11,489 research outputs found
Sensor performance analysis
The theory is described and the equations required to design are developed and the performance of electro-optical sensor systems that operate from the visible through the thermal infrared spectral regions are analyzed. Methods to compute essential optical and detector parameters, signal-to-noise ratio, MTF, and figures of merit such as NE delta rho and NE delta T are developed. A set of atmospheric tables are provided to determine scene radiance in the visible spectral region. The Planck function is used to determine radiance in the infrared. The equations developed were incorporated in a spreadsheet so that a wide variety of sensor studies can be rapidly and efficiently conducted
Symmetry Reduction of Optimal Control Systems and Principal Connections
This paper explores the role of symmetries and reduction in nonlinear control
and optimal control systems. The focus of the paper is to give a geometric
framework of symmetry reduction of optimal control systems as well as to show
how to obtain explicit expressions of the reduced system by exploiting the
geometry. In particular, we show how to obtain a principal connection to be
used in the reduction for various choices of symmetry groups, as opposed to
assuming such a principal connection is given or choosing a particular symmetry
group to simplify the setting. Our result synthesizes some previous works on
symmetry reduction of nonlinear control and optimal control systems. Affine and
kinematic optimal control systems are of particular interest: We explicitly
work out the details for such systems and also show a few examples of symmetry
reduction of kinematic optimal control problems.Comment: 23 pages, 2 figure
High sensitivity operation of discrete solid state detectors at 4 K
Techniques are described to allow operation of discrete, solid state detectors at 4 K with optimized JFET amplifiers. Three detector types cover the 0.6 to 4 mm spectral range with NEP approximately equal to 10 to the 16th power Hz (-1/2) for two of the types and potential improvement to this performance for the third. Lower NEP's are anticipated at longer infrared wavelengths
Velocity field distributions due to ideal line vortices
We evaluate numerically the velocity field distributions produced by a
bounded, two-dimensional fluid model consisting of a collection of parallel
ideal line vortices. We sample at many spatial points inside a rigid circular
boundary. We focus on ``nearest neighbor'' contributions that result from
vortices that fall (randomly) very close to the spatial points where the
velocity is being sampled. We confirm that these events lead to a non-Gaussian
high-velocity ``tail'' on an otherwise Gaussian distribution function for the
Eulerian velocity field. We also investigate the behavior of distributions that
do not have equilibrium mean-field probability distributions that are uniform
inside the circle, but instead correspond to both higher and lower mean-field
energies than those associated with the uniform vorticity distribution. We find
substantial differences between these and the uniform case.Comment: 21 pages, 9 figures. To be published in Physical Review E
(http://pre.aps.org/) in May 200
Measuring ^{12}C(&alpha,&gamma)^{16}O from White Dwarf Asteroseismology
During helium burning in the core of a red giant, the relative rates of the
3&alpha and ^{12}C(&alpha,&gamma)^{16}O reactions largely determine the final
ratio of carbon to oxygen in the resulting white dwarf star. The uncertainty in
the 3&alpha reaction at stellar energies due to the extrapolation from
high-energy laboratory measurements is relatively small, but this is not the
case for the ^{12}C(&alpha,&gamma)^{16}O reaction. Recent advances in the
analysis of asteroseismological data on pulsating white dwarf stars now make it
possible to obtain precise measurements of the central ratio of carbon to
oxygen, providing a more direct way to measure the ^{12}C(&alpha,&gamma)^{16}O
reaction rate at stellar energies. We assess the systematic uncertainties of
this approach and quantify small shifts in the measured central oxygen
abundance originating from the observations and from model settings that are
kept fixed during the optimization. Using new calculations of white dwarf
internal chemical profiles, we find a rate for the ^{12}C(&alpha,&gamma)^{16}O
reaction that is significantly higher than most published values. The accuracy
of this method may improve as we modify some of the details of our description
of white dwarf interiors that were not accessible through previous
model-fitting methods.Comment: 8 pages, 4 figures, 3 tables, uses emulateapj5.sty, Accepted for
publication in the Astrophysical Journa
The asymptotic quasi-stationary states of the two-dimensional magnetically confined plasma and of the planetary atmosphere
We derive the differential equation governing the asymptotic quasi-stationary
states of the two dimensional plasma immersed in a strong confining magnetic
field and of the planetary atmosphere. These two systems are related by the
property that there is an intrinsic constant length: the Larmor radius and
respectively the Rossby radius and a condensate of the vorticity field in the
unperturbed state related to the cyclotronic gyration and respectively to the
Coriolis frequency. Although the closest physical model is the
Charney-Hasegawa-Mima (CHM) equation, our model is more general and is related
to the system consisting of a discrete set of point-like vortices interacting
in plane by a short range potential. A field-theoretical formalism is developed
for describing the continuous version of this system. The action functional can
be written in the Bogomolnyi form (emphasizing the role of Self-Duality of the
asymptotic states) but the minimum energy is no more topological and the
asymptotic structures appear to be non-stationary, which is a major difference
with respect to traditional topological vortex solutions. Versions of this
field theory are discussed and we find arguments in favor of a particular form
of the equation. We comment upon the significant difference between the CHM
fluid/plasma and the Euler fluid and respectively the Abelian-Higgs vortex
models.Comment: Latex 126 pages, 7 eps figures included. Discussion on various forms
of the equatio
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A Status Report On A Planet Search Around White Dwarf Stars
We have continued monitoring a pilot sample of 15 isolated, pulsating DA white dwarfs for center-of-mass motion caused by a planetary companion. Roughly 7 years into our survey, we have preliminary evidence for periodic variations in pulse arrival times for at least two white dwarfs in our sample. The variations in these systems are unlikely to be caused by secular evolution and are possibly the result of motion of the white dwarf around a center of mass. We have yet to claim confirmation of a planet. GD66 is a previously published candidate system, with a modulation in pulse arrival times that could be caused by a 2.0 M-J sin i planetary companion with an 8.3 year orbital period. Another candidate system, WD1354+0108, has a phase modulation consistent with a 0.7 M-J sin i planet at 2.3 AU (a 4.5 year orbit). We see similar behavior in two independent frequencies within this star, and while a sinusoid is currently a marginally better fit to the data than a straight line (as we might expect from cooling alone in a DAV), we are hesitant to over-interpret our results. Finally, we have a third system, WD0018+0031, that shows a change in pulse arrival times inconsistent with cooling alone; a 2.7 M-J planet at an orbit of about 5 AU could cause the observed trend. Observations of these candidate systems are ongoing in order to constrain any planetary companions that may be present.Astronom
Laboratory and telescope use of the NICMOS2 128 x 128 HgCdTe array
The second generation of Hubble Space Telescope (HST) instruments will include a near-infrared instrument. This choice has driven the development of near-infrared arrays to larger sizes and lower read noises. Rockwell International has delivered an array for use in the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument; this array has been dubbed NICMOS2. NICMOS2 is a 128x128 array of HgCdTe diodes In-bonded to a switched MOSFET readout. The readout was specifically designed for astronomical use with the HST requirement of low read noise a prime goal. These arrays use detector material which is similar to that used by Rockwell in previous arrays (e.g., HgCdTe produced on a sapphire substrate), but the NICMOS2 devices differ substantially from other 128x128 arrays produced by Rockwell in having a read noise of only 30 electrons when read out using appropriate correlated sampling. NICMOS2 has now been characterized in the laboratory, and it has been used on groundbased telescopes
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