Skylight Research Articles

Commission 21: Light of the Night Sky

Commission 21, one of IAU's smallest commissions, consists of some hundred members and consultants working to understand and describe the light of the night sky with emphasis on the diffuse components. Many more work on these topics without being members of the commission. Light is here defined in its broader sense of electromagnetic radiation of any frequency. The diffuse components of the light of the night sky encompass a variety of physical phenomena over the full range of cosmic distance scales and include scattered light, thermal emission, line emission, and any other emission phenomena producing a diffuse light source. These attract interest not only as scientific topics of study in their own right but also as an unwanted foreground or background against which all other sky phenomena are observed. Commission 21 has for mandate to promote research and availability of results on issues related to the diffuse light of the night sky. This document is a report on activities in this field and is not confined to the activities of its members, no distinction is made between work carried out by commission members and non commission members. The report is organized starting with a summary of the state of broad surveys that provide most of the observations. The report on developments in the various disciplines start with the sources closest to the observer known as airglow and progresses by way of the interplanetary and interstellar mediums to the increasingly distant integrated starlight, diffuse galactic light and diffuse emission in other galaxies ending with the extragalactic background radiation.

EFFECTS OF HEAT TREATMENT ON BLUE SAPPHIRES AS MONITORED BY ESR SPECTROSCOPY

We report effects of heat treatments on physical properties and finding optimal heating condition to add value to Thai blue sapphires. The color of sapphire arises from the presence of trace 3d-transition ions in its crystal lattice. For blue sapphire, the color is due to a charge transfer mechanism between Fe 2+ and Ti 4+ ions. However, iron may adopt both Fe 3+ and Fe 2+ due to oxygen vacancies. Fe 3+ and Fe 2+ yield sapphire yellow and green colors, respectively. Therefore, we have to convert as many as possible of Fe 3+ to Fe 2+ by heating the blue sapphire in N 2 atmosphere for 12 h. Experimental results reveal that the ratio of lattice parameter c/a increases with the heating temperature and reaches maximum at 1700°C, which can be caused by displacement of Fe 3+ ions or more Fe 3+ ions being converted to Fe 2+. ESR signals show that the number of Fe 3+ ions decreases roughly linearly with the heating temperature. The intense sky blue color was achieved after the 1500°C heat treatment, having the [Formula: see text] ratio ~0.78. The optimal heat treatment should therefore be at 1500°C in flowing N 2 atmosphere for Thai blue sapphires which yield intense sky blue color and good crystal clarity. The blue sapphires exhibited good clarity but light sky blue due to the increase in lightness after the treatment at 1700°C. A monoclinic distortion of the corundum structure has been found to start at the 1600°C treatment by ESR spectrometer. This is also clearly evident from low angle shifts of XRD peaks after heating at 1700°C. We can therefore conclude that the color change of Thai blue sapphires arises from the conversion of Fe 3+ to Fe 2+ and thus the change in crystal field. The monoclinic distortion of the crystal structure may also play an important role in coloring the sapphires after the heat treatment at 1600–1700°C.

Tests of a prototype multiplexed fiber-optic ultra-fast FADC data acquisition system for the MAGIC telescope

Ground-based Atmospheric Air Cherenkov Telescopes (ACTs) are successfully used to observe very high energy (VHE) gamma rays from celestial objects. The light of the night sky (LONS) is a strong background for these telescopes. The gamma ray pulses being very short, an ultra-fast read-out of an ACT can minimize the influence of the LONS. This allows one to lower the so-called tail cuts of the shower image and the analysis energy threshold. It could also help to suppress other unwanted backgrounds. Fast ‘flash’ analog-to-digital converters (FADCs) with GSamples/s are available commercially; they are, however, very expensive and power consuming. Here we present a novel technique of Fiber-Optic Multiplexing which uses a single 2 GSamples/s FADC to digitize 16 read-out channels consecutively. The analog signals are delayed by using optical fibers. The multiplexed (MUX) FADC read-out reduces the cost by about 85% compared to using one ultra-fast FADC per read-out channel. Two prototype multiplexers, each digitizing data from 16 channels, were built and tested. The ultra-fast read-out system will be described and the test results will be reported. The new system will be implemented for the read-out of the 17 m diameter MAGIC telescope camera.

