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VLSS Redux version, 2012

RADIO SCIENCE, VOL. 47, RS0K04, 9 PP., 2012

doi:10.1029/2011RS004941 arXiv:1205.4695v1

VLSS Redux: Software Improvements applied to the Very Large Array Low-frequency Sky Survey

W. M. Lane, W. D. Cotton, J. F. Helmboldt, N. E. Kassim

We present details of improvements to data processing and analysis which were recently used for a re-reduction of the Very Large Array (VLA) Low-frequency Sky Survey (VLSS) data. Algorithms described are implemented in the data-reduction package Obit, and include smart-windowing to reduce clean bias, improved automatic radio frequency interference removal, improved bright-source peeling, and higher-order Zernike fits to model the ionospheric phase contributions. An additional, but less technical improvement was using the original VLSS catalog as a same-frequency/same-resolution reference for calculating ionospheric corrections, allowing more accuracy and a higher percentage of data for which solutions are found. We also discuss new algorithms for extracting a source catalog and analyzing ionospheric fluctuations present in the data. The improved reduction techniques led to substantial improvements including images of six previously unpublished fields (1% of the survey area) and reducing the clean bias by 50%. The largest angular size imaged has been roughly doubled, and the number of cataloged sources is increased by 35% to 95,000.

The 4-meter All-Sky Survey (4MASS): Methodology

Lane, W. M. Cohen, A. S. Cotton, W. D. Perley, R. A. Kassim, N. E. Lazio, T. J. W. Condon, J. J. Erickson, W. C. 

*Affiliation:*
AA(Naval Research Lab), AB(Naval Research Lab), AC(NRAO), AD(NRAO),
AE(Naval Research Lab), AF(Naval Research Lab), AG(NRAO), AH(Univ. of MD)
*Journal:*
American Astronomical Society Meeting 201, #08.06
*Publication Date:*
12/2002
*Origin:*
AAS
*Abstract Copyright:*
(c) 2002: American Astronomical Society
*Bibliographic Code:*
2002AAS...201.0806L

Abstract

We discuss the observational methodology of the ongoing 4 Meter All Sky Survey (4MASS). The first all-sky survey at such a low frequency, 4MASS will map the entire sky above ? = -30^o at 74 MHz with a resolution of 80'' using the Very Large Array (VLA). Arcminute resolution observations below 100 MHz, not possible before development of the 74 MHz system at the VLA, reflect an ongoing renaissance in low frequency radio astronomy which will open a new window on one of the most poorly explored regions of the EM spectrum. The observational challenges at this frequency include radio frequency interference (RFI), ionospheric phase distortions, and a large field of view filled with sources. Most RFI at 74 MHz at the VLA is generated internally, and we have developed semi-automatic means of excising it. We have also developed a "field-based" calibration scheme in which corrections for the ionospheric effects are determined in the image plane from multiple sources at known positions within the field of view. The survey is currently 10% complete, and preliminary data products and more information are available at http://lofar.nrl.navy.mil/acohen/4MASS/web/info.html.