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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including images of six previously unpublished fields (1% of the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.