Effects Of Non-coplanarity Research Articles

Joint deconvolution and blind source separation with non-coplanar interferometric data

With the fastly increasing development of multichannel imagers, blind source separation (BSS) algorithms are ubiquitous in astrophysics to unmix multispectral images. In this context, analyzing data from the forthcoming very large, continental-size, radio interferometers using BSS algorithms raises two challenges. Firstly, the data are incomplete and deteriorated by instrumental effects, which requires incorporating a deconvolution step to retrieve exploitable images. Secondly, the data are affected by non-coplanar effects that notably arise from the very large antenna baselines and which must be accounted for in the separation scheme. For this purpose, we introduce a joint non-coplanar deconvolution and BSS algorithm, called wGMCA. The algorithm is tested and characterized in many challenging configurations, showing remarkable robustness to initialization and inversion. It is compared to classical methods that process the deconvolution and separation separately; these tests demonstrate the advantage of performing the deconvolution and separation in a single pass.

Adaptive-scale wide-field reconstruction for radio synthesis imaging

Sky curvature and non-coplanar effects, caused by low frequencies, long baselines, or small apertures in wide field-of-view instruments such as the Square Kilometre Array (SKA), significantly limit the imaging performance of an interferometric array. High dynamic range imaging essentially requires both an excellent sky model and the correction of imaging factors such as non-coplanar effects. New CLEAN deconvolution with adaptive-scale modeling already has the ability to construct significantly better narrow-band sky models. However, the application of wide-field observations based on modern arrays has not yet been jointly explored. We present a new wide-field imager that can model the sky on an adaptive-scale basis, and the sky curvature and the effects of non-coplanar observations with the w-projection method. The degradation caused by the dirty beam due to incomplete spatial frequency sampling is eliminated during sky model construction by our new method, while the w-projection mainly removes distortion of sources far from the image phase center. Applying our imager to simulated SKA data and the real observation data of the Karl G. Jansky Very Large Array (an SKA pathfinder) suggested that our imager can handle the effects of wide-field observations well and can reconstruct more accurate images. This provides a route for high dynamic range imaging of SKA wide-field observations, which is an important step forward in the development of the SKA imaging pipeline.

W-Stackingw-projection hybrid algorithm for wide-field interferometric imaging: implementation details and improvements

AbstractWe present a detailed discussion of the implementation strategies for a recently developedw-stackingw-projection hybrid algorithm used to reconstruct wide-field interferometric images. In particular, we discuss the methodology used to deploy the algorithm efficiently on a supercomputer via use of a Message Passing Interface (MPI)k-means clustering technique to achieve efficient construction and application of non-coplanar effects. Additionally, we show that the use of conjugate symmetry can increase thew-stacking efficiency, decrease the time required to construction, and applyw-projection kernels for large data sets. We then demonstrate this implementation by imaging an interferometric observation of Fornax A from the Murchison Widefield Array (MWA). We perform an exact non-coplanar wide-field correction for 126.6 million visibilities using 50 nodes of a computing cluster. Thew-projection kernel construction takes only 15 min prior to reconstruction, demonstrating that the implementation is both fast and efficient.

Direct wide-field radio imaging in real-time at high time resolution using antenna electric fields

ABSTRACTThe recent demonstration of a real-time direct imaging radio interferometry correlator represents a new capability in radio astronomy. However, wide-field imaging with this method is challenging since wide-field effects and array non-coplanarity degrade image quality if not compensated for. Here, we present an alternative direct imaging correlation strategy using a direct Fourier transform (DFT), modelled as a linear operator facilitating a matrix multiplication between the DFT matrix and a vector of the electric fields from each antenna. This offers perfect correction for wide field and non-coplanarity effects. When implemented with data from the Long Wavelength Array (LWA), it offers comparable computational performance to previously demonstrated direct imaging techniques, despite having a theoretically higher floating point cost. It also has additional benefits, such as imaging sparse arrays and control over which sky coordinates are imaged, allowing variable pixel placement across an image. It is in practice a highly flexible and efficient method of direct radio imaging when implemented on suitable arrays. A functioning electric field direct imaging architecture using the DFT is presented, alongside an exploration of techniques for wide-field imaging similar to those in visibility-based imaging, and an explanation of why they do not fit well to imaging directly with the digitized electric field data. The DFT imaging method is demonstrated on real data from the LWA telescope, alongside a detailed performance analysis, as well as an exploration of its applicability to other arrays.

A Fast and Exact w-stacking and w-projection Hybrid Algorithm for Wide-field Interferometric Imaging

Abstract The standard wide-field imaging technique, the w-projection, allows correction for wide fields of view for non-coplanar radio interferometric arrays. However, calculating exact corrections for each measurement has not been possible due to the amount of computation required at high resolution and with the large number of visibilities from current interferometers. The required accuracy and computational cost of these corrections is one of the largest unsolved challenges facing next-generation radio interferometers such as the Square Kilometre Array. We show that the same calculation can be performed with a radially symmetric w-projection kernel, where we use one-dimensional adaptive quadrature to calculate the resulting Hankel transform, decreasing the computation required for kernel generation by several orders of magnitude, while preserving the accuracy. We confirm that the radial w-projection kernel is accurate to approximately 1% by imaging the zero-spacing with an added w-term. We demonstrate the potential of our radially symmetric w-projection kernel via sparse image reconstruction, using the software package PURIFY. We develop a distributed w-stacking and w-projection hybrid algorithm. We apply this algorithm to individually correct for non-coplanar effects in 17.5 million visibilities over a 25 by 25 degree FoV Murchison Widefield Array observation for image reconstruction. Such a level of accuracy and scalability is not possible with standard w-projection kernel generation methods. This demonstrates that we can scale to a large number of measurements with large image sizes while still maintaining both speed and accuracy.