Wide-angle polarimetry of the night sky: Measurements of atmospheric glow and zodiacal light

This paper is devoted to the analysis of the results of wide-angle polarimetric observations of the night sky, which has been carried out in December 2002 in the Southern Laboratory of Shternberg State Astronomical Institute (Crimea, Ukraine) in a color band close to the V band (the effective wavelength is 5250 A). We propose a method with the help of which the sky light was separated into basic components: the atmospheric glow, emission of weak stars, and the zodiacal light. The polarization map of extra-atmospheric emission of five sky zones close to the ecliptic is constructed, and it is shown that polarization of the sky background far from the antisolar point is determined, in the main, by the zodiacal light. For each observational period, the time dependence of the atmospheric luminosity and absorption is obtained.

DEVELOPMENT OF WIDE-FIELD IMAGING CAMERA FOR ZODIACAL LIGHT OBSERVATION

We have developed a wide-field imaging camera system, called WICZO, to monitor light of the night sky over extended period. Such monitoring is necessary for studying the morphology of interplanetary dust cloud and also the time and spatial variations of airglow emission. The system consists of an electric cooler a CCD camera with <TEX>$60\%$</TEX> quantum efficiency at 500nm, and a fish-eye lens with <TEX>$180^{\circ}$</TEX> field of view. Wide field imaging is highly desired in light of the night sky observations in general, because the zodiacal light and the airglow emission extend over the entire sky. This paper illustrates the design of WICZO, reports the result of its laboratory performance test, and presents the first night sky image, which was taken, under collaboration with Byulmaro Observatory, on top of Mt. Bongrae at Yongweol in January, 2004.

Polarization of sky light from a canopy atmosphere

Light from the clear sky is produced by the scattering of unpolarized sunlight by molecules of the atmosphere and is partially linearly polarized in the process. Singly scattered light, for instance, is fully polarized in viewing directions perpendicular to the sun direction and less and less so towards the parallel and antiparallel directions, where it is unpolarized. The true, multiple, scattering is much less tractable, but importantly different, changing the polarization pattern's topology by splitting the unpolarized directions into pairs. The underlying cause of this ‘symmetry breaking’ is that the atmosphere is ‘wider’ than it is deep. Simplifying as much as possible while retaining this feature leads to the caricature atmosphere analysed here: a flattened sheet atmosphere in the sky, a canopy. The multiple scattering is fully tractable and leads to a simple polarization pattern in the sky: the ellipses and hyperbolas of standard confocal ellipsoidal coordinates. The model realizes physically a mathematical pattern of polarization in terms of a complex function proposed by Berry, Dennis and Lee (2004 New J. Phys.6 162) as the simplest one which captures the topology.

Sky lights: Picture might show an extrasolar planet

Science NewsVolume 166, Issue 12 p. 179-179 This Week Sky lights: Picture might show an extrasolar planet Ron Cowen, Ron CowenSearch for more papers by this author Ron Cowen, Ron CowenSearch for more papers by this author First published: 02 July 2009 https://doi.org/10.2307/4015490AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume166, Issue1218 September 2004Pages 179-179 RelatedInformation

On the Influence of Asymmetry of Short Wind Waves on Statistical Characteristics of the Sea-Surface Image in Diffuse Sky Light

We develop an optical model of imaging of wind roughness on a sea surface in diffuse sky light with allowance for bound waves (parasitic capillary ripple) on the front slope of decimeter gravity waves. It is found that in the case of oblique observation, free and bound waves give different contribution to the statistical characteristics of the image of a rough sea surface.

Determination of the night sky background around the Crab pulsar using its optical pulsation

