Calibration and the Flux Density Scale

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 VLA Calibrator Manual for advice. Over the last several years, we have implemented accurate source models directly in AIPS for much improved calibration of the amplitude scales; see page 21 of the July 2006 NRAO Newsletter for details. At present, models are available for 3C48 and 3C 286 for all bands from 1.3 to 20 cm, for 3C138 at all bands from 1.3 to 6 cm, and for 3C 147 at all bands from 0.7 to 2.0 cm.

Since the standard source flux densities are slowly variable, we monitor and update their flux densities when the VLA is in its D configuration. As the VLA cannot measure absolute flux densities, the values obtained must be referenced to assumed or calculated standards, as described in the next paragraph. Table 13 shows the flux densities of these sources in November 2001 at our standard bands. The accuracy of these values, relative to the assumed standards, is set by the gain stability of the VLA. The estimated 1- $\sigma$ errors in the table, relative to the assumed standards, are less than 1% for frequencies up to 25 GHz, and about 2% for the 43 GHz band.

The flux densities shown in the table for frequencies below 10 GHz are based on the Baars et al. value for 3C295. For frequencies above 10 GHz, the flux densities are based on models of Mars and NGC7027 which are themselves based on the Baars scale below 10 GHz.

Polynomial coefficients describing the derived flux densities for the standard calibrators have been determined which permit accurate interpolation of the flux density at any VLA frequency. These coefficients are updated approximately every few years, and are implemented into the AIPS task SETJY. At present, a substantial new effort is under way to improve the long-term (past and present) accuracy of the VLA flux density scale; contact Rick Perley or Bryan Butler for information on this work.

**Table 13:** **Flux densities (Jy) of Standard Calibrators for November 2001**
Source/Frequency (MHz)	327.5	1465	4885	8435	14965	22460	43340
3C48 = J0137+3309	45.52	15.54	5.47	3.22	1.83	1.23	0.63
3C138 = J0521+1638	19.34	8.16	3.71	2.43	1.54	1.10	0.62
3C147 = J0542+4951	55.00	21.32	7.88	4.72	2.77	1.93	1.11
3C286 = J1331+3030	25.96	14.49	7.49	5.22	3.51	2.63	1.58
3C295 = J1411+5212	60.37	21.49	6.53	3.42	1.67	0.99	0.41
NGC7027	0.2	1.54	5.52	6.03	5.90	5.68	5.22
MARS	-	-	0.175	0.528	1.67	3.81	14.22

For most observing projects, the effects of atmospheric extinction will automatically be accounted for by regular calibration when using a nearby point source whose flux density has been determined by an observation of a flux density standard taken at a similar elevation. However, at high frequencies (most notably K-band and Q-band), both the antenna gain and the atmospheric absorption may be strong enough to make `simple' flux density bootstrapping unreliable. The AIPS task ELINT is now available to permit measurement of an elevation gain curve using your own observations, and subsequent adjustment of the derived gains to remove these elevation-dependent effects. The current calibration methodology does not require knowledge of the atmospheric extinction (since the true flux densities of the standard calibrators are believed known). However, if knowledge of the actual extinction is desired, a simple antenna tipping procedure is available (and is known to the JObserve program) which will provide both the vertical extinction and the total system temperature. For advice on these procedures, contact Bryan Butler, Rick Perley, or Claire Chandler.