PSRCHIVE and PSRFITS: Definition of the Stokes Parameters and Instrumental Basis Conventions

This paper defines the mathematical convention adopted to describe an electromagnetic wave and its polarisation state, as implemented in the PSRCHIVE software and represented in the PSRFITS definition. Contrast is made between the convention that has been widely accepted by pulsar astronomers and the IAU/IEEE definitions of the Stokes parameters. The former is adopted as the PSR/IEEE convention, and a set of useful parameters are presented for describing the differences between the PSR/IEEE standard and the conventions (either implicit or explicit) that form part of the design of observatory instrumentation. To aid in the empirical determination of instrumental convention parameters, well-calibrated average polarisation profiles of PSR J0304+1932 and PSR J0742-2822 are presented at radio wavelengths of approximately 10, 20, and 40 cm.Comment: 7 pages, 2 figures, to be published in PAS

The MeqTrees software system and its use for third-generation calibration of radio interferometers

The formulation of the radio interferometer measurement equation (RIME) by Hamaker et al. has provided us with an elegant mathematical apparatus for better understanding, simulation and calibration of existing and future instruments. The calibration of the new radio telescopes (LOFAR, SKA) would be unthinkable without the RIME formalism, and new software to exploit it. MeqTrees is designed to implement numerical models such as the RIME, and to solve for arbitrary subsets of their parameters. The technical goal of MeqTrees is to provide a tool for rapid implementation of such models, while offering performance comparable to hand-written code. We are also pursuing the wider goal of increasing the rate of evolution of radio astronomical software, by offering a tool for rapid experimentation and exchange of ideas. MeqTrees is implemented as a Python-based front-end called the meqbrowser, and an efficient (C++-based) computational back-end called the meqserver. Numerical models are defined on the front-end via a Python-based Tree Definition Language (TDL), then rapidly executed on the back-end. The use of TDL facilitates an extremely short turn-around time for experimentation with new ideas. This is also helped by unprecedented visualization capabilities for all final and intermediate results. A flexible data model and a number of important optimizations in the back-end ensures that the numerical performance is comparable to that of hand-written code. MeqTrees is already widely used as the simulation tool for new instruments (LOFAR, SKA) and technologies (focal plane arrays). It has demonstrated that it can achieve a noise-limited dynamic range in excess of a million, on WSRT data. It is the only package that is specifically designed to handle what we propose to call third-generation calibration (3GC), which is needed for the new generation of giant radio telescopes.Comment: 15 pages; 14 figure

An evaluation of a two year cognitive intervention programme in technology education for Key Stage 4

We reported (in Vol. 1 No 2 1996) an interim evaluation of the effects of a small-scale cognitive intervention programme in technology. The subjects in this study were 120 Year 10 students (15+) attending a girls comprehensive school in an inner London. Education Authority. The students were randomly placed in eight all ability classes according to the normal school practice and policy. Three experimental classes (45 students) and five control classes (75 students) were identified. The new head of design and technology had targeted this year group in order to try to raise achievement of a group of students in a domain which had not previously performed as effectively as had been expected. The teacher would be taking the three experimental groups designated 10.1, 10.2, 10.5. Two other teachers would have the responsibility of teaching the five control classes. Unfortunately we were unable to establish a control group being taught by the teacher of the experimental groups.The model that was adopted for the study utilised principles from Cognitive Acceleration through Science Education (CASE) (the five pillars), from Instrumental Enrichment and from the Somerset Thinking Skills project. The essential feature of the study was to concentrate on enhancing the students' thinking, reasoning and problem solving capability with the teacher acting as a mediator and director of the activities and of the discussion that occurred. (For details see pp 121-124 Vol. 1 No 21996). The results suggested that the intenention was having a positive but modest effect in technology achievement on the experimental classes, but that there was little or no effect in other areas of the curriculum that we were investigating. This article is a follow-up to the original paper and presents further data showing the effects of the inteNention after the full two years of the programme. The subjects, design and methodology adopted throughout this project are described in the original paper (Hamaker et al 1996).The established groups with their corresponding teachers are summarised in Table 1. Each of the experimental classes had one core technology lesson replaced by an inteNention lesson each fortnight. Class 10.2 had a further graphical communication option replaced by an intervention lesson each fortnight. The five control classes received a normal allocation of technology core and option lessons. The tests and measures used for analyses are similar to that already described (Hamaker et al 1996). Further clarification on the nature of these tests and measures can be obtained from the authors of this article

Radio astronomical polarimetry and phase-coherent matrix convolution

A new phase-coherent technique for the calibration of polarimetric data is presented. Similar to the one-dimensional form of convolution, data are multiplied by the response function in the frequency domain. Therefore, the system response may be corrected with arbitrarily high spectral resolution, effectively treating the problem of bandwidth depolarization. As well, the original temporal resolution of the data is retained. The method is therefore particularly useful in the study of radio pulsars, where high time resolution and polarization purity are essential requirements of high-precision timing. As a demonstration of the technique, it is applied to full-polarization baseband recordings of the nearby millisecond pulsar, PSR J0437-4715.Comment: 8 pages, 4 figures, accepted for publication in Ap

Radio Astronomical Polarimetry and Point-Source Calibration

A mathematical framework is presented for use in the experimental determination of the polarimetric response of observatory instrumentation. Elementary principles of linear algebra are applied to model the full matrix description of the polarization measurement equation by least-squares estimation of non-linear, scalar parameters. The formalism is applied to calibrate the center element of the Parkes Multibeam receiver using observations of the millisecond pulsar, PSR J0437-4715, and the radio galaxy, 3C 218 (Hydra A).Comment: 8 pages, 4 figures, to be published in ApJ

High-efficiency GaAs concentrator space cells

High efficiency Al sub x Ga sub 1-x As/GaAs heteroface solar concentrator cells have been developed for space applications. The cells, which were grown using metalorganic chemical vapor deposition (MOCVD), have been fabricated in both the p-n and n-p configurations. Magnesium and zinc are used as the p-type dopants, and Se is used as the n-type dopant. The space cells, which are designed for use in a Cassegrainian concentrator operating at 100 suns, AMO, have a circular illuminated area 4 mm in diameter on a 5 mm by 5 mm cell. These cells have exhibited flash-tested efficiencies as high as 23.6 percent at 28 C and 21.6 percent at 80 C