Calibration and Flux Density Scale

by Stephan W. Witz last modified Sep 21, 2016 by Tony Perreault

The VLA Calibrator List contains information on 1860 sources sufficiently unresolved and bright to permit their use as calibrators; and is also available from within the Observation Preparation Tool (OPT).

Accurate flux densities can be obtained by observing one of 3C286, 3C147, 3C48 or 3C138 during the observing run. Not all of these are suitable for every observing band and configuration—consult the Guide to Observing at the VLA for advice. Over the last several years, accurate source models have been implemented directly in AIPS and CASA for much improved calibration of the amplitude scales. Models are available for 3C48, 3C138, 3C147, and 3C286 at L, S, C, X, Ku, K, Ka, and Q-bands.

Since the standard source flux densities are slowly variable, we monitor their flux densities when the array is in its D configuration. As the VLA cannot accurately measure absolute flux densities, the values obtained must be referenced to assumed or calculated standards as described in the next paragraph. Table 3.11.1 shows the flux densities of these sources in January 2012 at the standard VLA bands. The flux density scale for the VLA, from 1 through 50 GHz, is based on emission models of the planet Mars, which is then calibrated to the CMB dipole using WMAP (Wilkinson Microwave Anisotropy Probe) observations (for details see Perley and Butler, 2013a). The source 3C286 (J1331+3030) is known to be non-variable, and has thus been adopted as the prime flux density calibrator source for the VLA. The adopted polynomial expression for the spectral flux density for 3C286 is:

[display]\log(S) = 1.2515 - 0.4605 \log(f) - 0.1715 \log^2(f) + 0.0336 \log^3(f)[/display]

where S is the flux density in Jy, and f is the frequency in GHz.

The absolute accuracy of our flux density scale is estimated to be about 2%. With care, the internal accuracy in flux density bootstrapping is better than 1% at all bands except Q-band, where pointing errors limit bootstrap accuracy to perhaps 3%. Note that such high internal accuracies are only possible in long-duration observations where the antenna gain curves and atmospheric opacity can be directly measured, and where there is good elevation overlap between the target source(s) and the flux density standard calibrator.

Table 3.11.1: Flux densities (Jy) of Standard Calibrators for January 2012
Source

1465 MHz

2565 MHz4885 MHz8435 MHz14965 MHz22460 MHz36435 MHz43340 MHz
3C48 = J0137+3309 15.56 9.80 5.39 3.14 1.77 1.19 0.73 0.63
3C138 = J0521+1638 8.71 6.17 4.02 2.78 1.89 1.46 1.03 0.92
3C147 = J0542+4951 21.85 13.75 7.59 4.49 2.59 1.77 1.10 0.94
3C286 = J1331+3030 14.90 10.03 7.34 5.09 3.39 2.52 1.75 1.53
3C295 = J1411+5212 22.15 12.95 6.41 3.34 1.62 0.957 0.507 0.403
NGC7027 1.62 3.59 5.38 5.79 5.62 5.42 5.18 5.04

The sources 3C48, 3C147, and 3C138 are all slowly variable. VLA staff monitor these variations on timescales of a year or two;  suitable polynomial coefficients are determined for them which should allow accurate flux density bootstrapping. These coefficients are updated approximately every other year and are used in the AIPS task SETJY and in the CASA task setjy.

The VLA antennas have elevation-dependent gain variations which are important to account for at the four highest frequency bands. Gain curves are determined by VLA staff, and the necessary corrections are applied to the visibility data when these data are downloaded from the archive. Additionally, atmospheric opacity will also cause an elevation-dependent gain which is also particularly notable at these four highest frequency bands. Currently, we do not have an atmospheric opacity monitoring procedure; users should utilize the appropriate tasks available in both AIPS and CASA to estimate and correct for the opacity using ground-based weather data. Correction of these gain dependencies, plus regular calibration using a nearby phase calibrator, should enable good amplitude gain calibration for most users. Please note that extraordinary attenuation by clouds can only be approximately corrected for by regular observation of a nearby calibrator.

A better procedure for removing elevation gain dependencies uses the AIPS task ELINT. This task will generate a 2nd order polynomial gain correction utilizing your own calibrator observations. This will remove both the antenna and opacity gain variations, and has the decided advantage of not utilizing opacity models or possibly outdated antenna gain curves. Use of this procedure is only practical if your observations span a wide range in elevation.

By far the most important gain variation effect is that due to pointing. Daytime observations on sunny days can suffer pointing errors, primarily in elevation, of up to one arcminute. This effect can be largely removed by utilizing the referenced pointing procedure which determines the pointing offset of a nearby calibrator. This offset is then applied to subsequent target source observations. It is recommended that this local offset be determined at least hourly, utilizing an object within 15 degrees of the target source—preferentially at an earlier R.A.. Studies show that the maximum pointing error will be reduced to about 7 arcseconds or better. VLA staff continue to work on improving this essential methodology.

The VLA's post-amplifiers are not temperature stabilized and exhibit significant gain changes between night and day, particularly at the four highest frequency bands. Changes as large as 30% have been seen between night and day in calm, clear conditions. These gain changes, and others caused by possible changes in attenuator settings, are monitored and will be removed with excellent accuracy by application of the internal calibration signal, whose results are recorded in the switched power table (SY table in AIPS). These corrections are not applied by default—users who wish to correct for these gain changes must utilize the appropriate tasks in AIPS or CASA. For the most accurate flux density bootstrapping, this table must be applied to the visibility data before calibration. Gain bootstrapping better than 1% can be accomplished for the 8-bit sampler system after application of the switched power data. For the 3-bit system there is an additional complication, as the values of the switched power data are sensitive to the total power as well as the system gain. VLA staff are currently working on a methodology to remove the total power dependency. Not applying the switched power data will reduce bootstrapping accuracy to perhaps 10%, and possibly worse, if the observation of the flux density calibrator is not close in time to the local complex gain calibrator (amplitude and phase).