32,124 research outputs found
Emissivity measurements of reflective surfaces at near-millimeter wavelengths
We have developed an instrument for directly measuring the emissivity of reflective surfaces at near-millimeter wavelengths. The thermal emission of a test sample is compared with that of a reference surface, allowing the emissivity of the sample to be determined without heating. The emissivity of the reference surface is determined by oneâs heating the reference surface and measuring the increase in emission. The instrument has an absolute accuracy of Îe = 5 x 10^-4 and can reproducibly measure a difference in emissivity as small as Îe = 10^-4 between flat reflective samples. We have used the instrument to measure the emissivity of metal films evaporated on glass and carbon fiber-reinforced plastic composite surfaces. We measure an emissivity of (2.15 ± 0.4) x 10^-3 for gold evaporated on glass and (2.65 ± 0.5) x 10^-3 for aluminum evaporated on carbon fiber-reinforced plastic composite
Planck pre-launch status: The HFI instrument, from specification to actual performance
Context. The High Frequency Instrument (HFI) is one of the two focal instruments of the Planck mission. It will observe the whole sky in six
bands in the 100 GHzâ1 THz range.
Aims. The HFI instrument is designed to measure the cosmic microwave background (CMB) with a sensitivity limited only by fundamental
sources: the photon noise of the CMB itself and the residuals left after the removal of foregrounds. The two high frequency bands will provide
full maps of the submillimetre sky, featuring mainly extended and point source foregrounds. Systematic effects must be kept at negligible levels
or accurately monitored so that the signal can be corrected. This paper describes the HFI design and its characteristics deduced from ground tests
and calibration.
Methods. The HFI instrumental concept and architecture are feasible only by pushing new techniques to their extreme capabilities, mainly:
(i) bolometers working at 100 mK and absorbing the radiation in grids; (ii) a dilution cooler providing 100 mK in microgravity conditions;
(iii) a new type of AC biased readout electronics and (iv) optical channels using devices inspired from radio and infrared techniques.
Results. The Planck-HFI instrument performance exceeds requirements for sensitivity and control of systematic effects. During ground-based
calibration and tests, it was measured at instrument and system levels to be close to or better than the goal specification
Magnetooptical sum rules close to the Mott transition
We derive new sum rules for the real and imaginary parts of the
frequency-dependent Hall constant and Hall conductivity. As an example, we
discuss their relevance to the doped Mott insulator that we describe within the
dynamical mean-field theory of strongly correlated electron systems.Comment: 4 pages, 4 ps figures; accepted for publication in PR
Measuring Planck beams with planets
Aims. Accurate measurement of the cosmic microwave background (CMB) anisotropy requires precise knowledge of the instrument beam. We explore how well the Planckâbeams will be determined from observations of planets, developing techniques that are also appropriate for other experiments.
Methods. We simulate planet observations with a Planck-like scanning strategy, telescope beams, noise, and detector properties. Then we employ both parametric and non-parametric techniques, reconstructing beams directly from the time-ordered data. With a faithful parameterization of the beam shape, we can constrain certain detector properties, such as the time constants of the detectors, to high precision. Alternatively, we decompose the beam using an orthogonal basis. For both techniques, we characterize the errors in the beam reconstruction with Monte Carlo realizations. For a simplified scanning strategy, we study the impact on estimation of the CMB power spectrum. Finally, we explore the consequences for measuring cosmological parameters, focusing on the spectral index of primordial scalar perturbations, n_s.
Results. The quality of the power spectrum measurement will be significantly influenced by the optical modeling of the telescope. In our most conservative case, using no information about the optics except the measurement of planets, we find that a single transit of Jupiter across the focal plane will measure the beam window functions to better than 0.3% for the channels at 100â217 GHz that are the most sensitive to the CMB. Constraining the beam with optical modeling can lead to much higher quality reconstruction.
Conclusions. Depending on the optical modeling, the beam errors may be a significant contribution to the measurement systematics for n_s
Markov Chain Beam Randomization: a study of the impact of PLANCK beam measurement errors on cosmological parameter estimation
We introduce a new method to propagate uncertainties in the beam shapes used
to measure the cosmic microwave background to cosmological parameters
determined from those measurements. The method, which we call Markov Chain Beam
Randomization, MCBR, randomly samples from a set of templates or functions that
describe the beam uncertainties. The method is much faster than direct
numerical integration over systematic `nuisance' parameters, and is not
restricted to simple, idealized cases as is analytic marginalization. It does
not assume the data are normally distributed, and does not require Gaussian
priors on the specific systematic uncertainties. We show that MCBR properly
accounts for and provides the marginalized errors of the parameters. The method
can be generalized and used to propagate any systematic uncertainties for which
a set of templates is available. We apply the method to the Planck satellite,
and consider future experiments. Beam measurement errors should have a small
effect on cosmological parameters as long as the beam fitting is performed
after removal of 1/f noise.Comment: 17 pages, 23 figures, revised version with improved explanation of
the MCBR and overall wording. Accepted for publication in Astronomy and
Astrophysics (to appear in the Planck pre-launch special issue
Parameter Estimation from Improved Measurements of the Cosmic Microwave Background from QUaD
We evaluate the contribution of cosmic microwave background (CMB) polarization spectra to cosmological parameter constraints. We produce cosmological parameters using high-quality CMB polarization data from the ground-based QUaD experiment and demonstrate for the majority of parameters that there is significant improvement on the constraints obtained from satellite CMB polarization data. We split a multi-experiment CMB data set into temperature and polarization subsets and show that the best-fit confidence regions for the ÎCDM six-parameter cosmological model are consistent with each other, and that polarization data reduces the confidence regions on all parameters. We provide the best limits on parameters from QUaD EE/BB polarization data and we find best-fit parameters from the multi-experiment CMB data set using the optimal pivot scale of k_p = 0.013 Mpc^(â1) to be {h^2Ω_c, h^2Ω_b, H_0, A_s, n_s, Ï} = {0.113, 0.0224, 70.6, 2.29 Ă 10^(â9), 0.960, 0.086}
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