The Design of a New Ultrasonic Horn for Flip-Chip Bonding

In the manufacturing process of flip-chips, ultrasonic bonding has been widely used. In the past, however, the ultrasonic horn was designed to move horizontally while the bonding force was applied vertically, the motion control of the horn thus cannot be controlled accurately. In addition, non-coplanar effects were unavoidably occurred and the quality of bonding became unacceptable. In this paper, the non-coplanar phenomenon between the horn and the substrate was attempted to improve by designing the horn with the following features: 1. The resonant mode of the horn is purely longitudinal; 2. The contact surface between the horn and the flip-chip in the substrate remain plane; The finite element method (FEM) was used to design the ultrasonic horn with the above features. In the future, the amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) will be used to verify the FEM results.

Noncoplanarity in proton-proton bremsstrahlung

Using the soft-photon approximation, we address the issue of the importance of noncoplanarity effects in proton-proton bremsstrahlung. We investigate the noncoplanar cross section as a function of the noncoplanarity angle $\overline{\ensuremath{\varphi}}$ for the entire range of the photon polar angle ${\ensuremath{\psi}}_{\ensuremath{\gamma}}.$ The $\overline{\ensuremath{\varphi}}$ dependence is shown to provide a significant variation in the cross section, for a given ${\ensuremath{\psi}}_{\ensuremath{\gamma}}.$ Thus, there can be some uncertainty in determining experimental coplanar cross sections. To avoid the phase-space singularities of spherical geometry, we utilize the Harvard noncoplanar geometry. A detailed explication of the Harvard geometry is provided. Comparison of our calculations with experimental data is included.

Neutron-proton bremsstrahlung calculation: Noncoplanarity effects

Coplanar and noncoplanar $\mathrm{np}\ensuremath{\gamma}$ cross sections and coplanar $\mathrm{np}\ensuremath{\gamma}$ analyzing powers are calculated in the energy region between 130 and 200 MeV for various nucleon scattering angles. Both pseudoscalar and pseudovector $\ensuremath{\pi}N$ couplings have been considered in our calculations. The results are used to investigate noncoplanarity effects. Agreement with available $\mathrm{np}\ensuremath{\gamma}$ data is good for most cases, but discrepancies do exist for some data points. We show that the dependence of the noncoplanar cross sections upon the noncoplanarity angle is significantly different at different photon angles. This suggests that this more general approach should be used to investigate the important noncoplanarity effects. Similar to what has previously been observed in potential-model calculations, the meson-exchange effects dominate the $\mathrm{np}\ensuremath{\gamma}$ process. To the best of our knowledge, results for $\mathrm{np}\ensuremath{\gamma}$ analyzing powers have heretofore not been calculated. As far as we know, no $\mathrm{np}\ensuremath{\gamma}$ analyzing power experimental data are presently available for comparison, but these results should prove useful for future experiments. The differences in either cross sections or analyzing powers calculated using the two couplings are very small for all cases studied, in contradistinction to our previous calculations for the $\mathrm{pp}\ensuremath{\gamma}$ process where the differences were found to be greater.

Comment on “Noncoplanarity effects in proton-proton bremsstrahlung”

Li, Liou, Timmermans, and Gibson conclude in their paper [Phys. Rev. C 58, R 1880 (1998)] that the effect of noncoplanarity in proton-proton bremsstrahlung is significant. To overcome the problems with kinematic singularities in the cross section, they recommend the use of the Harvard noncoplanar coordinate system in the analysis of data. We show that this coordinate system introduces other experimental problems. Alternatively, we propose to present data as the square of the invariant matrix elements, along with cross sections.

Reply to “Comment on ‘Noncoplanarity effects in proton-proton bremsstrahlung’ ”

We respond to the Comment by Huisman and Kalantar-Nayestanaki [Phys. Rev. C 64, 029801 (2001)]. In particular, we point out that the proposal to present noncoplanar proton-proton bremsstrahlung data as squared matrix elements involves unresolved theoretical problems.

Noncoplanarity effects in proton-proton bremsstrahlung

Noncoplanarity in proton-proton bremsstrahlung is investigated. Significant effects are observed for certain photon polar angles, psi(gamma). Such noncoplanarity effects, not of dynamical origin, are possibly responsible for past disagreements between theory and experiment. The Harvard noncoplanar coordinate system, which avoids kinematic singularities in the cross section, is used in our calculations and is recommended for use in the analysis of experimental data. Alternative methods of presenting cross sections are discussed. [S0556-2813(98)50110-0].

PREPARATION AND CIRCULAR DICHROISM OF OPTICAL ACTIVE COBALT(III) COMPLEX CONTAINING 3,3′-DIMETHYL-2,2′-BIPYRIDINE

Abstract Cobalt(III) complex of a bulky ligand 3,3′-dimethyl-2,2′-bipyridine (dmbpy), [Coen2dmbpy] Cl(ClO4)2·H2O was synthesized and resolved. Stereospecific coordination was found. The effects of non-coplanarity of coordinated dmbpy were observed in the absorption (AB) and circular dichroism (CD) spectra of the resolved complex.

Comment on the Forward-AngleppγData at 156 MeV

Proton-proton bremsstrahlung cross sections at 156 MeV are calculated and compared to the Orsay experimental data. Both relativistic effects and noncoplanar effects are found to be important for forward-angle scattering. Agreement with Orsay's data is much better if these effects are included.