34,951 research outputs found
High Accuracy Near-infrared Imaging Polarimetry with NICMOS
The findings of a nine orbit calibration plan carried out during HST Cycle
15, to fully determine the NICMOS camera 2 (2.0 micron) polarization
calibration to high accuracy, are reported. Recently Ueta et al. and Batcheldor
et al. have suggested that NICMOS possesses a residual instrumental
polarization at a level of 1.2-1.5%. This would completely inhibit the data
reduction in a number of GO programs, and hamper the ability of the instrument
to perform high accuracy polarimetry. We obtained polarimetric calibration
observations of three polarimetric standards at three spacecraft roll angles
separated by ~60deg. Combined with archival data, these observations were used
to characterize the residual instrumental polarization in order for NICMOS to
reach its full potential of accurate imaging polarimetry at p~1%. Using these
data, we place an 0.6% upper limit on the instrumental polarization and
calculate values of the parallel transmission coefficients that reproduce the
ground-based results for the polarimetric standards. The uncertainties
associated with the parallel transmission coefficients, a result of the
photometric repeatability of the observations, are seen to dominate the
accuracy of p and theta. However, the updated coefficients do allow imaging
polarimetry of targets with p~1.0% at an accuracy of +/-0.6% and +/-15deg. This
work enables a new caliber of science with HST.Comment: 13 pages, 9 figures, PASP accepte
3 mm GMVA Observations of Total and Polarized Emission from Blazar and Radio Galaxy Core Regions
We present total and linearly polarized 3 mm Global mm-VLBI Array images of a
sample of blazars and radio galaxies from the VLBA-BU-BLAZAR 7 mm monitoring
program designed to probe the innermost regions of active galactic nuclei (AGN)
jets and locate the sites of gamma-ray emission observed by the Fermi-LAT. The
lower opacity at 3 mm and improved angular resolution, on the order of 50
microarcseconds, allow us to distinguish features in the jet not visible in the
7 mm VLBA data. We also compare two different methods used for the calibration
of instrumental polarisation and we analyze the resulting images for some of
the sources in the sample.Comment: Polarised Emission from Astrophysical Jets, June 12-16, 2017,
Ierapetra, Greec
Observing the Sun with the Atacama Large Millimeter-submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
The Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope has
commenced science observations of the Sun starting in late 2016. Since the Sun
is much larger than the field of view of individual ALMA dishes, the ALMA
interferometer is unable to measure the background level of solar emission when
observing the solar disk. The absolute temperature scale is a critical
measurement for much of ALMA solar science, including the understanding of
energy transfer through the solar atmosphere, the properties of prominences,
and the study of shock heating in the chromosphere. In order to provide an
absolute temperature scale, ALMA solar observing will take advantage of the
remarkable fast-scanning capabilities of the ALMA 12m dishes to make
single-dish maps of the full Sun. This article reports on the results of an
extensive commissioning effort to optimize the mapping procedure, and it
describes the nature of the resulting data. Amplitude calibration is discussed
in detail: a path that utilizes the two loads in the ALMA calibration system as
well as sky measurements is described and applied to commissioning data.
Inspection of a large number of single-dish datasets shows significant
variation in the resulting temperatures, and based on the temperature
distributions we derive quiet-Sun values at disk center of 7300 K at lambda=3
mm and 5900 K at lambda=1.3 mm. These values have statistical uncertainties of
order 100 K, but systematic uncertainties in the temperature scale that may be
significantly larger. Example images are presented from two periods with very
different levels of solar activity. At a resolution of order 25 arcsec, the 1.3
mm wavelength images show temperatures on the disk that vary over about a 2000
K range.Comment: Solar Physics, accepted: 24 pages, 13 figure
POLOCALC: a Novel Method to Measure the Absolute Polarization Orientation of the Cosmic Microwave Background
We describe a novel method to measure the absolute orientation of the
polarization plane of the CMB with arcsecond accuracy, enabling unprecedented
measurements for cosmology and fundamental physics. Existing and planned CMB
polarization instruments looking for primordial B-mode signals need an
independent, experimental method for systematics control on the absolute
polarization orientation. The lack of such a method limits the accuracy of the
detection of inflationary gravitational waves, the constraining power on the
neutrino sector through measurements of gravitational lensing of the CMB, the
possibility of detecting Cosmic Birefringence, and the ability to measure
primordial magnetic fields. Sky signals used for calibration and direct
measurements of the detector orientation cannot provide an accuracy better than
1 deg. Self-calibration methods provide better accuracy, but may be affected by
foreground signals and rely heavily on model assumptions. The POLarization
Orientation CALibrator for Cosmology, POLOCALC, will dramatically improve
instrumental accuracy by means of an artificial calibration source flying on
balloons and aerial drones. A balloon-borne calibrator will provide far-field
source for larger telescopes, while a drone will be used for tests and smaller
polarimeters. POLOCALC will also allow a unique method to measure the
telescopes' polarized beam. It will use microwave emitters between 40 and 150
GHz coupled to precise polarizing filters. The orientation of the source
polarization plane will be registered to sky coordinates by star cameras and
gyroscopes with arcsecond accuracy. This project can become a rung in the
calibration ladder for the field: any existing or future CMB polarization
experiment observing our polarization calibrator will enable measurements of
the polarization angle for each detector with respect to absolute sky
coordinates.Comment: 15 pages, 5 figures, Accepted by Journal of Astronomical
Instrumentatio
J Fluorescence
The scope of this paper is to illustrate the need for an improved quality assurance in fluorometry. For this purpose, instrumental sources of error and their influences on the reliability and comparability of fluorescence data are highlighted for frequently used photoluminescence techniques ranging from conventional macro- and microfluorometry over fluorescence microscopy and flow cytometry to microarray technology as well as in vivo fluorescence imaging. Particularly, the need for and requirements on fluorescence standards for the characterization and performance validation of fluorescence instruments, to enhance the comparability of fluorescence data, and to enable quantitative fluorescence analysis are discussed. Special emphasis is dedicated to spectral fluorescence standards and fluorescence intensity standards
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