Patch Of Sky Research Articles

Extragalactic Fields Optimized for Adaptive Optics

ABSTRACTIn this article we present the coordinates of 67 55′ × 55′ patches of sky that have the rare combination of both high stellar surface density (≥0.5 arcmin-2 with 13 < R < 16.5 mag) and low extinction (E(B - V)≤0.1). These fields are ideal for adaptive-optics-based follow-up of extragalactic targets. One region of sky, situated near Baade’s Window, contains most of the patches we have identified. Our optimal field, centered at R.A.: 7h24m3s, decl.: -1°27′15″, has an additional advantage of being accessible from both hemispheres. We propose a figure of merit for quantifying real-world adaptive optics performance and use this to analyze the performance of multiconjugate adaptive optics in these fields. We also compare our results with those that would be obtained in existing deep fields. In some cases adaptive optics observations undertaken in the fields given in this article would be orders of magnitude more efficient than equivalent observations undertaken in existing deep fields.

Efficient power spectrum estimation for high resolution CMB maps

Estimation of the angular power spectrum of the cosmic microwave background on a small patch of sky is usually plagued by serious spectral leakage, especially when the map has a hard edge. Even on a full-sky map, point source masks can alias power from large scales to small scales producing excess variance at high multipoles. We describe a new fast, simple, and local method for estimation of power spectra on small patches of the sky that minimizes spectral leakage and reduces the variance of the spectral estimate. For example, when compared with the standard uniform sampling approach on a $8\text{ }\text{ }\mathrm{degree}\ifmmode\times\else\texttimes\fi{}8\text{ }\text{ }\mathrm{degree}$ patch of the sky with 2% area masked due to point sources, our estimator halves the error bars at $\ensuremath{\ell}=2000$ and achieves a more than fourfold reduction in error bars at $\ensuremath{\ell}=3500$. Thus, a properly analyzed experiment will have error bars at $\ensuremath{\ell}=3500$ equivalent to those of an experiment analyzed with the now standard technique with $\ensuremath{\sim}16--25$ times the integration time.

Cross-calibration of cluster mass observables

This paper is a first step towards developing a formalism to optimally extract dark energy information from number counts using multiple cluster observation techniques. We use a Fisher matrix analysis to study the improvements in the joint dark energy and cluster mass-observables constraints resulting from combining cluster counts and clustering abundances measured with different techniques. We use our formalism to forecast the constraints in Ω DE and w from combining optical and SZ cluster counting on a 4000 sq. degree patch of sky. We find that this cross-calibration approach yields ∼2 times better constraints on Ω DE and w compared to simply adding the Fisher matrices of the individually self-calibrated counts. The cross-calibrated constraints are less sensitive to variations in the mass threshold or maximum redshift range. A by-product of our technique is that the correlation between different mass-observables is well constrained without the need of additional priors on its value.

Cosmic magnetic fields in galaxies, groups and clusters

AbstractThe distribution of the Faraday depth induced by galaxies, groups and clusters on a patch of sky is investigated. For instance, we utilise a halo model approach to obtain synthetic Faraday skies. Moreover, our modelling includes cluster physical as well as cosmological aspects. A SKA sky survey will provide a large sample of rotation measures of polarised sources. Hence, we examine to what extent statistics of rotation measures of these sources can yield information about cosmic magnetic fields and cosmology.

TRIS. III. The Diffuse Galactic Radio Emission at δ = +42°

We present values of temperature and spectral index of the Galactic diffuse radiation measured at 600 and 820 MHz along a 24h right ascension circle at declination δ = + 42°. They have been obtained from a subset of absolute measurements of the sky temperature made with TRIS, an experiment devoted to the measurement of the CMB temperature at decimetric wavelengths with an angular resolution of about 20°. Our analysis confirms the preexisting picture of the Galactic diffuse emission at decimetric wavelength and improves the accuracy of the measurable quantities. In particular, the signal coming from the halo has a spectral index in the range 2.9-3.1 above 600 MHz, depending on the sky position. In the disk, at TRIS angular resolution, the free-free emission accounts for the 11% of the overall signal at 600 MHz and 21% at 1420 MHz. The polarized component of the Galactic emission, evaluated from the survey by Brouw and Spoelstra, affects the observations at TRIS angular resolution by less than 3% at 820 MHz and less than 2% at 600 MHz. Within the uncertainties, our determination of the Galactic spectral index is practically unaffected by the correction for polarization. Since the overall error budget of the sky temperatures measured by TRIS at 600 MHz, that is, 66 (systematic) + 18 (statistical) mK, is definitely smaller than those reported in previous measurements at the same frequency, our data have been used to discuss the zero levels of the sky maps at 150, 408, 820, and 1420 MHz in literature. Concerning the 408 MHz survey, limiting our attention to the patch of sky corresponding to the region observed by TRIS, we suggest a correction of the base level of +3.9 ± 0.6 K.

