sky color varies with time of day and is reddest at local noon. We present a true color picture of the Martian surface and color enhancement techniques that increase image saturation, maximize color discriminability while preserving hue, and eliminate brightness variations while preserving the chromaticity of the scene. Although Mars has long been called the “red” planet, quantitative measurements of the surface color from telescopic and surface observations indicate a light to moderate yellowish brown color. The Pathfinder camera measurements presented here support the claim that the red planet is not red but indeed yellowish brown.

Twilight Sky Photometry and Polarimetry: The Problem of Multiple Scattering at the Twilight Time

Abstract: This work is devoted to the study of solar light that experiences multiple scattering in the atmo- sphere at the twilight time, from the setting of the sun to nightfall. The work is based on polarimetric twilight sky observations at 3560 A that were conducted in the summer of 1997 at the Astronomical Observatory at Odessa University. These data were used to determine the fraction of multiple-scattered light in the total sky background for various points of the sky, various depths of the sun under the horizon, and various polarization directions. A critical analysis is performed of some methods previously employed to take into account multiple scattering in twilight observations. The altitude distribution of small-sized atmospheric aerosol particles in the stratosphere is also examined.

Determining orientation of images containing blue sky

Abstract: A method, image recognition system, computer program, etc., for determining image orientation. The invention classifies potential sky pixels in the image by color, identifies spatially contiguous regions of the potential sky pixels, identifies actual sky regions by eliminating ones of the spatially contiguous regions that have a texture above a predetermined texture threshold, computes desaturation gradients of the actual sky regions, classifies the image as one of portrait and landscape based on average absolute values of horizontal and vertical desaturation gradient of pixels within each of the actual sky regions, determines orientation of the image based on a polarity of the average horizontal and vertical desaturation gradients, and confirms that the actual sky regions are true sky regions by comparing the desaturation gradients with a predetermined desaturation gradient for sky.

The ROCSAT-1 IPEI Preliminary Results: Vertical Ion Drift Statistics

Abstract: A number of ground-based instruments,including 19.5GHz radiometer,optical raingauge,portable weather station,and high resolutiondisdrometer,were set up to conduct the Ka band propagation experimentof the Experimental Communication Payload(ECP)for ROCSAT-1.Inthis article,19.5GHz background sky noise temperatures measured atChung-Li and Tainan sites are presented and investigated.Long-term statisticsof the 19.5GHz background sky noise temperature observed by avertically pointed radiometer in precipitation-free condition over the Taiwanarea shows that the percentages of time that the sky noise temperatureexceeds 20K,30K,40K and 50K are,respectively,98%,85%,53%,and27%.However,in precipitating environments,statistics shows that the percentageof time that the sky noise temperature exceeds 55K,100K,150K,and 200K are,respectively,22%,13%,4.5%,and 2%.The statistics of skynoise temperatures observed at different zenith angles under environmentswithout precipitation is also made.The results show that 80% of the observedsky noise temperatures at zenith angles of 10°,30°,50°and 70°are,respectively,in the ranges of 92-180K,39-52K,26-33K,and 21-27K.In addition,a comparison between surface rainfall rate recorded by theoptical raingauge and sky noise temperature measured by 19.5GHz radiometershows that the former lags behind the latter by about 5 minutes,implying non-uniform and inhomogeneous distribution of precipitation inthe air.In order to measure the precipitation aloft,the Chung-Li VHF radarwas operated simultaneously.Champaign observation shows that thereis no latency between sky noise temperature and VHF backscatter fromprecipitation.This result implies that the VHF backscatter from precipitationcan be employed to validate the observed sky noise temperature.Inaddition,we also find that the sky noise temperature may be as high as 155K(corresponding to 3.6dB attenuation)under an environment withoutsurface precipitation.This feature is attributed to the dense water vaporand heavy cloud.

Extinction Curves, Distances, and Clumpiness of Diffuse Interstellar Dust Clouds

Abstract: We present CCD photometry in UBVRI of several thousand Galactic field stars in four large (>1 deg2) regions centered on diffuse interstellar dust clouds, commonly referred to as "cirrus" clouds (with optical depth AV less than unity). Our goal in studying these stars is to investigate the properties of the cirrus clouds. A comparison of the observed stellar surface density between on-cloud and off-cloud regions as a function of apparent magnitude in each of the five bands effectively yields a measure of the extinction through each cloud. For two of the cirrus clouds, this method is used to derive UBVRI star-count–based extinction curves, and U-band counts are used to place constraints on the cloud distance. The color distribution of stars and their location in (U-B, B-V) and (B-V, V-I) color-color space are analyzed in order to determine the amount of selective extinction (reddening) caused by the cirrus. The color excesses, Aλ-AV, derived from stellar color histogram offsets for the four clouds, are better fitted by a reddening law that rises steeply toward short wavelengths [RV ≡ AV/E(B - V) 2] than by the standard law (RV = 3.1). This may be indicative of a higher than average abundance of small dust grains relative to larger grains in diffuse cirrus clouds. The shape of the count-based effective extinction curve and a comparison of different estimates of the dust optical depth (extinction optical depth derived from background star counts/colors; emission optical depth derived from far infrared measurements) are used to measure the degree of clumpiness in clouds. The set of techniques explored in this paper can be readily adapted to the Sloan Digital Sky Survey data set in order to carry out a systematic, large-scale study of cirrus clouds.