The poor angular resolution of imaging γ-ray telescopes is offset by the large reflector areas of next generation telescopes such as MAGIC (17 m diameter), which makes the study of optical emission associated with some γ-ray sources feasible. Furthermore, the extremely fast time response of photomultipliers (PMs) makes them ideal detectors for fast (subsecond) optical transients and periodic sources like pulsars. The optical pulse of the Crab pulsar was detected with the HEGRA CT1 central pixel using a modified PM, similar to the future MAGIC camera PMs. The HEGRA CT1 telescope is a 2.1 m radius Čerenkov telescope, which has seen the Crab optical pulsations. The corresponding observation time required for a detection is 200 s, but MAGIC will only require ∼30 s if the same strategy with standard PMs is used. The constancy of the pulsed fraction (with a relatively small error) during the 2 h CT1 measurements shows that the pointing did not change during the observations. The purpose of these periodic observations was to determine the light of the night sky (LONS) for the galactic anticenter Crab region, and to compare it with the LONS for a nearby bright star ( ζ Tauri). These obtained LONS values vary between 4.6 and 4.3 × 10 12 ph m −2 s −1 sr −1 (with a coarse estimated error of 10%), being the first value from the timing signature of the Crab pulsar, and the second one was derived from the technique using the ζ Tauri standard star. Our results are between 2.5 and 3 times larger than the previously measured LONS (outside the galactic plane) of (1.7 ± 0.4) × 10 12 ph m −2 s −1 sr −1, as expected since the Crab pulsar is in the galactic plane, which implies a slightly higher energy threshold for Crab observations, if the higher value of CT1 measured LONS rate for galactic sources is used.

Spectroscopic measurements of NO2 in a Colorado thunderstorm: Determination of the mean production by cloud‐to‐ground lightning flashes

Ground‐based visible absorption spectroscopy of zenith‐scattered sky light was used to measure changes in the column abundance of NO2 during the passage of a thunderstorm over Boulder, Colorado, on September 12, 2002. The measurements showed a 10‐fold rise in slant column NO2 within the thunderstorm cell, consistent with an increase of about 7 ppbv in the lower core updrafts and main precipitation shaft. Mean mixing ratios of about 3 and 1.5 ppbv were inferred for the upper cell and anvil, respectively. These observations, together with measurements from the Denver NEXRAD and the Colorado State University (CSU) CHILL radars, and cloud‐to‐ground (CG) lightning flash data from the National Lightning Detection Network (NLDN), are used to estimate the flash mean production of NOx by CG lightning. The measurements suggest a mean production rate of (5.8 ± 2.9) × 1026 NOx molecules per CG flash, intermediate between the values derived from previous spectroscopic studies and in good agreement with the widely used value of 6.7 × 1026 NOx molecules per CG flash estimated by Price et al. [1997]. This agreement may be fortuitous, however, since the lightning in the present study was about a factor of two less energetic than the mean assumed by Price et al. and the production of (2.3 ± 1.4) × 1017 NOx/J inferred here is significantly larger than the value of 1 × 1017 NOx/J assumed in their calculations. These differences can be reduced, however, if the estimates of Price et al. are recalculated using newer peak current data from the NLDN, and more recently published flash energy production rates of Wang et al. [1998].

SATELLITE FLUORESCENCE AS A MEASURE OF OCEAN SURFACE CHLOROPHYLL

Both the MODIS optical sensors on the NASA Aqua and Terra spacecraft and the MERIS instrument on the ESA Envisat measure spectral radiance in bands designed to allow detection of the fluorescence signal from surface chlorophyll in sea water, stimulated by ambient sun and sky light. We present images of fluorescence as derived from the level 1 MERIS data and compare the signal levels of the fluorescence with measurements of extracted chlorophyll from research cruises during 2002 to 2004, with satellite estimates of chlorophyll from the blue to green ratio observed by MERIS, and with fluorescence measurements made by MODIS. We show that the fluorescence signal is consistent between MERIS and MODIS, and that relation between fluorescence and chlorophyll concentration in surface waters is consistent with a simple model accounting for absorption of stimulating and emitted radiation by chlorophyll pigments. Considerable scatter is observed, suggesting variable fluorescence efficiency and effects of coloured dissolved organic matter

