3,197 research outputs found
Revisiting the radio interferometer measurement equation. I. A full-sky Jones formalism
Since its formulation by Hamaker et al., the radio interferometer measurement
equation (RIME) has provided a rigorous mathematical basis for the development
of novel calibration methods and techniques, including various approaches to
the problem of direction-dependent effects (DDEs). This series of papers aims
to place recent developments in the treatment of DDEs into one RIME-based
mathematical framework, and to demonstrate the ease with which the various
effects can be described and understood. It also aims to show the benefits of a
RIME-based approach to calibration.
Paper I re-derives the RIME from first principles, extends the formalism to
the full-sky case, and incorporates DDEs. Paper II then uses the formalism to
describe self-calibration, both with a full RIME, and with the approximate
equations of older software packages, and shows how this is affected by DDEs.
It also gives an overview of real-life DDEs and proposed methods of dealing
with them. Applying this to WSRT data (Paper III) results in a noise-limited
image of the field around 3C 147 with a very high dynamic range (1.6 million),
and none of the off-axis artifacts that plague regular selfcal. The resulting
differential gain solutions contain significant information on DDEs, and can be
used for iterative improvements of sky models.
Perhaps most importantly, sources as faint as 2 mJy have been shown to yield
meaningful differential gain solutions, and thus can be used as potential
calibration beacons in other DDE-related schemes.Comment: 12 pages, no figures, published in A&
ATMOSPHERE ENTRY SIMULATOR AND ITS APPLICATION TO THE DETERMINATION OF ABLATIVE PROPERTIES OF MATERIALS FOR BALLISTIC MISSILES
Ames atmosphere entry simulator and its use in determining characteristics of ethyl cellulose as ablative heat shield for ballistic missile
Revisiting the radio interferometer measurement equation. IV. A generalized tensor formalism
The radio interferometer measurement equation (RIME), especially in its 2x2
form, has provided a comprehensive matrix-based formalism for describing
classical radio interferometry and polarimetry, as shown in the previous three
papers of this series. However, recent practical and theoretical developments,
such as phased array feeds (PAFs), aperture arrays (AAs) and wide-field
polarimetry, are exposing limitations of the formalism. This paper aims to
develop a more general formalism that can be used to both clearly define the
limitations of the matrix RIME, and to describe observational scenarios that
lie outside these limitations. Some assumptions underlying the matrix RIME are
explicated and analysed in detail. To this purpose, an array correlation matrix
(ACM) formalism is explored. This proves of limited use; it is shown that
matrix algebra is simply not a sufficiently flexible tool for the job. To
overcome these limitations, a more general formalism based on tensors and the
Einstein notation is proposed and explored both theoretically, and with a view
to practical implementations. The tensor formalism elegantly yields generalized
RIMEs describing beamforming, mutual coupling, and wide-field polarimetry in
one equation. It is shown that under the explicated assumptions, tensor
equations reduce to the 2x2 RIME. From a practical point of view, some methods
for implementing tensor equations in an optimal way are proposed and analysed.
The tensor RIME is a powerful means of describing observational scenarios not
amenable to the matrix RIME. Even in cases where the latter remains applicable,
the tensor formalism can be a valuable tool for understanding the limits of
such applicability.Comment: 16 pages, no figures, accepted by A&
Single Dish Polarization Calibration
Using the formalism of Hamaker et al. (1996), I derive a method for the
polarization calibration of observations made with a single radio telescope.
This method is particularly appropriate for observations of pulsars, where the
sign and magnitude of the circular polarization are useful for understanding
the emission processes at work. I apply the method to observations of PSR
J1359-6038 made using the multibeam receiver on the Parkes radio telescope.Comment: Accepted by PASA. 11 pages. 3 figure
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
Reduced Ambiguity Calibration for LOFAR
Interferometric calibration always yields non unique solutions. It is
therefore essential to remove these ambiguities before the solutions could be
used in any further modeling of the sky, the instrument or propagation effects
such as the ionosphere. We present a method for LOFAR calibration which does
not yield a unitary ambiguity, especially under ionospheric distortions. We
also present exact ambiguities we get in our solutions, in closed form. Casting
this as an optimization problem, we also present conditions for this approach
to work. The proposed method enables us to use the solutions obtained via
calibration for further modeling of instrumental and propagation effects. We
provide extensive simulation results on the performance of our method.
Moreover, we also give cases where due to degeneracy, this method fails to
perform as expected and in such cases, we suggest exploiting diversity in time,
space and frequency.Comment: Draft version. Final version published on 10 April 201
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