458 research outputs found
Millimeter Wave Scattering from Neutral and Charged Water Droplets
We investigated 94GHz millimeter wave (MMW) scattering from neutral and
charged water mist produced in the laboratory with an ultrasonic atomizer.
Diffusion charging of the mist was accomplished with a negative ion generator
(NIG). We observed increased forward and backscattering of MMW from charged
mist, as compared to MMW scattering from an uncharged mist. In order to
interpret the experimental results, we developed a model based on classical
electrodynamics theory of scattering from a dielectric sphere with
diffusion-deposited mobile surface charge. In this approach, scattering and
extinction cross-sections are calculated for a charged Rayleigh particle with
effective dielectric constant consisting of the volume dielectric function of
the neutral sphere and surface dielectric function due to the oscillation of
the surface charge in the presence of applied electric field. For small
droplets with (radius smaller than 100nm), this model predicts increased MMW
scattering from charged mist, which is qualitatively consistent with the
experimental observations. The objective of this work is to develop indirect
remote sensing of radioactive gases via their charging action on atmospheric
humid air.Comment: 18 pages, 8 figure
A template of atmospheric O2 circularly polarized emission for CMB experiments
We compute the circularly polarized signal from atmospheric molecular oxygen.
Polarization of O2 rotational lines is caused by Zeeman effect in the Earth
magnetic field. We evaluate the circularly polarized emission for various sites
suitable for CMB measurements: South Pole and Dome C (Antarctica), Atacama
(Chile) and Testa Grigia (Italy). An analysis of the polarized signal is
presented and discussed in the framework of future CMB polarization
experiments. We find a typical circularly polarized signal (V Stokes parameter)
of ~ 50 - 300 {\mu}K at 90 GHz looking at the zenith. Among the other sites
Atacama shows the lower polarized signal at the zenith. We present maps of this
signal for the various sites and show typical elevation and azimuth scans. We
find that Dome C presents the lowest gradient in polarized temperature: ~ 0.3
{\mu}K/\circ at 90 GHz. We also study the frequency bands of observation:
around {\nu} \simeq 100 GHz and {\nu} \simeq 160 GHz we find the best
conditions because the polarized signal vanishes. Finally we evaluate the
accuracy of the templates and the signal variability in relation with the
knowledge and the variability of the Earth magnetic field and the atmospheric
parameters.Comment: 10 pages, 12 figures, accepted for publication on Mon. Not. R.
Astron. So
MITO measurements of the Sunyaev-Zeldovich Effect in the Coma cluster of galaxies
We have measured the Sunyaev-Zeldovich effect towards the Coma cluster
(A1656) with the MITO experiment, a 2.6-m telescope equipped with a 4-channel
17 arcminute (FWHM) photometer. Measurements at frequency bands 143+/-15,
214+/-15, 272+/-16 and 353+/-13 GHz, were made during 120 drift scans of Coma.
We describe the observations and data analysis that involved extraction of the
S-Z signal by employing a spatial and spectral de-correlation scheme to remove
a dominant atmospheric component. The deduced values of the thermal S-Z effect
in the first three bands are DT_{0} = -179+/-38,-33+/-81,170+/-35 microKelvin
in the cluster center. The corresponding optical depth, tau=(4.1+/-0.9)
10^{-3}, is consistent (within errors) with both the value from a previous low
frequency S-Z measurement, and the value predicted from the X-ray deduced gas
parameters.Comment: Ap.J.Letters accepted, 4 pages, 2 figure
Tropospheric Phase Calibration in Millimeter Interferometry
We review millimeter interferometric phase variations caused by variations in
the precipitable water vapor content of the troposphere, and we discuss
techniques proposed to correct for these variations. We present observations
with the Very Large Array at 22 GHz and 43 GHz designed to test these
techniques. We find that both the Fast Switching and Paired Array calibration
techniques are effective at reducing tropospheric phase noise for radio
interferometers. In both cases, the residual rms phase fluctuations after
correction are independent of baseline length for b > b_{eff}. These techniques
allow for diffraction limited imaging of faint sources on arbitrarily long
baselines at mm wavelengths. We consider the technique of tropospheric phase
correction using a measurement of the precipitable water vapor content of the
troposphere via a radiometric measurement of the brightness temperature of the
atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz
for the MMA, and 120 mK for the VLA at 22 GHz. The minimum gain stability
requirement is 200 at 185 GHz at the MMA assuming that the astronomical
receivers are used for radiometry. This increases to 2000 for an uncooled
system. The stability requirement is 450 for the cooled system at the VLA at 22
GHz. To perform absolute radiometric phase corrections also requires knowledge
of the tropospheric parameters and models to an accuracy of a few percent. It
may be possible to perform an `empirically calibrated' radiometric phase
correction, in which the relationship between fluctuations in brightness
temperature differences with fluctuations in interferometric phases is
calibrated by observing a celestial calibrator at regular intervals.Comment: AAS LATEX preprint format. to appear in Radio Science 199
Small multi-purpose reservoir ensemble planning
People living in arid areas with highly variable rainfall, experience droughts and floods and often have insecure livelihoods. Small multi-purpose reservoirs are a widely used form of infrastructure for the provision of water. They supply water for domestic use, livestock watering, small scale irrigation, and other beneficial uses. The reservoirs are hydrologically linked by the streams that have been dammed. Although reservoirs store a large quantity of water and have a significant effect on downstream flows, they have rarely been considered as systems, with synergies and tradeoffs resulting from the number and density of their structures. Often reservoirs were constructed in a series of projects funded by different agencies, at different times, with little or no coordination among the implementing partners. A significant number are functioning sub-optimally and/or are falling into disrepair. This indicates that there is room for improvement in the planning, operation, and maintenance of small reservoirs. The water management institutions in Volta, Limpopo, and Sao Francisco Basins are being revamped to better serve their constituencies. We have an opportunity to collaborate with government officials, stakeholders, and farmers who are actively looking for ways to improve the planning process.
