Calibrating the Flux Density Scale

by Stephan W. Witz last modified Jun 22, 2017 by Emmanuel Momjian

Normal calibration of the flux density scale for VLA observations is effected by including a scan on a source of presumed known flux density in each Scheduling Block (SB). Using that known flux density source, the flux density of the complex gain calibrator(s) can be determined, and then transferred to your target source(s). Historically, 3C48 and 3C286 have been the standard sources for which NRAO has assumed flux densities are known as a function of frequency, and which have been recommended as flux density scale calibrator sources for the VLA. Restrictions on baseline length as a function of VLA configuration and observing band were supplied, which if followed allowed relatively accurate flux density scale calibration. More recently, we have improved the ability to calibrate the flux density scale by providing sky brightness models for these sources in CASA and AIPS, which loosens the restrictions on configurations and bands. In addition, we have added the sources 3C138 and 3C147 to the list of calibrators which can be used for this. However, all three of 3C48, 3C138, and 3C147 have spectral flux densities which vary with time (3C286 is constant, along with 3C295 and 3C196), so some care should still be taken if the most accurate flux density scale calibration is desired.

A single observation of a few minutes of one of the above-mentioned flux density scale calibrators will suffice for most observers. If possible, the flux density scale calibrator should be observed at a time when it is nearly at the same elevation as the complex gain calibrator, especially for the highest four bands (Ku-, K-, Ka-, and Q-band).  This is not always possible because of timing and geometry of sources, and the fact that it is not typically known specifically when an SB will be executed (so elevations vs. time are uncertain). However, flux density scale calibration accuracy in this case should be of order 10% at 4- and P-band, 5% at L- through Ku-bands, increasing to 10-15% for the three higher bands. If more accuracy is needed, a more careful strategy should be adopted, potentially using multiple flux density scale calibrators. The fundamental accuracy of the scale is ~5% at 4- and P-bands, 3% at L- through Ku-bands, increasing to 5% at Q-band. See Perley and Butler (2017) for more details on how the spectral flux densities of 3C48, 3C138, 3C147, and 3C286 (and many other sources) have been determined, and how they vary vs. time, along with information on the fundamental accuracy of the flux density scale when using these sources.

If less accuracy is needed in the flux density scale calibration, an observation of one of these standard sources need not necessarily be included in an SB. For example, for short triggered observations, where a simple detection is desired, and the time spent slewing back and forth to the flux density scale calibrator can make the SB significantly longer than it could be otherwise. In this case, the switched power measurement can be used to calibrate the flux density scale (see EVLA Memo 120 for some background on this). This is not a standard path of calibration and is not supported in an easy way in CASA or AIPS, but it is possible. Flux density scale accuracy in this case is ~10% for L- through Ku-bands, increasing to ~20% at Q-band, so not nearly as good as if using the "standard" method of flux density scale calibration, but it may be sufficient for some observers.