Imaging polarimetry of forest canopies: how the azimuth direction of the sun, occluded by vegetation, can be assessed from the polarization pattern of the sunlit foliage

Radiance, color, and polarization of the light in forests combine to create complex optical patterns. Earlier sporadic polarimetric studies in forests were limited by the narrow fields of view of the polarimeters used in such studies. Since polarization patterns in the entire upper hemisphere of the visual environment of forests could be important for forest-inhabiting animals that make use of linearly polarized light for orientation, we measured 180 degrees field-of-view polarization distributions in Finnish forests. From a hot air balloon we also measured the polarization patterns of Hungarian grasslands lit by the rising sun. We found that the pattern of the angle of polarization alpha of sunlit grasslands and sunlit tree canopies was qualitatively the same as that of the sky. We show here that contrary to an earlier assumption, the alpha-pattern characteristic of the sky always remains visible underneath overhead vegetation, independently of the solar elevation and the sky conditions (clear or partly cloudy with visible sun's disc), provided the foliage is sunlit and not only when large patches of the clear sky are visible through the vegetation. Since the mirror symmetry axis of the alpha-pattern of the sunlit foliage is the solar-antisolar meridian, the azimuth direction of the sun, occluded by vegetation, can be assessed in forests from this polarization pattern. Possible consequences of this robust polarization feature of the optical environment in forests are briefly discussed with regard to polarization-based animal navigation.

Initial results from Harvard all-sky optical SETI

We report initial results from the Harvard/Planetary Society all-sky search for pulsed optical signals from other civilizations, which saw “first light” on 11 April 2006 after 6 years of planning and construction. To survey the northern sky ( - 20 ∘ < δ < + 70 ∘ ) , our 1.8-m spherical f / 2.5 optical telescope images a 1 . 6 ∘ × 0 . 2 ∘ patch of sky on two matched focal planes with a total of 1024 photomultiplier tube pixels. Each pair of pixels images the same 2.3 square arcminute patch of sky, and fast electronics filters the incoming visual band light for nanosecond pulses. Coincident optical flashes in a pair of pixels triggers one of 32 PulseNet full-custom chips to record the pulse profiles at nanosecond resolution. This experiment is an all-sky, kilopixel evolution of our targeted search. Its meridian transit survey mode requires ∼ 150 clear nights to cover the northern sky with 1-min dwell time per source point. Focus in this talk will be given to describing the capabilities of the all-sky search instrument, the first few months observations, and constraints that these observations place on the density of pulsed optical signals in the galaxy.

Separating polarized cosmological and galactic emissions for cosmic microwave background B-mode polarization experiments

The detection and characterization of the B mode of Cosmic Microwave Background (CMB) polarization anisotropies will not be possible without a high precision removal of the foreground contamination present in the microwave band. In this work we study the relevance of the component separation technique based on the Independent Component Analysis (ICA) for this purpose and investigate its performance in the context of a limited sky coverage observation and from the viewpoint of our ability to differentiate between cosmological models with different primordial B-mode content. We focus on the low Galactic emission sky patch centered at 40 degrees in right ascension and -45 in declination, corresponding to the target of several operating and planned CMB experiments and which, in many respects, adequately represents a typical “clean” high latitude sky. We consider two fiducial observations, one operating at low (40, 90 GHz) and one at high (150, 350 GHz) frequencies and thus dominated by the synchrotron and thermal dust emission, respectively. We use foreground templates simulated in accordance with the existing observations in the radio and infrared bands, as well as the Wilkinson Microwave Anisotropy Probe (WMAP) and Archeops data and model the CMB emission adopting the current best fit cosmological model, with an amplitude of primordial gravitational waves either set to zero or 10%. We use a parallel version of the FastICA code to explore a substantial parameter space including Gaussian pixel noise level, observed sky area and the amplitude of the foreground emission and employ large Monte Carlo simulations to quantify errors and biases pertinent to the reconstruction for different choices of the parameter values. We identify a large subspace of the parameter space for which the quality of the CMB reconstruction is excellent, i.e., where the errors and biases introduced by the separation are found to be comparable or lower than the uncertainty due to the cosmic variance and instrumental noise. For both the cosmological models, with and without the primordial gravitational waves, we find that FastICA performs extremely well even in the cases when the B mode CMB signal is up to a few times weaker than the foreground contamination and the noise amplitude is comparable with the total CMB polarized emission. In addition we discuss limiting cases of the noise and foreground amplitudes, for which the ICA approach fails. Although our conclusions are limited by the absence of systematics in the simulated data, these results indicate that these component separation techniques could play a

