Point source

About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.

Analytical calculation of scatter distributions in SPECT projections

Abstract: In this paper the authors present a method for analytically calculating the distribution of photons detected in single photon emission computed tomography (SPECT) projections. The technique is applicable to sources in homogeneous and nonhomogeneous media. The photon distribution (primary, first-, and second-order Compton scatter) is computed using a precalculated camera-dependent look-up table in conjunction with an attenuation map of the scattering object and a map of the activity distribution. The speed and accuracy of this technique is compared to that of Monte Carlo simulations. The cases considered are a point source in a homogeneous and also in a nonhomogeneous scattering medium, an extended source in a nonhomogeneous medium, and a homogeneous cylinder filled uniformly with activity. The method is quantitatively accurate and faithfully reproduces the spatial distribution of the unscattered and scattered photons. For comparable statistical precision in the peak of the calculated distribution, their approach can result in a gain in calculation time over Monte Carlo simulators. For point sources, the computation times are improved by a factor of 20-150. However, this gain depends on the source configuration, and calculation times become comparable for an 800 voxel source and are five times slower for a 55000 voxel source. The method also offers an increase in the speed of computation of higher order Compton scatter events over a similar analytical technique.

A 189 MHz, 2400 square degree polarization survey with the Murchison Widefield Array 32-element prototype

Abstract: We present a Stokes I, Q and U survey at 189 MHz with the Murchison Widefield Array 32-element prototype covering 2400 square degrees. The survey has a 15.6 arcmin angular resolution and achieves a noise level of 15 mJy/beam. We demonstrate a novel interferometric data analysis that involves calibration of drift scan data, integration through the co-addition of warped snapshot images and deconvolution of the point spread function through forward modeling. We present a point source catalogue down to a flux limit of 4 Jy. We detect polarization from only one of the sources, PMN J0351-2744, at a level of 1.8 \pm 0.4%, whereas the remaining sources have a polarization fraction below 2%. Compared to a reported average value of 7% at 1.4 GHz, the polarization fraction of compact sources significantly decreases at low frequencies. We find a wealth of diffuse polarized emission across a large area of the survey with a maximum peak of ~13 K, primarily with positive rotation measure values smaller than +10 rad/m^2. The small values observed indicate that the emission is likely to have a local origin (closer than a few hundred parsecs). There is a large sky area at 2^h30^m where the diffuse polarized emission rms is fainter than 1 K. Within this area of low Galactic polarization we characterize the foreground properties in a cold sky patch at $(\alpha,\delta) = (4^h,-27^\circ.6)$ in terms of three dimensional power spectra

Depth sensing camera systems and methods

Abstract: This invention relates to a method and apparatus for sensing a three-dimensional (depth and illuminance) image of a scene It is based on the inverse-square law relating the incident brightness on an area illuminated by a light point source, to its distance from the point source In the preferred embodiment of the invention the scene is sequentially illuminated by more than one light point source each at a pre calibrated location in the reference coordinate system The resulting reflections from the field of-view are sensed by a stationary digital camera that maps each scene element into a corresponding image pixel, to provide a 2-dimensional brightness map that contains the photometric values of each image pixel for each specific illumination Each pixel photometric value depends on the illumination incident on the corresponding scene element which, in itself, is further determined by the element inherent Lambertian reflectance-coefficient at the illumination wavelength, the element orientation relative to the coordinate system, and the element illuminance as determined by the point source brightness and the distance separating the point source and the scene element Each brightness map is different from its sequel due to the differing point-source locations By manipulating the brightness maps the spatial location of each scene element relative to the fixed point sources is determined, thus yielding a depth-image as well as a brightness-image

Structure of Sagittarius A* at 86 GHz using VLBI Closure Quantities

Abstract: At radio wavelengths, images of the compact radio source Sagittarius A* (Sgr A*) in the Galactic center are scatter broadened with a j2 dependence due to an intervening ionized medium. We present VLBI observations of Sgr A* at 86 GHz using a six station array, including the VLBA antennas at Pie Town, Fort Davis, and Los Alamos, the 12 m antenna at Kitt Peak, and the millimeter arrays at Hat Creek and Owens Valley. To avoid systematic errors due to imperfect antenna calibration, the data were modeled using interferometric closure information. The data are best modeled by a circular Gaussian brightness distribution of FWHM 0.18 ^ 0.02 mas. The data are also shown to be consistent with an elliptical model corresponding to the scattering of a point source. The source structure in the northsouth direction, which is less well determined than in the east-west direction because of the limited north-south u-v coverage of the array, is constrained to be less than 0.27 mas by these measurements. These results are consistent with extrapolations of intrinsic structure estimates obtained with VLBI at a 7 mm wavelength, assuming the intrinsic size of Sgr A* has a greater dependence than j0.9 with wavelength.