1,753 research outputs found
Tropospheric limitations on the accuracy of phase measurement of coordinates in astronomy
The effect of tropospheric fluctuation on the accuracy of phase measurements of coordinates is discussed. The nature of the averaging of the tropospheric effects, if N coordinate measurements of duration T with period mu are made, is investigated. Various averaging modes depending on the relation of the various time parameters are investigated. Equations taking into account the correlations between individual observations are presented. It is shown that the correlation interval between the individual observations is always greater than the fluctuation period of tropospheric inhomogeneities typical for a given baseline
A method of detecting radio transients
Radio transients are sporadic signals and their detection requires that the
backends of radio telescopes be equipped with the appropriate hardware and
software to undertake this. Observational programs to detect transients can be
dedicated or they can piggy-back on observations made by other programs. It is
the single-dish single-transient (non-periodical) mode which is considered in
this paper. Because neither the width of a transient nor the time of its
arrival is known, a sequential analysis in the form of a cumulative sum (cusum)
algorithm is proposed here. Computer simulations and real observation data
processing are included to demonstrate the performance of the cusum. The use of
the Hough transform is here proposed for the purpose of non-coherent
de-dispersion. It is possible that the detected transients could be radio
frequency interferences (RFI) and a procedure is proposed here which can
distinguish between celestial signals and man-made RFI. This procedure is based
on an analysis of the statistical properties of the signals
RFI mitigation with phase-only adaptive beamforming
Connected radio interferometers are sometimes used in the tied-array mode:
signals from antenna elements are coherently added and the sum signal applied
to a VLBI backend or pulsar processing machine. Usually there is no
computer-controlled amplitude weighting in the existing radio interferometer
facilities. Radio frequency interference (RFI) mitigation with phase-only
adaptive beamforming is proposed for this mode of observation. Small phase
perturbations are introduced in each of the antenna's signal. The values of
these perturbations are optimized in such a way that the signal from a radio
source of interest is preserved and RFI signals suppressed. An evolutionary
programming algorithm is used for this task. Computer simulations, made for
both one-dimensional and two-dimensional array set-ups, show considerable
suppression of RFI and acceptable changes to the main array beam in the radio
source direction.Comment: 7 pages, 11 figure
Pressurized rf cavities in ionizing beams
A muon collider or Higgs factory requires significant reduction of the six dimensional emittance of the beam prior to acceleration. One method to accomplish this involves building a cooling channel using high pressure gas filled radio frequency cavities. The performance of such a cavity when subjected to an intense particle beam must be investigated before this technology can be validated. To this end, a high pressure gas filled radio frequency (rf) test cell was built and placed in a 400 MeV beam line from the Fermilab linac to study the plasma evolution and its effect on the cavity. Hydrogen, deuterium, helium and nitrogen gases were studied. Additionally, sulfur hexafluoride and dry air were used as dopants to aid in the removal of plasma electrons. Measurements were made using a variety of beam intensities, gas pressures, dopant concentrations, and cavity rf electric fields, both with and without a 3 T external solenoidal magnetic field. Energy dissipation per electron-ion pair, electron-ion recombination rates, ion-ion recombination rates, and electron attachment times to SF6 and O-2 were measured.ope
Statistically Stable Estimates of Variance in Radioastronomical Observations as Tools for RFI Mitigation
A selection of statistically stable (robust) algorithms for data variance
calculating has been made. Their properties have been analyzed via computer
simulation. These algorithms would be useful if adopted in radio astronomy
observations in the presence of strong sporadic radio frequency interference
(RFI). Several observational results have been presented here to demonstrate
the effectiveness of these algorithms in RFI mitigation
Spectral density asymptotics for Gaussian and Laguerre -ensembles in the exponentially small region
The first two terms in the large asymptotic expansion of the
moment of the characteristic polynomial for the Gaussian and Laguerre
-ensembles are calculated. This is used to compute the asymptotic
expansion of the spectral density in these ensembles, in the exponentially
small region outside the leading support, up to terms . The leading form
of the right tail of the distribution of the largest eigenvalue is given by the
density in this regime. It is demonstrated that there is a scaling from this,
to the right tail asymptotics for the distribution of the largest eigenvalue at
the soft edge.Comment: 19 page
Mechanisms of the Vertical Secular Heating of a Stellar Disk
We investigate the nonlinear growth stages of bending instability in stellar
disks with exponential radial density profiles.We found that the unstable modes
are global (the wavelengths are larger than the disk scale lengths) and that
the instability saturation level is much higher than that following from a
linear criterion. The instability saturation time scales are of the order of
one billion years or more. For this reason, the bending instability can play an
important role in the secular heating of a stellar disk in the direction.
In an extensive series of numerical -body simulations with a high spatial
resolution, we were able to scan in detail the space of key parameters (the
initial disk thickness , the Toomre parameter , and the ratio of dark
halo mass to disk mass ). We revealed three distinct
mechanisms of disk heating in the direction: bending instability of the
entire disk, bending instability of the bar, and heating on vertical
inhomogeneities in the distribution of stellar matter.Comment: 22 pages including 8 figures. To be published in Astronomy Letters
(v.29, 2003
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