Analyzing sunlight duration and optimum shading using a sky map

A method of predicting the sunlight duration on windows in a densely packed building environment is presented. This method is based on dividing the hemispherical sky into small patches of 5°×5° and determining (using long-term climatic data) the probable sunlight duration with the sun locating in each sky patch. Hence, a sky map is constructed which gives the sunlight duration from every patch of the sky. A graphical tool and a spreadsheet calculation method have also been developed for designing external shading and calculation of probable sunlight duration. Optimum external shading configurations are suggested for different façade orientations of an isolated building in Hong Kong.

Evaluating an Indoor Daylight Illuminance Calculation Tool Against Full-Scale Measured Data

The determination of daylight illuminance in internal spaces is a key issue in daylighting analysis. The recently introduced concept of the daylight coefficient (DC), which considers changes in the luminance of sky patches, provides an accurate way for calculating indoor daylight illuminance under various sky patterns and sun positions. This paper proposes a simplified numerical procedure to estimate interior daylight illuminance using the DC concept. The paper presents an evaluation of the new method via full-scale field measurements under different sky conditions. The findings indicate that the daylight illuminances obtained by the proposed approach were in agreement with measured readings. The simple nature of the proposed method offers building professionals and students a reliable and convenient alternative to extensive calculations that incorporates sky luminance variations to predict the daylight illuminance and assess visual performance under various sky conditions.

The Mexican hat wavelet family: application to point-source detection in cosmic microwave background maps

We propose a new detection technique in the plane based on an isotropic wavelet family. This family is naturally constructed as an extension of the Gaussian–Mexican hat wavelet pair and for that reason we call it the Mexican hat wavelet family (MHWF). We show the performance of these wavelets when dealing with the detection of extragalactic point sources in cosmic microwave background (CMB) maps: a very important issue within the most general problem of the component separation of the microwave sky. Specifically, flat two-dimensional simulations of the microwave sky comprising all astrophysical components plus instrumental noise have been analysed for the channels at 30, 44 and 70 GHz of the forthcoming ESA Planck mission Low Frequency Instrument (LFI). We adopt up-to-date cosmological evolution models of extragalactic sources able to fit well the new data on high-frequency radio surveys and we discuss our current results on point-source detection by comparing them with those obtained using the Mexican hat wavelet (MHW) technique, which has been already proven a suitable tool for detecting point sources. By assuming a 5 per cent reliability level, the first new members of the MHWF, at their ‘optimal scale’, provide three point-source catalogues on half of the sky (at Galactic latitude |b| > 30°) at 30, 44 and 70 GHz of 639, 387 and 340 extragalactic sources, respectively. The corresponding flux detection limits are 0.38, 0.45 and 0.47 Jy. By using the same simulated sky patches and at the same frequencies as before, the MHW at its optimal scale provides 543, 322 and 311 sources with flux detection limits of 0.44, 0.51 and 0.50 Jy, respectively (5 per cent reliability level). These results show a clear improvement when we use the new members of the MHWF and, in particular, the MHW2 with respect to the MHW.

May day celebrations

May Day, the public holiday, created in honour of working people - and a typically rainy start to the day. But there is good news about the NHS on the radio for a change. From today nurses will be able to prescribe all products listed in the British National Formulary (BNF) in order to provide an improved service to patients. I felt a warm glow that at last we have achieved what has been described as sense over tradition (Cumberlege, 2003). Today's weather report rather reflects the development of prescribing by nurses, a showery start, an overcast and cloudy day with patches of blue sky and promise of warmer weather on the horizon!

Deflections of cosmic rays in a random component of the Galactic magnetic field

We express the mean square deflections of the ultra-high energy cosmic rays (UHECR) caused by the random component of the Galactic magnetic field (GMF) in terms of the GMF power spectrum. We use recent measurements of the GMF spectra in several sky patches to estimate the deflections quantitatively. We find that deflections due to the random field constitute 0.03–0.3 of the deflections which are due to the regular component and depend on the direction on the sky. They are small enough not to preclude the identification of UHECR sources, but large enough to be detected in the new generation of UHECR experiments.

