Cosmic-Ray Rejection by Laplacian Edge Detection

Conventional algorithms for rejecting cosmic-rays in single CCD exposures rely on the contrast between cosmic-rays and their surroundings, and may produce erroneous results if the Point Spread Function (PSF) is smaller than the largest cosmic-rays. This paper describes a robust algorithm for cosmic-ray rejection, based on a variation of Laplacian edge detection. The algorithm identifies cosmic-rays of arbitrary shapes and sizes by the sharpness of their edges, and reliably discriminates between poorly sampled point sources and cosmic-rays. Examples of its performance are given for spectroscopic and imaging data, including HST WFPC2 images.Comment: Accepted for publication in the PASP (November 2001 issue). The algorithm is implemented in the program L.A.Cosmic, which can be obtained from http://www.astro.caltech.edu/~pgd/lacosmic

Hidden Cues in Random Line Stereograms

Successful fusion of random-line stereograms with breaks in the vernier acuity range has been interpreted to suggest that the interpolation process underlying hyperacuity is parallel and preliminary to stereomatching. In this paper (a) we demonstrate with computer experiments that vernier cues are not needed to solve the stereomatching problem posed by these stereograms and (b) we provide psychophysical evidence that human stereopsis probably does not use vernier cues alone to achieve fusion of these random-line stereograms.MIT Artificial Intelligence Laborator

Analysis of the wind tunnel test of a tilt rotor power force model

Two series of wind tunnel tests were made to determine performance, stability and control, and rotor wake interaction on the airframe, using a one-tenth scale powered force model of a tilt rotor aircraft. Testing covered hover (IGE/OCE), helicopter, conversion, and airplane flight configurations. Forces and moments were recorded for the model from predetermined trim attitudes. Control positions were adjusted to trim flight (one-g lift, pitching moment and drag zero) within the uncorrected test data balance accuracy. Pitch and yaw sweeps were made about the trim attitudes with the control held at the trimmed settings to determine the static stability characteristics. Tail on, tail off, rotors on, and rotors off configurations were testes to determine the rotor wake effects on the empennage. Results are presented and discussed

The fundamental solution of the unidirectional pulse propagation equation

The fundamental solution of a variant of the three-dimensional wave equation known as "unidirectional pulse propagation equation" (UPPE) and its paraxial approximation is obtained. It is shown that the fundamental solution can be presented as a projection of a fundamental solution of the wave equation to some functional subspace. We discuss the degree of equivalence of the UPPE and the wave equation in this respect. In particular, we show that the UPPE, in contrast to the common belief, describes wave propagation in both longitudinal and temporal directions, and, thereby, its fundamental solution possesses a non-causal character.Comment: accepted to J. Math. Phy

Direct calculation of the hard-sphere crystal/melt interfacial free energy

We present a direct calculation by molecular-dynamics computer simulation of the crystal/melt interfacial free energy, $\gamma$, for a system of hard spheres of diameter $\sigma$. The calculation is performed by thermodynamic integration along a reversible path defined by cleaving, using specially constructed movable hard-sphere walls, separate bulk crystal and fluid systems, which are then merged to form an interface. We find the interfacial free energy to be slightly anisotropic with $\gamma$ = 0.62$\pm 0.01$, 0.64$\pm 0.01$ and 0.58$\pm 0.01 k_BT/\sigma^2$ for the (100), (110) and (111) fcc crystal/fluid interfaces, respectively. These values are consistent with earlier density functional calculations and recent experiments measuring the crystal nucleation rates from colloidal fluids of polystyrene spheres that have been interpreted [Marr and Gast, Langmuir {\bf 10}, 1348 (1994)] to give an estimate of $\gamma$ for the hard-sphere system of $0.55 \pm 0.02 k_BT/\sigma^2$, slightly lower than the directly determined value reported here.Comment: 4 pages, 4 figures, submitted to Physical Review Letter