The optical calibration of the MAGIC telescope camera

A 17-m-diameter air Cherenkov telescope, dubbed MAGIC, for ground-based gamma-ray astronomy above 30 GeV is under construction on the Canary Island of La Palma. The 577-pixel photomultiplier camera requires precise and regular calibration over a large dynamic range. A system for the optical calibration consisting of a number of very fast and powerful light emitting diode light pulsers is presented. We intend to calibrate each individual pixel up to 2000-3000 photoelectrons with different wavelengths, e.g. 370, 460, and 520 nm. We aim to achieve an absolute calibration at these three wavelengths by comparing the signal of the pixels with the one of specially prepared and calibrated photomultipliers and, at a later stage, hybrid photon detectors operated in single photon counting mode and with the use of well-calibrated attenuation filters (blind pixels). The light flux of the pulser is cross calibrated by a 1 cm/sup 2/ PIN diode (Hamamatsu), read out via a charge sensitive preamplifier. The PIN diode is calibrated with 60-keV gammas (from an Am 241 source) producing a precise signal of 16 570 electron-hole pairs. In addition, there will be a computer adjustable continuous light source to simulate and calibrate the response of the PMTs in the camera to the moon and the light of the night sky. This will also help to handle starlight in the field of view of the camera during analysis. Special attention is paid toward building a robust field usable device complying with IP 65 standards (sealed against heavy jet spray and dust).

The Politics of Sky Battles in Early Hanoverian Britain

We need not wonder at Aerial Knights, At elemental combats, and strange fights, when earthly monarchs thus renew their jars, and even all Europe is involv'd in wars. (William Lux,Poems on Several Occasions)On 6 March 1716, theHistorical Registerfeatured a description of an extravagant “display” of sky lights above London. The newspaper reported that during the evening, a number of Londoners noticed something that at first appeared like a huge and motionless body of light that in a curtain-like shape hung over the sky. They then saw it starting to move in a random fashion and dissolve into parts, spreading toward the west and changing into gigantic “pillars of flame.” From the west horizon and with an incredible velocity the lights continued to dart southeastward. After a series of undulatory motions and vibrations, they turned into a “continual fulguration, interspersed with green, red, blue, and yellow.” What were they? The celebrated astronomer Edmund Halley wrote that he was afraid that in delivering the “Etiology of a Matter so uncommon, never before seen by myself, nor fully described by any of the Ancient and Moderns, I fail to answer [philosophers'] Expectation or my own Desires.” But the less scholarly sources had less trouble. In Oxford, people saw swords and cavalries in the sky. In Upton upon Severn, Worcestershire, two heavenly armies waged a war that was so close that the air smelled of gunpowder.

Brewster-angle measurements of sea-surface reflectance using a high resolution spectroradiometer

This paper describes the design, construction and testing of a ship-borne spectroradiometer base do n a ni maging spectrograph and cooled CCD array with a wavelength range of 350–800 nm and 4 nm spectral sampling. The instrument had a minimum spectral acquisition time of 0.1 s, but in practice data were collected over periods of 10 s to allow averaging of wave effects. It was mounted on a ship’s superstructure so that it viewed the sea surface from a height of several metres at the Brewster angle (53 ◦ )t hrough a linear polarizing filter. Comparison of sea-leaving spectra acquired with the polarizer oriented horizontally and vertically enabled estimation of the spectral composition of sky light reflected directly from the sea surface. A semi-empirical correction procedure was devised for retrieving water-leaving radiance spectra from these measurements while minimizing the influence of reflected sky light. Sea trials indicated that reflectance spectra obtained by this method were consistent with the results of radiance transfer modelling of case 2 waters with similar concentrations of chlorophyll and coloured dissolved organic matter. Surface reflectance signatures measured at three locations containing blooms of different phytoplankton species were easily discriminated and the instrument was sufficiently sensitive to detect solar-stimulated fluorescence from surface chlorophyll concentrations down to 1 mg m −3 .

Simulation and visualization of atmospheric light phenomena induced by light scattering

This paper presents a method of visualizing some of the light phenomena visible in the outdoors such as sunsets, rainbows, purple lights or halos, These phenomena are produced by the scattering of sunlight and sky light on atmospherical particles. The enormous increase in computing power in the last ten years has made it possible to correctly simulate some of these light phenomena and render them visible, using computer-graphics techniques. The created visualization environment combines the mathematical basis of the scattering process (Rayleigh- and Mie-scattering) with real world properties for the atmosphere (pure air, aerosols, ozone) and its particles (soot, dust, water, ice crystals) to reproduce light effects by specifying sun position and the atmospheric composition.

