1,589 research outputs found
Hydrogen maser development at Laval University
The physical construction of two hydrogen masers is described and results of measurements made on one of the masers are given. These include: cavity Q, thermal time constant, line Q, signal power output, magnetic shielding factor. Preliminary results indicate that the frequency stability will be mainly affected by the thermal of the cavity. The magnetic field and the barometric fluctuations should not affect the maser at the stability level above a few parts in 10 to the 15th power, which is the goal for averaging times of several hours
Preliminary vegetation map of the Espenberg Peninsula, Alaska, based on an Earth Resources Technology Satellite image
There are no author-identified significant results in this report
Future Prospects: Deep Imaging of Galaxy Outskirts using Telescopes Large and Small
The Universe is almost totally unexplored at low surface brightness levels.
In spite of great progress in the construction of large telescopes and
improvements in the sensitivity of detectors, the limiting surface brightness
of imaging observations has remained static for about forty years. Recent
technical advances have at last begun to erode the barriers preventing
progress. In this Chapter we describe the technical challenges to low surface
brightness imaging, describe some solutions, and highlight some relevant
observations that have been undertaken recently with both large and small
telescopes. Our main focus will be on discoveries made with the Dragonfly
Telephoto Array (Dragonfly), which is a new telescope concept designed to probe
the Universe down to hitherto unprecedented low surface brightness levels. We
conclude by arguing that these discoveries are probably only scratching the
surface of interesting phenomena that are observable when the Universe is
explored at low surface brightness levels.Comment: 27 pages, 10 figures, Invited review, Book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
Liquid Hydrogen Target Experience at SLAC
Liquid hydrogen targets have played a vital role in the physics program at SLAC for the past 40 years. These targets have ranged from small "beer can" targets to the 1.5 m long E158 target that was capable of absorbing up to 800 W without any significant density changes. Successful use of these targets has required the development of thin-wall designs, liquid hydrogen pumps, remote positioning and alignment systems, safety systems, control and data acquisition systems, cryogenic cooling circuits and heat exchangers. Detailed operating procedures have been created to ensure safety and operational reliability.This paper surveys the evolution of liquid hydrogen targets at SLAC and discusses advances in several of the enabling technologies that made these targets possible
Retrospective analysis of the impact of respiratory motion in treatment margins for frameless lung SBRT based on respiratory-correlated CBCT data-sets.
To investigate the impact of respiratory motion in the treatment margins for lung SBRT frameless treatments and to validate our treatment margins using 4D CBCT data analysis.
Two hundred and twenty nine fractions with early stage NSCLC were retrospectively analyzed. All patients were treated in frameless and free breathing conditions. The treatment margins were calculated according to van Herk equation in Mid-Ventilation. For each fraction, three 4D CBCT scans, pre- and postcorrection, and posttreatment, were acquired to assess target baseline shift, target localization accuracy and intra-fraction motion errors. A bootstrap analysis was performed to assess the minimum number of patients required to define treatment margins.
The retrospectively calculated target-baseline shift, target localization accuracy and intra-fraction motion errors agreed with the literature. The best tailored margins to our cohort of patients were retrospectively computed and resulted in agreement with already published data. The bootstrap analysis showed that fifteen patients were enough to assess treatment margins.
The treatment margins applied to our patient's cohort resulted in good agreement with the retrospectively calculated margins based on 4D CBCT data. Moreover, the bootstrap analysis revealed to be a promising method to verify the reliability of the applied treatment margins for safe lung SBRT delivery
Canonical formulation of self-gravitating spinning-object systems
Based on the Arnowitt-Deser-Misner (ADM) canonical formulation of general
relativity, a canonical formulation of gravitationally interacting classical
spinning-object systems is given to linear order in spin. The constructed
position, linear momentum and spin variables fulfill standard Poisson bracket
relations. A spatially symmetric time gauge for the tetrad field is introduced.