The Interstellar Rubidium Isotope Ratio toward Rho Ophiuchi A

The isotope ratio, 85Rb/87Rb, places constraints on models of the nucleosynthesis of heavy elements, but there is no precise determination of the ratio for material beyond the Solar System. We report the first measurement of the interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 A for the diffuse gas toward rho Oph A yields a value of 1.21 +/- 0.30 (1-sigma) that differs significantly from the meteoritic value of 2.59. The Rb/K elemental abundance ratio for the cloud also is lower than that seen in meteorites. Comparison of the 85Rb/K and 87Rb/K ratios with meteoritic values indicates that the interstellar 85Rb abundance in this direction is lower than the Solar System abundance. We attribute the lower abundance to a reduced contribution from the r-process. Interstellar abundances for Kr, Cd, and Sn are consistent with much less r-process synthesis for the solar neighborhood compared to the amount inferred for the Solar System.Comment: 12 pages with 2 figures and 1 table; will appear in ApJ Letter

Troubles with Bayesianism: An introduction to the psychological immune system

A Bayesian mind is, at its core, a rational mind. Bayesianism is thus well-suited to predict and explain mental processes that best exemplify our ability to be rational. However, evidence from belief acquisition and change appears to show that we do not acquire and update information in a Bayesian way. Instead, the principles of belief acquisition and updating seem grounded in maintaining a psychological immune system rather than in approximating a Bayesian processor

Neurofeedback: principles, appraisal and outstanding issues

Neurofeedback is a form of brain training in which subjects are fed back information about some measure of their brain activity which they are instructed to modify in a way thought to be functionally advantageous. Over the last twenty years, NF has been used to treat various neurological and psychiatric conditions, and to improve cognitive function in various contexts. However, despite its growing popularity, each of the main steps in NF comes with its own set of often covert assumptions. Here we critically examine some conceptual and methodological issues associated with the way general objectives and neural targets of NF are defined, and review the neural mechanisms through which NF may act, and the way its efficacy is gauged. The NF process is characterised in terms of functional dynamics, and possible ways in which it may be controlled are discussed. Finally, it is proposed that improving NF will require better understanding of various fundamental aspects of brain dynamics and a more precise definition of functional brain activity and brain-behaviour relationships.Comment: 12 page

Partial report is the wrong paradigm

Is consciousness independent of the general-purpose information processes known as ‘cognitive access’? The dominantmethodology for supporting this independence hypothesis appeals to partial report experiments as evidence for perceptual consciousness in the absence of cognitive access. Using a standard model of evidential support, and reviewing recent elaborations of the partial report paradigm, this article argues that the paradigm has the wrong structure to support the independence hypothesis. Like reports in general, a subject’s partial report is evidence that she is conscious of information only where that information is cognitively accessed. So partial report experiments could dissociate consciousness from cognitive access only if there were uncontroversial evidence forconsciousness which did not imply reportability. There is no such evidence. An alternative, broadly Marrian methodology for supporting the independence hypothesis is suggested, and some challenges to it outlined. This methodology does not require experimental evidence for consciousness in the absence of cognitive access. Instead it focuses on a function of perceptual consciousness when a stimulus is cognitively accessed. If the processes best suited to implement this function exclude cognitive access, the independence hypothesis will be supported. One relevant function of consciousness may be reflected in reason-based psychological explanations of a subject’s behaviour

Separation and identification of dominant mechanisms in double photoionization

Double photoionization by a single photon is often discussed in terms of two contributing mechanisms, {\it knock-out} (two-step-one) and {\it shake-off} with the latter being a pure quantum effect. It is shown that a quasi-classical description of knock-out and a simple quantum calculation of shake-off provides a clear separation of the mechanisms and facilitates their calculation considerably. The relevance of each mechanism at different photon energies is quantified for helium. Photoionization ratios, integral and singly differential cross sections obtained by us are in excellent agreement with benchmark experimental data and recent theoretical results.Comment: 4 pages, 5 figure