Evaluation of a Ground-Based Sky Camera System for Use in Surface Irradiance Measurement

Abstract: This paper describes the evaluation of a ground-based sky camera system for studying the effect of clouds on the level of the ambient ultraviolet radiation. The system has been developed for research in the characterization of the effect of clouds around the sun. It is the first sky camera system to be used for the assessment of cloud conditions in the vicinity of the sun, rather than a whole-sky assessment. The system features a sun-tracking sky camera with an integrated measurement of horizontally received radiation at the same location. The imageprocessing algorithm uses solar radiation readings to reduce reflections from the sun on the camera system being mistaken for cloud in the images. Cloud amount was estimated in an angular region of between 12.58 and 37.58 around the sun. The algorithm also estimates the amount of solar obstruction by cloud (sun not covered, partially or totally covered). The system was evaluated during September 1997 at Toowoomba, Australia (27.68S latitude). Compared to manual assessment of 592 images, 76.5% were identified correctly by the algorithm for the degree of solar obstruction and 81.9% for cloud amount. The behavior of ultraviolet radiation levels with cloud conditions is discussed.

Toward a Continuous Record of the Sky

Abstract: It is currently feasible to start a continuous digital record of the entire sky sensitive to any visual magnitude brighter than 15 each night. Such a record could be created with a modest array of small telescopes, collectively generating no more than a few gigabytes of data daily. Alternatively, a few small telescopes could continually repoint to scan and record the entire sky down to any visual magnitude brighter than 15 with a recurrence epoch of at most a few weeks, again always generating less than 1 Gbyte of data each night. These estimates derive from CCD ability and budgets typical of university research projects. As a prototype, we have developed and are utilizing an inexpensive single-telescope system that obtains optical data from about 1500 deg2. We discuss the general case of creating and storing data from both an epochal survey, in which a small number of telescopes continually scan the sky, and a continuous survey, composed of a constellation of telescopes each dedicated to continually inspect a designated section of the sky. We compute speciÐc limitations of canonical surveys in visible light, and estimate that all-sky continuous visual light surveys could be sensitive to magnitude 20 in a single night by about 2010. Possible scientiÐc returns of continuous and epochal sky surveys include continued monitoring of most known variable stars, estab- lishing case histories for variables of future interest, uncovering new forms of stellar variability, discovering the brightest cases of microlensing, discovering new novae and supernovae, discovering new counterparts to gamma-ray bursts, monitoring known solar system objects, discovering new solar system objects, and discovering objects that might strike the Earth.

Rossi X-Ray Timing Explorer All-Sky Monitor Localization of SGR 1627-41

Abstract: The fourth unambiguously identified soft gamma repeater (SGR), SGR 1627-41, was discovered with the BATSE instrument on 1998 June 15. Interplanetary Network (IPN) measurements and BATSE data constrained the location of this new SGR to a 6° segment of a narrow (19'') annulus. We present two bursts from this source observed by the All-Sky Monitor (ASM) on the Rossi X-Ray Timing Explorer. We use the ASM data to further constrain the source location to a 5' long segment of the BATSE/IPN error box. The ASM/IPN error box lies within 03 of the supernova remnant G337.0-0.1. The probability that a supernova remnant would fall so close to the error box purely by chance is ~5%.

Structure in the first quadrant of the Galaxy: an analysis of TMGS star counts using the SKY model

Abstract: We analyse the stellar content of almost 300 deg2 of the sky close to the Galactic plane by directly comparing the predictions of the SKY model (Cohen and Wainscoat et al.) with star counts taken from the Two Micron Galactic plane Survey (TMGS: Garzon et al.). Through these comparisons we can examine discrepancies between counts and model and thereby elicit an understanding of Galactic structure. Over the vast majority of areas in which we have compared the TMGS data with the SKY predictions we find very good accord; so good that we are able to remove the disc source counts to highlight structure in the plane. The exponential disc is usually dominant, but by relying on the predicted disc counts of SKY we have been able to probe the molecular ring, spiral arms, and parts of the bulge. The latter is clearly triaxial. We recognize a number of off-plane dust clouds not readily included in models. However, we find that, whilst the simple exponential extinction function works well in the outer Galaxy, closer than about 4 kpc to the Galactic Centre the extinction drops dramatically. We also examine the shape of the luminosity function of the bulge and argue that the cores of all spiral arms we have observed contain a significant population of supergiants that provides an excess of bright source counts over those of a simple model of the arms. Analysis of one relatively isolated cut through an arm near longitude 65 degrees categorically precludes any possibility of a sech2z stellar density function for the disc.