The Small Reservoir Project team developed a tool kit to support the planning, development, and management of small reservoir ensembles on the basin level and the use of small multi-purpose reservoirs that are properly located, well designed, operated and maintained in sustainable fashion, and economically viable on the local/community level. There are tools to improve intervention planning, storage estimation and the analysis of the hydrology, ecology and health of small reservoirs. There ara also tools for the analysis of institutional and economic aspects of the reservoirs. The toolkit not only includes the necessary analytical instruments, but also a set of process oriented tools for improved participatory decision making. The Tool Kit is meant to be a living “document” with additional tools and experiences to be added as they are developed
The Nuclear Spectroscopic Telescope Array (NuSTAR)
The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer
mission that will carry the first focusing hard X-ray (5 -- 80 keV) telescope
to orbit. NuSTAR will offer a factor 50 -- 100 sensitivity improvement compared
to previous collimated or coded mask imagers that have operated in this energy
band. In addition, NuSTAR provides sub-arcminute imaging with good spectral
resolution over a 12-arcminute field of view. After launch, NuSTAR will carry
out a two-year primary science mission that focuses on four key programs:
studying the evolution of massive black holes through surveys carried out in
fields with excellent multiwavelength coverage, understanding the population of
compact objects and the nature of the massive black hole in the center of the
Milky Way, constraining explosion dynamics and nucleosynthesis in supernovae,
and probing the nature of particle acceleration in relativistic jets in active
galactic nuclei. A number of additional observations will be included in the
primary mission, and a guest observer program will be proposed for an extended
mission to expand the range of scientific targets. The payload consists of two
co-aligned depth-graded multilayer coated grazing incidence optics focused onto
solid state CdZnTe pixel detectors. To be launched in early 2012 on a Pegasus
rocket into a low-inclination Earth orbit. Data will be publicly available at
GSFC's High Energy Astrophysics Science Archive Research Center (HEASARC)
following validation at the science operations center located at Caltech.Comment: 9 pages, 5 figures, to appear in Proceedings of the SPIE, Space
Telescopes and Instrumentation 2010: Ultraviolet to Gamma Ra
Focal Plane Alignment Utilizing Optical CMM
In many applications, an optical detector has to be located relative to mechanical reference points. One solution is to specify stringent requirements on (1) mounting the optical detector relative to the chip carrier, (2) soldering the chip carrier onto the printed circuit board (PCB), and (3) installing the PCB to the mechanical structure of the subsystem. Figure 1 shows a sketch of an optical detector mounted relative to mechanical reference with high positional accuracy. The optical detector is typically a fragile wafer that cannot be physically touched by any measurement tool. An optical coordinate measuring machine (CMM) can be used to position optical detectors relative to mechanical reference points. This approach will eliminate all requirements on positional tolerances. The only requirement is that the PCB is manufactured with oversized holes. An exaggerated sketch of this situation is shown in Figure 2. The sketch shows very loose tolerances on mounting the optical detector in the chip carrier, loose tolerance on soldering the chip carrier to the PCB, and finally large tolerance on where the mounting screws are located. The PCB is held with large screws and oversized holes. The PCB is mounted loosely so it can move freely around. The optical CMM measures the mechanical reference points. Based on these measurements, the required positions of the optical detector corners can be calculated. The optical CMM is commanded to go to the position where one detector corner is supposed to be. This is indicated with the cross-hairs in Figure 2(a). This figure is representative of the image of the optical CMM monitor. Using a suitable tapping tool, the PCB is manually tapped around until the corner of the optical detector is at the crosshairs of the optical CMM. The CMM is commanded to another corner, and the process is repeated a number of times until all corners of the optical detector are within a distance of 10 to 30 microns of the required position. The situation is sketched in Figure 2(b) (the figure also shows the tapping tool and where to tap). At this point the fasteners for the PCB are torqued slightly so the PCB can still move. The PCB location is adjusted again with the tapping tool. This process is repeated 3 to 4 times until the final torque is achieved. The oversized mounting holes are then filled with a liquid bonding agent to secure the board in position (not shown in the sketch). A 10- to 30-micron mounting accuracy has been achieved utilizing this method.
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