Secondary Calibrators

Slow variations of the antenna based complex gains occur on time scales of a few tens of minutes. Therefore, the relative phase and amplitude variations between the antennas needs to be corrected so that the phase and amplitude of all the antennas of the array is constant over several hours of observations. These slow variations are measured and corrected using periodic observations of a secondary calibrator.

It is advisable to choose a secondary calibrator which is not too far from the target source; otherwise, the antenna based complex gains obtained from the secondary calibrator may not reflect the phase variations in the direction of the target field. Hence, if the secondary calibrator is within 10 to 15 deg. from the source, atmospheric phase fluctuations will be better calibrated.

The task CLCAL was used with parameters CALSOURCE='3C286, '3C48', '3C147', ''; INTERPOL='2PT'; REFANT=9, to as to generate CL (ver 2) table containing solutions derived from SN table by linear vector interpolation.

To further identify the presence of bad data, if any, the data was examined once again using UVPLT task and VPLOT task with DOCALIB=1 (this option applies the derived calibration for the data). After editing the bad data, if any, the tables SN (ver 1) and CL (ver 2) were deleted and the tasks CALIB and CLCAL were made to run once again for a few more iterations, until the bad data was completely, as best as possible!, identified and flagged.

The BP table was derived by time averaging the data to determine bandpass response function of the antennas and calibrate the bandpass. The task BPASS was run with DOCALIB=1; BPASSPRM(5)=1; ICHANSEL=40, 40, 1; SOLINT=0. BPASSPRM(5)=1 was used to do `NO' normalization before determining the solutions, and the SOLINT=0 is for averaging the UV-data over the whole scan defined by NX table.