63,164 research outputs found
Application of distorted models in developing scaled structural models
In the design and development of dynamically similar structural models a distorted model of the panel is used. The panel thickness is made larger than that dictated by geometric scaling, and the mass of the panel is decreased by adding mass to the surface of the panel to counteract the additional stiffness obtained by the thickness increase
Optimal Compression of Floating-point Astronomical Images Without Significant Loss of Information
We describe a compression method for floating-point astronomical images that
gives compression ratios of 6 -- 10 while still preserving the scientifically
important information in the image. The pixel values are first preprocessed by
quantizing them into scaled integer intensity levels, which removes some of the
uncompressible noise in the image. The integers are then losslessly compressed
using the fast and efficient Rice algorithm and stored in a portable FITS
format file. Quantizing an image more coarsely gives greater image compression,
but it also increases the noise and degrades the precision of the photometric
and astrometric measurements in the quantized image. Dithering the pixel values
during the quantization process can greatly improve the precision of
measurements in the images. This is especially important if the analysis
algorithm relies on the mode or the median which would be similarly quantized
if the pixel values are not dithered. We perform a series of experiments on
both synthetic and real astronomical CCD images to quantitatively demonstrate
that the magnitudes and positions of stars in the quantized images can be
measured with the predicted amount of precision. In order to encourage wider
use of these image compression methods, we have made available a pair of
general-purpose image compression programs, called fpack and funpack, which can
be used to compress any FITS format image.Comment: Accepted PAS
Lossless Astronomical Image Compression and the Effects of Noise
We compare a variety of lossless image compression methods on a large sample
of astronomical images and show how the compression ratios and speeds of the
algorithms are affected by the amount of noise in the images. In the ideal case
where the image pixel values have a random Gaussian distribution, the
equivalent number of uncompressible noise bits per pixel is given by Nbits
=log2(sigma * sqrt(12)) and the lossless compression ratio is given by R =
BITPIX / Nbits + K where BITPIX is the bit length of the pixel values and K is
a measure of the efficiency of the compression algorithm.
We perform image compression tests on a large sample of integer astronomical
CCD images using the GZIP compression program and using a newer FITS
tiled-image compression method that currently supports 4 compression
algorithms: Rice, Hcompress, PLIO, and GZIP. Overall, the Rice compression
algorithm strikes the best balance of compression and computational efficiency;
it is 2--3 times faster and produces about 1.4 times greater compression than
GZIP. The Rice algorithm produces 75%--90% (depending on the amount of noise in
the image) as much compression as an ideal algorithm with K = 0.
The image compression and uncompression utility programs used in this study
(called fpack and funpack) are publicly available from the HEASARC web site. A
simple command-line interface may be used to compress or uncompress any FITS
image file.Comment: 20 pages, 9 figures, to be published in PAS
Evaluating implicit feedback models using searcher simulations
In this article we describe an evaluation of relevance feedback (RF) algorithms using searcher simulations. Since these algorithms select additional terms for query modification based on inferences made from searcher interaction, not on relevance information searchers explicitly provide (as in traditional RF), we refer to them as implicit feedback models. We introduce six different models that base their decisions on the interactions of searchers and use different approaches to rank query modification terms. The aim of this article is to determine which of these models should be used to assist searchers in the systems we develop. To evaluate these models we used searcher simulations that afforded us more control over the experimental conditions than experiments with human subjects and allowed complex interaction to be modeled without the need for costly human experimentation. The simulation-based evaluation methodology measures how well the models learn the distribution of terms across relevant documents (i.e., learn what information is relevant) and how well they improve search effectiveness (i.e., create effective search queries). Our findings show that an implicit feedback model based on Jeffrey's rule of conditioning outperformed other models under investigation
Observed variability in the Fraunhofer line spectrum of solar flux, 1975 - 1980
Over the five years double-pass spectrometer observations of the Sun-as-a-star revealed significant changes in line intensities. The photospheric component weakened linearly with time 0 to 2.3%. From a lack of correlation between these line weakenings and solar activity indicators like sunspots and plage, a global variation of surface properties is inferred. Model-atmosphere analysis suggests a slight reduction in the lower-photospheric temperature gradient corresponding to a 15% increase in the mixing length within the granulation layer. Chromospheric lines such as Ca II H and K, Ca II 8543 and the CN band head weaken synchronously with solar activity. Thus, the behavior of photospheric and chromospheric lines is markedly different, with the possibility of secular change for the former
Workshop on web information seeking and interaction
The World Wide Web has provided access to a diverse range of information sources and systems. People engaging with this rich network of information may need to interact with different technologies, interfaces, and information providers in the course of a single search task. These systems may offer different interaction affordances and require users to adapt their informationseeking strategies. Not only is this challenging for users, but it also presents challenges for the designers of interactive systems, who need to make their own system useful and usable to broad user groups. The popularity of Web browsing and Web search engines has given rise to distinct forms of information-seeking behaviour, and new interaction styles, but we do not yet fully understand these or their implications for the development of new systems
Double window viewing chamber assembly
A viewing chamber which permits observation of a sample retained therein includes a pair of double window assemblies mounted in opposed openings in the walls thereof so that a light beam can directly enter and exit from the chamber. A flexible mounting arrangement for the outer windows of the window assemblies enables the windows to be brought into proper alignment. An electrical heating arrangement prevents fogging of the outer windows whereas desiccated air in the volume between the outer and inner windows prevents fogging of the latter
Spin-tunnel investigation of a 1/25-scale model of the General Dynamics F-16XL airplane
A spin-tunnel investigation of the spin and recovery characteristics of a 1/25-scale model to the General Dynamics F-16XL aircraft was conducted in the Langley Spin Tunnel. Tests included erect and inverted spins at various symmetric and asymmetric loading conditions. The required size of an emergency spin-recovery parachute was determined
Some integration formulae which simplify the evaluation of certain integrals in common use by engineers
Integration formulas to simplify evaluation of certain commonly used integral
Experimental study of vibro-acoustic response of stiffened cylindrical shells
Vibrational response of stiffened cylindrical shell to reverberant acoustic field
- …