A Method to Construct Virtual Sky Domes for Use in Standard CAD-based Light Simulation Software

A Virtual Sky Dome (VSD) is a computational model of a hemisphere resembling the daylight conditions of a particular location, date and time. VSD's allow users of standard CAD-based simulation software to simulate architecture and assess daylight performance criteria making full use of the 15 standard sky luminance distribution models, which have just recently been accepted as a CIE standard [1]. This paper introduces the method to construct VSD. Prior to this any daylight simulation using standard CAD based light simulation software was limited to a smaller set of standard skies: clear, overcast and intermediate with non-changeable parameters as in the case of the software Lightscape. The all-weather-sky tool developed for the software Radiance follows the sky definitions defined by Perez. Neither of these tools considers the recently developed standard of 15 sky types, which is to replace previous standards[2] to further differentiate the skies to better match the real daylight conditions. This is of particular significance for predominately cloudy regions like Singapore, where a wide range of overcast and intermediate skies with significant different luminance distributions occur. A VSD is a CAD model of the hemisphere comprising 145 point lights of controllable parameters, each one sitting in the centre of a particular sky patch. Their luminous flux is derived from the luminance of the individual sky patch calculated using Kittler's equation[3]. A computational method has been developed that creates a VSD for a given sky type based on the input of solar azimuth and elevation, hence representing a particular time and location. This paper explains the method and shows results using VSD's to simulate daylight with the standard CAD-based light simulation software Lightscape.

The Second Data Release of the Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) has validated and made publicly available its Second Data Release. This data release consists of 3324 deg2 of five-band (ugriz) imaging data with photometry for over 88 million unique objects, 367,360 spectra of galaxies, quasars, stars, and calibrating blank sky patches selected over 2627 deg2 of this area, and tables of measured parameters from these data. The imaging data reach a depth of r ≈ 22.2 (95% completeness limit for point sources) and are photometrically and astrometrically calibrated to 2% rms and 100 mas rms per coordinate, respectively. The imaging data have all been processed through a new version of the SDSS imaging pipeline, in which the most important improvement since the last data release is fixing an error in the model fits to each object. The result is that model magnitudes are now a good proxy for point-spread function magnitudes for point sources, and Petrosian magnitudes for extended sources. The spectroscopy extends from 3800 to 9200 Å at a resolution of 2000. The spectroscopic software now repairs a systematic error in the radial velocities of certain types of stars and has substantially improved spectrophotometry. All data included in the SDSS Early Data Release and First Data Release are reprocessed with the improved pipelines and included in the Second Data Release. Further characteristics of the data are described, as are the data products themselves and the tools for accessing them.

Daylight Models—Singapore Sky

The daylight factor approach, based the assumption of uniform sky luminance distribution, has served the purpose of daylighting design in buildings for a long time. With the availability of cheaper and abundant computing powers, it is now possible to make use of more sophisticated algorithms, which can make use of the graded distribution of luminance in the sky vault. The variation of luminance distribution in the sky vault is of particular interest in ascertaining the glare problem in the application of daylight in the tropics. The daylight data of horizontal and vertical illuminances and irradiances in Singapore were analyzed by Ullah [1,2]. The illuminance data were applied to predict the potential of energy conservation with the integration of daylight with the indoor artificial lights[3]. The study of daylight in Singapore continued with more detailed observations using the sky scanner, which divides typically the sky into 145 circular patches. The luminance value of every single patch is captured by a computer-controlled scanner fitted with a luminance meter; simultaneous solar irradiance is also recorded with a solarimeter. There is a built-in safety device in the scanner that closes the meter aperture, whenever the scanner directly points to the sun. Thus, the scanner data are meant for evaluating the diffuse luminance distribution in the sky vault. The scanner is being used to collect the data continuously from 1996 till now at National University of Singapore, except for some lapses for repairs and maintenance. In 1998, Lam et al4 made use of part of these data to evaluate six sky luminance prediction models as an effort to adopt a sky luminance models for Singapore. Kittler, Darula and Perez5 proposed a set of 15 sky models for the world-wide use. Sky patch luminance and horizontal diffuse illuminance models were formulated as functions of solar and sky patch geometry for the sky luminance and of solar geometry for the horizontal diffuse illuminance, both normalized as the ratios to the zenith luminance. While the basic morphology of the function was kept, various sky types were represented by assigning differing values to the constants used in the functions. These models were also evaluated by Tregenza6 with the data for Fukuoka (Japan), Garston (UK) and Sheffield (UK) and again with a limited amount of data for Singapore. This paper attempts to make a thorough evaluation of the 15 sky models proposed by Kittler etal5 by calculating the sky patch luminance and horizontal diffuse illuminane using their equations and comparing them against the sky scanned measured data in Singapore and to obtain the equations which fit the best of these data.

