The Flux Density Scale Used at the VLA

The flux density scale used by CASA and AIPS between 1 and 50 GHz is based on the Perley-Butler scale (ApJS, vol 204, 19, 2013). In turn, this scale is based on the absolute Wilkinson Microwave Anisotropy Probe (WMAP) measurements of the brightness of the planet Mars. The corresponding flux densities of Mars were then transferred to four stable calibrator sources (3C123, 3C196, 3C286 and 3C295) through careful VLA measurements of their ratios to Mars spanning more than one decade. The new scale is very close to the traditional Baars et al. (1977 Astron. Astrophys., 61, 99) scale between 1 and 15 GHz.

Below 1 GHz, AIPS and CASA have adopted the 'Scaife and Heald' scale. The VLA has made careful measurements of the ratios of a small set of standard calibrators to Cygnus A, whose absolute flux density is believed to be known to within ~3%.  Early results suggest the 'Scaife and Heald' scale is consistent with the known flux density of Cygnus A between 220 and 480 MHz.

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