1,239 research outputs found
Sky Variability in the y Band at the LSST Site
We have measured spatial and temporal variability in the y band sky
brightness over the course of four nights above Cerro Tololo near Cerro Pachon,
Chile, the planned site for the Large Synoptic Survey Telescope (LSST). Our
wide-angle camera lens provided a 41 deg field of view and a 145 arcsec pixel
scale. We minimized potential system throughput differences by deploying a deep
depletion CCD and a filter that matches the proposed LSST y_3 band (970 nm-1030
nm). Images of the sky exhibited coherent wave structure, attributable to
atmospheric gravity waves at 90 km altitude, creating 3%-4% rms spatial sky
flux variability on scales of about 2 degrees and larger. Over the course of a
full night the y_3 band additionally showed highly coherent temporal
variability of up to a factor of 2 in flux. We estimate the mean absolute sky
level to be approximately y_3 = 17.8 mag (Vega), or y_3 = 18.3 mag (AB). While
our observations were made through a y_3 filter, the relative sky brightness
variability should hold for all proposed y bands, whereas the absolute levels
should more strongly depend on spectral response. The spatial variability
presents a challenge to wide-field cameras that require illumination correction
strategies that make use of stacked sky flats. The temporal variability may
warrant an adaptive y band imaging strategy for LSST, to take advantage of
times when the sky is darkest.Comment: 8 pages, 5 figures, accepted to PASP. Minor changes from referee
report and editor's revisions
Stellar Locus Regression: Accurate Color Calibration, and the Real-time Determination of Galaxy Cluster Photometric Redshifts
We present Stellar Locus Regression (SLR), a method of directly adjusting the
instrumental broadband optical colors of stars to bring them into accord with a
universal stellar color-color locus, producing accurately calibrated colors for
both stars and galaxies. This is achieved without first establishing individual
zeropoints for each passband, and can be performed in real-time at the
telescope. We demonstrate how SLR naturally makes one wholesale correction for
differences in instrumental response, for atmospheric transparency, for
atmospheric extinction, and for Galactic extinction. We perform an example SLR
treatment of SDSS data over a wide range of Galactic dust values and
independently recover the direction and magnitude of the canonical Galactic
reddening vector with 14--18 mmag RMS uncertainties. We then isolate the effect
of atmospheric extinction, showing that SLR accounts for this and returns
precise colors over a wide of airmass, with 5--14 mmag RMS residuals. We
demonstrate that SLR-corrected colors are sufficiently accurate to allow
photometric redshift estimates for galaxy clusters (using red sequence
galaxies) with an uncertainty sigma_z/(1+z) = 0.6% per cluster for redshifts
0.09<z<0.25. Finally, we identify our objects in the 2MASS all-sky catalog, and
produce i-band zeropoints typically accurate to 18 mmag using only SLR. We
offer open-source access to our IDL routines, validated and verified for the
implementation of this technique, at
http://stellar-locus-regression.googlecode.comComment: Submitted to AJ. The public code is available at
http://stellar-locus-regression.googlecode.co
New insights into black bodies
Planck's law describes the radiation of black bodies. The study of its
properties is of special interest, as black bodies are a good description for
the behavior of many phenomena. In this work a new mathematical study of
Planck's law is performed and new properties of this old acquaintance are
obtained. As a result, the exact form for the locus in a color-color diagrams
has been deduced, and an analytical formula to determine with precision the
black body temperature of an object from any pair of measurements has been
developed. Thus, using two images of the same field obtained with different
filters, one can compute a fast estimation of black body temperatures for every
pixel in the image, that is, a new image of the black body temperatures for all
the objects in the field. Once these temperatures are obtained, the method
allows, as a consequence, a quick estimation of their emission in other
frequencies, assuming a black body behavior. These results provide new tools
for data analysis.Comment: 6 pages, 5 figures. Accepted in EP
Method of Fabricating a Composite Apparatus
A method for fabricating a piezoelectric macro-fiber composite actuator comprises making a piezoelectric fiber sheet by providing a plurality of wafers of piezoelectric material, bonding the wafers together with an adhesive material to from a stack of alternating layers of piezoelectric material and adhesive material, and cutting through the stack in a direction substantially parallel to the thickness of the stack and across the alternating layers of piezoelectric material and adhesive material to provide at least one piezoelectric fiber sheet having two sides comprising a plurality of piezoelectric fibers in juxtaposition to the adhesive material. The method further comprises bonding two electrically conductive films to the two sides of the piezoelectric fiber sheet. At least one conductive film has first and second conductive patterns formed thereon which are electrically isolated from one another and in electrical contact with the piezoelectric fiber sheet
Optical followup of galaxy clusters detected by the South Pole Telescope
The South Pole Telescope (SPT) is a 10 meter telescope operating at mm
wavelengths. It has recently completed a three-band survey covering 2500 sq.