Simulation and Visualization of Atmospheric Light Phenomena Induced by Light Scattering

This paper presents a method of visualizing some of the light phenomena visible in the outdoors such as sunsets, rainbows, purple lights or halos. These phenomena are produced by the scattering of sunlight and sky light on atmospherical particles. The enormous increase in computing power in the last ten years has made it possible to correctly simulate some of these light phenomena and render them visible, using computer-graphics techniques. The created visualization environment combines the mathematical basis of the scattering process (Rayleigh- and Mie-scattering) with real world properties for the atmosphere (pure air, aerosols, ozone) and its particles (soot, dust, water, ice crystals) to reproduce light effects by specifying sun position and the atmospheric composition.

MULTIPLE REFERENCE USAGE IN THE ZONAL RECOVERY BEHAVIOR BY THE FIDDLER CRAB UCA CUMULANTA

Abstract Prevalence of certain celestial and terrestrial cues was tested in the zonal recovery behavior and orientation of the fiddler crab Uca cumulanta. Experiments were conducted in the field bordering a mangrove forest on the eastern Venezuelan coast. Uca cumulanta exhibited zonal recovery behavior based on different orientational references, including celestial cues (sun and polarized sky light) and terrestrial cues (landscape vision and substrate slope). Sun azimuth orientation was time-compensated. Apparent solar position and polarized sunlight pattern were celestial cues which, as tested, prevailed over landscape references and beach slope—terrestrial cues—in cueing the direction of the orientation response shown by crabs.

Multiple Reference Usage in the Zonal Recovery Behavior by the Fiddler Crab Uca Cumulanta

Prevalence of certain celestial and terrestrial cues was tested in the zonal recovery behavior and orientation of the fiddler crab Uca cumulanta. Experiments were conducted in the field bordering a mangrove forest on the eastern Venezuelan coast. Uca cumulanta exhibited zonal recovery behavior based on different orientational references, including celestial cues (sun and polarized sky light) and terrestrial cues (landscape vision and substrate slope). Sun azimuth orientation was time-compensated. Apparent solar position and polarized sunlight pattern were celestial cues which, as tested, prevailed over landscape references and beach slope—terrestrial cues—in cueing the direction of the orientation response shown by crabs.

Retrieval of the scattering and microphysical properties of aerosols from ground-based optical measurements including polarization I Method

A method has been developed for retrieving the scattering and microphysical properties of atmospheric aerosol from measurements of solar transmission, aureole, and angular distribution of the scattered and polarized sky light in the solar principal plane. Numerical simulations of measurements have been used to investigate the feasibility of the method and to test the algorithm's performance. It is shown that the absorption and scattering properties of an aerosol, i.e., the single-scattering albedo, the phase function, and the polarization for single scattering of incident unpolarized light, can be obtained by use of radiative transfer calculations to correct the values of scattered radiance and polarized radiance for multiple scattering, Rayleigh scattering, and the influence of ground. The method requires only measurement of the aerosol's optical thickness and an estimate of the ground's reflectance and does not need any specific assumption about properties of the aerosol. The accuracy of the retrieved phase function and polarization of the aerosols is examined at near-infrared wavelengths (e.g., 0.870 mum). The aerosol's microphysical properties (size distribution and complex refractive index) are derived in a second step. The real part of the refractive index is a strong function of the polarization, whereas the imaginary part is strongly dependent on the sky's radiance and the retrieved single-scattering albedo. It is demonstrated that inclusion of polarization data yields the real part of the refractive index.

Estimation of global illuminance for clear skies at Madrid

Two elementary empirical models for the dependence of global illuminance on solar elevation for clear skies are provided. The models are developed from hourly data obtained from 1 year data collected at the International Daylight Measurement Programme (IDMP) general class station at the Escuela Técnica Superior de Arquitectura in Madrid (ETSAM), and are statistically assessed with data obtained during another period of 6 months. Values of the mean bias deviation (MBD) and root mean square deviation (RMSD) of respectively less than 5% and 7% of the average illuminance measured were obtained. The qualitative dependence of global illuminance on solar elevation, common to that found by other researchers for other locations is justified, and found to be different to that given by Vázquez and Bernabeu [D. Vázquez, E. Bernabeu, Quantitative estimation of clear sky light in Madrid, Energy and Buildings, 26 (1997) 331] using data obtained at a site which is in close proximity to our station.