783 research outputs found
Water vapor lidar
The feasibility was studied of measuring atmospheric water vapor by means of a tunable lidar operated from the space shuttle. The specific method evaluated was differential absorption, a two-color method in which the atmospheric path of interest is traversed by two laser pulses. Results are reported
Adaptive Filters Revisited - RFI Mitigation in pulsar observations
Pulsar detection and timing experiments are applications where adaptive
filters seem eminently suitable tools for radio-frequency-interference (RFI)
mitigation. We describe a novel variant which works well in field trials of
pulsar observations centred on an observing frequency of 675 MHz, a bandwidth
of 64 MHz and with 2-bit sampling. Adaptive filters have generally received bad
press for RFI mitigation in radio astronomical observations with their most
serious drawback being a spectral echo of the RFI embedded in the filtered
signals. Pulsar observations are intrinsically less sensitive to this as they
operate in the (pulsar period) time domain. The field trials have allowed us to
identify those issues which limit the effectiveness of the adaptive filter. We
conclude that adaptive filters can significantly improve pulsar observations in
the presence of RFI.Comment: Accepted for publication in Radio Scienc
Catalog of Galaxy Morphology in Four Rich Clusters: Luminosity Evolution of Disk Galaxies at 0.33<z<0.83
Hubble Space Telescope (HST) imaging of four rich, X-ray luminous, galaxy
clusters (0.33<z<0.83) is used to produce quantitative morphological
measurements for galaxies in their fields. Catalogs of these measurements are
presented for 1642 galaxies brighter than F814W(AB)=23.0 . Galaxy luminosity
profiles are fitted with three models: exponential disk, de Vaucouleurs bulge,
and a disk-plus-bulge hybrid model. The best fit is selected and produces a
quantitative assessment of the morphology of each galaxy: the principal
parameters derived being B/T, the ratio of bulge to total luminosity, the scale
lengths and half-light radii, axial ratios, position angles and surface
brightnesses of each component. Cluster membership is determined using a
statistical correction for field galaxy contamination, and a mass normalization
factor (mass within boundaries of the observed fields) is derived for each
cluster. In the present paper, this catalog of measurements is used to
investigate the luminosity evolution of disk galaxies in the rich-cluster
environment. Examination of the relations between disk scale-length and central
surface brightness suggests, under the assumption that these clusters represent
a family who share a common evolutionary history and are simply observed at
different ages, that there is a dramatic change in the properties of the small
disks (h < 2 kpc). This change is best characterized as a change in surface
brightness by about 1.5 magnitude between z=0.3 and z=0.8 with brighter disks
at higher redshifts.Comment: 53 pages, including 13 figures and 7 tables. Accepted for publication
in the Astrophysical Journal Supplement Serie
Serial MR diffusion to predict treatment response in high-grade pediatric brain tumors: a comparison of regional and voxel-based diffusion change metrics
Background
Assessment of treatment response by measuring tumor size is known to be a late and potentially confounded response index. Serial diffusion MRI has shown potential for allowing earlier and possibly more reliable response assessment in adult patients, with limited experience in clinical settings and in pediatric brain cancer. We present a retrospective study of clinical MRI data in children with high-grade brain tumors to assess and compare the values of several diffusion change metrics to predict treatment response.
Methods
Eighteen patients (age range, 1.9–20.6 years) with high-grade brain tumors and serial diffusion MRI (pre- and posttreatment interval range, 1–16 weeks posttreatment) were identified after obtaining parental consent. The following diffusion change metrics were compared with the clinical response status assessed at 6 months: (1) regional change in absolute and normalized apparent diffusivity coefficient (ADC), (2) voxel-based fractional volume of increased (fiADC) and decreased ADC (fdADC), and (3) a new metric based on the slope of the first principal component of functional diffusion maps (fDM).
Results
Responders (n = 12) differed significantly from nonresponders (n = 6) in all 3 diffusional change metrics demonstrating higher regional ADC increase, larger fiADC, and steeper slopes (P < .05). The slope method allowed the best response prediction (P < .01, η2 = 0.78) with a classification accuracy of 83% for a slope of 58° using receiver operating characteristic (ROC) analysis.
Conclusions
We demonstrate that diffusion change metrics are suitable response predictors for high-grade pediatric tumors, even in the presence of variable clinical diffusion imaging protocols
The Luminosity Function of Field Galaxies in the CNOC1 Redshift Survey
We have computed the luminosity function for 389 field galaxies from the
Canadian Network for Observational Cosmology cluster redshift survey (CNOC1),
over redshifts z = 0.2-0.6. We find Schechter parameters M^* - 5 log h = -19.6
\pm 0.3 and \alpha = -0.9 \pm 0.2 in rest-frame B_{AB}. We have also split our
sample at the color of a redshifted but nonevolving Sbc galaxy, and find
distinctly different luminosity functions for red and blue galaxies. Red
galaxies have a shallow slope \alpha \approx -0.4 and dominate the bright end
of the luminosity function, while blue galaxies have a steep \alpha \approx
-1.4 and prevail at the faint end. Comparisons of the CNOC1 results to those
from the Canada-France (CFRS) and Autofib redshift surveys show broad agreement
among these independent samples, but there are also significant differences
which will require larger samples to resolve. Also, in CNOC1 the red galaxy
luminosity density stays about the same over the range z = 0.2-0.6, while the
blue galaxy luminosity density increases steadily with redshift. These results
are consistent with the trend of the luminosity density vs. redshift relations
seen in the CFRS, though the normalizations of the luminosity densities appear
to differ for blue galaxies. Comparison to the local luminosity function from
the Las Campanas redshift survey (LCRS) shows that the luminosity density at z
\approx 0.1 is only about half that seen at z \approx 0.4. A change in the
luminosity function shape, particularly at the faint end, appears to be
required to match the CNOC1 and LCRS luminosity functions, if galaxy evolution
is the sole cause of the differences seen. However, it should be noted that the
specific details of the construction of different surveys may complicate the
comparison of results and so may need to be considered carefully.Comment: 22 pages, including 6 postscript figures, uses AASTEX v4.0 style
files. Corrected minor typos and updated references. Results and conclusions
unchanged. Final version to appear in the Astrophysical Journa
Characterization of Melt Infiltrated SiC/SiC Composite Combustor Liners Using Meso- and Micro-NDE Techniques
Melt-infiltrated ceramic matrix composite SiC/SiC material systems are under development for use in combustor liners for low-emission advanced gas turbines. Uncertainty in repeatability of processing methods for these large components (33--76 cm diameter), and hence possible reduced reliability for the end user. This requires that appropriate test methods, at both meso- and micro-scale, be used to ensure that the liners are acceptable for use. Nondestructive evaluation (NDE) methods, if demonstrated to reliably detect changes caused by processing, would be of significant benefit to both manufacturer and end user. This paper describes the NDE methods and their applications in detecting a process upset in a melt-infiltrated 33 cm combustor liner and how high-resolution scanning electron microscopy was used to verify the NDE data
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