The achieved formulation is of fully reduced form without unresolved
constraints, supplementary, gauge, or coordinate conditions. The canonical
field momentum is not related to the extrinsic curvature of spacelike
hypersurfaces in standard ADM form. A new reduction of the tetrad degrees of
freedom to the Einstein form of the metric field is suggested.Comment: 6 pages. v2: extended version; identical to the published one. v3:
corrected misprints in (24) and (39); improved notation; added note regarding
a further reference
Adaptive optics in high-contrast imaging
The development of adaptive optics (AO) played a major role in modern
astronomy over the last three decades. By compensating for the atmospheric
turbulence, these systems enable to reach the diffraction limit on large
telescopes. In this review, we will focus on high contrast applications of
adaptive optics, namely, imaging the close vicinity of bright stellar objects
and revealing regions otherwise hidden within the turbulent halo of the
atmosphere to look for objects with a contrast ratio lower than 10^-4 with
respect to the central star. Such high-contrast AO-corrected observations have
led to fundamental results in our current understanding of planetary formation
and evolution as well as stellar evolution. AO systems equipped three
generations of instruments, from the first pioneering experiments in the
nineties, to the first wave of instruments on 8m-class telescopes in the years
2000, and finally to the extreme AO systems that have recently started
operations. Along with high-contrast techniques, AO enables to reveal the
circumstellar environment: massive protoplanetary disks featuring spiral arms,
gaps or other asymmetries hinting at on-going planet formation, young giant
planets shining in thermal emission, or tenuous debris disks and micron-sized
dust leftover from collisions in massive asteroid-belt analogs. After
introducing the science case and technical requirements, we will review the
architecture of standard and extreme AO systems, before presenting a few
selected science highlights obtained with recent AO instruments.Comment: 24 pages, 14 figure
On The Halo-Mass And Radial Scale Dependence Of The Lensing Is Low Effect
The canonical Lambda cold dark matter (ΛCDM) cosmological model makes precise predictions for the clustering and lensing properties of galaxies. It has been shown that the lensing amplitude of galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS) is lower than expected given their clustering properties. We present new measurements and modelling of galaxies in the BOSS LOWZ sample. We focus on the radial and stellar mass dependence of the lensing amplitude mismatch. We find an amplitude mismatch of around 35 per cent when assuming ΛCDM with Planck Cosmological Microwave Background (CMB) constraints. This offset is independent of halo mass and radial scale in the range Mhalo ∼ 1013.3−1013.9h−1 M⊙ and r=0.1−60h−1Mpc (k≈0.05−20hMpc−1). The observation that the offset is both mass and scale independent places important constraints on the degree to which astrophysical processes (baryonic effects, assembly bias) can fully explain the effect. This scale independence also suggests that the ‘lensing is low’ effect on small and large radial scales probably have the same physical origin. Resolutions based on new physics require a nearly uniform suppression, relative to ΛCDM predictions, of the amplitude of matter fluctuations on these scales. The possible causes of this are tightly constrained by measurements of the CMB and of the low-redshift expansion history
Palomar 13: An Unusual Stellar System in the Galactic Halo
We have measured Keck/HIRES radial velocities for 30 candidate red giants in
the direction of Palomar 13: an object traditionally cataloged as a compact,
low-luminosity globular cluster. From a sample of 21 confirmed members, we find
a systemic velocity of 24.1 km/s and a projected, intrinsic velocity dispersion
of 2.2 km/s. Although small, this dispersion is several times larger than that
expected for a globular cluster of this luminosity and central concentration.
Taken at face value, this dispersion implies a mass-to-light ratio of ~ 40 (in
solar units) based on the best-fit King-Michie model. The surface density
profile of Palomar 13 also appears to be anomalous among Galactic globular
clusters -- depending upon the details of background subtraction and
model-fitting, Palomar 13 either contains a substantial population of
"extra-tidal" stars, or it is far more spatially extended than previously
suspected. The full surface density profile is equally well-fit by a
King-Michie model having a high concentration and large tidal radius, or by a
NFW model. We examine -- and tentatively reject -- a number of possible
explanations for the observed characteristics of Palomar 13 (e.g., velocity
"jitter" among the red giants, spectroscopic binary stars, non-standard mass
functions, modified Newtonian dynamics), and conclude that the two most
plausible scenarios are either catastrophic heating during a recent
perigalacticon passage, or the presence of a massive dark halo. Thus, the
available evidence suggests that Palomar 13 is either a globular cluster which
is now in the process of dissolving into the Galactic halo, or a faint,
dark-matter-dominated stellar system (ABRIDGED).Comment: 31 pages, 13 postscript figures and 1 color gif image. Also available
at http://www.physics.rutgers.edu/ast/ast-rap.html. Accepted for publication
in the Astrophysical Journa
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