The Sensitivity of Room Daylight to Sky Brightness

Abstract: The fundamental equation of daylighting links the daylight illuminance of a surface with the sky luminance distribution. The key element in the equation is the ‘daylight coefficient’ function. This indicates the sensitivity of illuminance to changes in the sky luminance distribution. Examples are given for mean and point illuminances in a room.

Extinction Curves, Distances, and Clumpiness of Diffuse Interstellar Dust Clouds

Abstract: We present CCD photometry in UBVRI of several thousand Galactic field stars in four large (>1 degree^2) regions centered on diffuse interstellar dust clouds, commonly referred to as ``cirrus'' clouds (with optical depth A_V less than unity). Our goal in studying these stars is to investigate the properties of the cirrus clouds. A comparison of the observed stellar surface density between on-cloud and off-cloud regions as a function of apparent magnitude in each of the five bands effectively yields a measure of the extinction through each cloud. For two of the cirrus clouds, this method is used to derive UBVRI star counts-based extinction curves, and U-band counts are used to place constraints on the cloud distance. The color distribution of stars and their location in (U-B, B-V) and (B-V, V-I) color-color space are analyzed in order to determine the amount of selective extinction (reddening) caused by the cirrus. The color excesses, A_lambda-A_V, derived from stellar color histogram offsets for the four clouds, are better fit by a reddening law that rises steeply towards short wavelengths [R_V==A_V/E(B-V)<=2] than by the standard law (R_V=3.1). This may be indicative of a higher-than-average abundance of small dust grains relative to larger grains in diffuse cirrus clouds. The shape of the counts-based effective extinction curve and a comparison of different estimates of the dust optical depth (extinction optical depth derived from background star counts/colors; emission optical depth derived from far infrared measurements), are used to measure the degree of clumpiness in clouds. The set of techniques explored in this paper can be readily adapted to the Sloan Digital Sky Survey data set in order to carry out a systematic, large-scale study of cirrus clouds.

First Results from the Parkes Multibeam High-velocity Cloud Survey

Abstract: With the onset of the HI Parkes All-Sky Survey (HIPASS), a new view of the high-velocity cloud (HVC) distribution in the southern sky is being revealed. The dense spatial sampling and unbiased coverage of HIPASS gives it multiple advantages over previous surveys of the southern sky. Detailed views of the clouds' structure and large mosaics of the HI sky allow us to link individual concentrations to larger structures (such as the Magellanic Clouds and the Galaxy), providing pictorial clues as to the origins of HVCs. It is clear that HVCs cannot be uniformly assigned a single origin scenario, and must be categorised appropriately

The First Results from the Parkes Multibeam High-Velocity Cloud Survey

Abstract: With the onset of the HI Parkes All-Sky Survey, a new view of the high-velocity cloud (HVC) distribution in the southern sky is being revealed. The dense spatial sampling and unbiased coverage of HIPASS gives it multiple advantages over previous surveys of the southern sky. Detailed views of the clouds' structure and large mosaics of the HI sky allow us to link individual concentrations to larger structures (such as the Magellanic Clouds and the Galaxy), providing pictorial clues as to the origins of HVCs. It is clear that HVCs cannot be uniformly assigned a single origin scenario, and must be categorised appropriately.

Search for Molecular Clouds toward Intermediate-to-High Latitude IRAS Sources in the Southern Sky

Abstract: We have conducted a search for molecular clouds toward southern IRAS point sources at intermediateto-high galactic latitude (|6| > 10°) in CO (J = 1-0) emission with the NANTEN telescope. The main purpose of the survey was to find a new sample of star-forming clouds unknown to date. Of the 29 targets, we detected CO emission toward 5 IRAS sources, i.e., 04591-0856, 05044-0325, 05050-0614, 06345-3023, and 13543—3941. Among 5 detections, the molecular cloud associated with 06345—3023 (G 239.2—16.3) was detected for the first time. If we assume the distance as being 1.5 kpc, this source is ~ 430 pc away from the galactic plane. Three sources 04591-0856 (G 208.3-28.4), 05044-0325 (G 203.5-24.7), and 05050-0614 (G 206.4-25.9) are located west to the Orion molecular clouds, and 13543-3941 (G 316.4+21.2) is located in the cometary globule CG 12. Four of the 5 clouds show a cometary shape with a "head-tail" distribution, and three of them have the IRAS source located at the edge opposite to the tail. We find that all of the CO clouds are associated with the optical nebulosities, which are likely to be reflection nebulae. A comparison of the present CO clouds with those in Ophiuchus, Taurus, Chamaeleon, and L 1333 indicates that starforming clouds tend to have a high column density, as well as a smaller ratio of the virial mass to the LTE mass.