J/AJ/131/1872       Fourth VLBA calibrator survey: VCS4     (Petrov+, 2006)
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Beginning of ReadMe : J/AJ/131/1872 Fourth VLBA calibrator survey: VCS4 (Petrov+, 2006) ================================================================================ The fourth VLBA calibrator survey: VCS4. Petrov L., Kovalev Y.Y., Fomalont E.B., Gordon D. <Astron. J., 131, 1872-1879 (2006)> =2006AJ....131.1872P ================================================================================ ADC_Keywords: Surveys ; Radio sources ; Positional data Keywords: astrometry - catalogs - surveys Abstract: This paper presents the fourth extension to the VLBA Calibrator Survey, containing 258 new sources not previously observed with very long baseline interferometry (VLBI). This survey, based on three 24hr Very Long Baseline Array observing sessions, fills remaining areas on the sky above declination -40deg where the calibrator density is less than one source within a 4deg radius disk in any given direction. The share of these areas was reduced from 4.6% to 1.9%. Source positions were derived from astrometric analysis of group delays determined at 2.3 and 8.6GHz frequency bands using the Calc/Solve software package. The VCS4 catalog of source positions, plots of correlated flux density versus projected baseline length, contour plots, and fits files of naturally weighted CLEAN images, as well as calibrated visibility function files, are available online at http://vlbi.gsfc.nasa.gov/vcs4 . Description: The VCS4 observations were carried out in three 24hr observing sessions with the VLBA on 2005 May 12, June 12, and June 30. Each target source was observed in two scans. The scan duration was 120s for sources from the list of 107 bright objects and 235s for other target sources. The target sources were observed in a sequence designed to minimize loss of time from antenna slewing. In addition to these objects, 93 strong sources were taken from the Goddard Space Flight Center astrometric and geodetic catalog 2004f_astro (See http://vlbi.gsfc.nasa.gov/solutions/astro). Observations of three to four strong sources from this list were made every 11.5hr during observing sessions.