The First Data Release of the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has validated and made publicly available its First Data Release. This consists of 2099 square degrees of five-band (u, g, r, i, z) imaging data, 186,240 spectra of galaxies, quasars, stars and calibrating blank sky patches selected over 1360 square degrees of this area, and tables of measured parameters from these data. The imaging data go to a depth of r ~ 22.6 and are photometrically and astrometrically calibrated to 2% rms and 100 milli-arcsec rms per coordinate, respectively. The spectra cover the range 3800--9200 A, with a resolution of 1800--2100. Further characteristics of the data are described, as are the data products themselves.

An analysis of all-sky zenith luminance data for Hong Kong

Sky luminance distribution data are of great importance to estimate internal daylight illuminance for building energy modelling. Sky luminance of a particular sky patch is always expressed as a ratio of the corresponding zenith luminance. For places where measured zenith luminance is not available, generating the required data from conversion models would be an appropriate alternative. This paper describes the sky luminance measurements made in 1999 at the City University of Hong Kong and reports the findings. A simple model relating zenith luminance and horizontal diffuse illuminance has been proposed. An evaluation of the proposed model and the Perez zenith luminance model has been carried out. It has been found that the proposed model gives reasonably good agreement with the measured data and outperforms the Perez model. The simplicity nature of the proposed model provides a convenient and reliable approach for building designers in estimating the zenith luminance as well as the whole sky luminance distribution.

Filtering techniques for the detection of Sunyaev-Zel'dovich clusters in multifrequency maps

The problem of detecting Sunyaev-Zel'dovich (SZ) clusters in multifrequency CMB observations is investigated using a number of filtering techniques. A multifilter approach is introduced, which optimizes the detection of SZ clusters on microwave maps. An alternative method is also investigated, in which maps at different frequencies are combined in an optimal manner so that existing filtering techniques can be applied to the single combined map. The SZ profiles are approximated by the circularly-symmetric template $\tau (x) = [1 +(x/r_c)^2]^{-\lambda}$, with $\lambda \simeq \tfrac{1}{2}$ and $x\equiv |\vec{x}|$, where the core radius $r_c$ and the overall amplitude of the effect are not fixed a priori, but are determined from the data. The background emission is modelled by a homogeneous and isotropic random field, characterized by a cross-power spectrum $P_{\nu_1 \nu_2}(q)$ with $q\equiv |\vec{q}|$. The filtering methods are illustrated by application to simulated Planck observations of a $12.8^\circ \times 12.8^\circ$ patch of sky in 10 frequency channels. Our simulations suggest that the Planck instrument should detect $\approx 10000$ SZ clusters in 2/3 of the sky. Moreover, we find the catalogue to be complete for fluxes $S > 170$ mJy at 300 GHz.

A sample of 6C radio sources designed to find objects at redshift z &gt; 4 - II. Spectrophotometry and emission-line properties

This is the second in a series of three papers that present and interpret basic observational data on the 6C* 151-MHz radio sample: a low-frequency selected sample that exploits filtering criteria based on radio properties (steep spectral index and small angular size) to find radio sources at redshift within a 0.133-sr patch of sky. We present results of a programme of optical spectroscopy that has yielded redshifts in the range for the 29 sources in the sample, all but six of which are secure. We find that the filtering criteria used for 6C* are very effective in excluding the low-redshift, low-luminosity radio sources: the median redshift of 6C* is compared with for a complete sample matched in 151-MHz flux density. By combining the emission-line data set for the 6C* radio sources with those for the 3CRR, 6CE and 7CRS samples we establish that radio galaxies follow a rough proportionality between Lyα and 151-MHz luminosity, which, like similar correlations seen in samples of lower redshift radio sources, is indicative of a primary link between the power in the source of the photoionizing photons (most likely a hidden quasar nucleus) and the power carried by the radio jets. We argue that radio sources modify their environments and that the range of emission-line properties seen is determined more by the range of source age than by the range in ambient environment. The smallest radio galaxies have all the properties expected if the size distribution of luminous high-redshift steep-spectrum radio sources reflects a broad range (∼2 dex) of source ages with a narrower range (≲ 1.5 dex) of environmental densities, namely: (1) high-ionization lines, e.g. Lyα, of relatively low luminosity; (2) boosted low-ionization lines, e.g. C ii]; (3) spatially compact emission-line regions; and (4) H i-absorbed Lyα profiles. This is in accord with the idea that all high-redshift, high-luminosity radio sources are triggered in similar environments, presumably recently collapsed massive structures.