degrees. One of the survey's main goals is to detect galaxy clusters using
Sunyaev-Zeldovich effect and use these clusters for a variety of cosmological
and astrophysical studies such as the dark energy equation of state, the
primordial non-gaussianity and the evolution of galaxy populations. Since 2005,
we have been engaged in a comprehensive optical and near-infrared followup
program (at wavelengths between 0.4 and 5 {\mu}m) to image high-significance
SPT clusters, to measure their photometric redshifts, and to estimate the
contamination rate of the candidate lists. These clusters are then used for
various cosmological and astrophysical studies.Comment: For TAUP 2011 proceeding
Detection of X-ray galaxy clusters based on the Kolmogorov method
The detection of clusters of galaxies in large surveys plays an important
part in extragalactic astronomy, and particularly in cosmology, since cluster
counts can give strong constraints on cosmological parameters. X-ray imaging is
in particular a reliable means to discover new clusters, and large X-ray
surveys are now available. Considering XMM-Newton data for a sample of 40 Abell
clusters, we show that their analysis with a Kolmogorov distribution can
provide a distinctive signature for galaxy clusters. The Kolmogorov method is
sensitive to the correlations in the cluster X-ray properties and can therefore
be used for their identification, thus allowing to search reliably for clusters
in a simple way
Ultra-High Energy Cosmic Ray Nuclei from Individual Magnetized Sources
We investigate the dependence of composition, spectrum and angular
distributions of ultra-high energy cosmic rays above 10^19 eV from individual
sources on their magnetization. We find that, especially for sources within a
few megaparsecs from the observer, observable spectra and composition are
severely modified if the source is surrounded by fields of ~ 10^-7 Gauss on
scales of a few megaparsecs. Low energy particles diffuse over larger distances
during their energy loss time. This leads to considerable hardening of the
spectrum up to the energy where the loss distance becomes comparable to the
source distance. Magnetized sources thus have very important consequences for
observations, even if cosmic rays arrive within a few degrees from the source
direction. At the same time, details in spectra and chemical composition may be
intrinsically unpredictable because they depend on the unknown magnetic field
structure. If primaries are predominantly nuclei of atomic mass A accelerated
up to a maximum energy E_max with spectra not much softer than E^-2, secondary
protons from photo-disintegration can produce a conspicuous peak in the
spectrum at energy ~ E_max/A. A related feature appears in the average mass
dependence on energy.Comment: 15 pages, 16 ps figures, published version with minor changes, see
http://stacks.iop.org/1475-7516/2004/i=08/a=01
The Blanco Cosmology Survey: Data Acquisition, Processing, Calibration, Quality Diagnostics and Data Release
The Blanco Cosmology Survey (BCS) is a 60 night imaging survey of 80
deg of the southern sky located in two fields: (,)= (5 hr,
) and (23 hr, ). The survey was carried out between
2005 and 2008 in bands with the Mosaic2 imager on the Blanco 4m
telescope. The primary aim of the BCS survey is to provide the data required to
optically confirm and measure photometric redshifts for Sunyaev-Zel'dovich
effect selected galaxy clusters from the South Pole Telescope and the Atacama
Cosmology Telescope. We process and calibrate the BCS data, carrying out PSF
corrected model fitting photometry for all detected objects. The median
10 galaxy (point source) depths over the survey in are
approximately 23.3 (23.9), 23.4 (24.0), 23.0 (23.6) and 21.3 (22.1),
respectively. The astrometric accuracy relative to the USNO-B survey is
milli-arcsec. We calibrate our absolute photometry using the stellar
locus in bands, and thus our absolute photometric scale derives from
2MASS which has % accuracy. The scatter of stars about the stellar locus
indicates a systematics floor in the relative stellar photometric scatter in
that is 1.9%, 2.2%, 2.7% and2.7%, respectively.
A simple cut in the AstrOmatic star-galaxy classifier {\tt spread\_model}
produces a star sample with good spatial uniformity. We use the resulting
photometric catalogs to calibrate photometric redshifts for the survey and
demonstrate scatter with an outlier fraction %
to . We highlight some selected science results to date and provide a
full description of the released data products.Comment: 23 pages, 23 figures . Response to referee comments. Paper accepted
for publication. BCS catalogs and images available for download from
http://www.usm.uni-muenchen.de/BC
Towards More Precise Survey Photometry for PanSTARRS and LSST: Measuring Directly the Optical Transmission Spectrum of the Atmosphere
Motivated by the recognition that variation in the optical transmission of
the atmosphere is probably the main limitation to the precision of ground-based
CCD measurements of celestial fluxes, we review the physical processes that
attenuate the passage of light through the Earth's atmosphere. The next
generation of astronomical surveys, such as PanSTARRS and LSST, will greatly
benefit from dedicated apparatus to obtain atmospheric transmission data that
can be associated with each survey image. We review and compare various
approaches to this measurement problem, including photometry, spectroscopy, and
LIDAR. In conjunction with careful measurements of instrumental throughput,
atmospheric transmission measurements should allow next-generation imaging
surveys to produce photometry of unprecedented precision. Our primary concerns
are the real-time determination of aerosol scattering and absorption by water
along the line of sight, both of which can vary over the course of a night's
observations.Comment: 41 pages, 14 figures. Accepted PAS
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