The antenna based complex gains vary across the passband, primarily due to the antenna based band-shape and residual fixed delay errors. These variations in the complex gains must be corrected before the visibilities from individual frequency channels are averaged. Therefore, we also used a bandpass calibrator to estimate and correct for the bandpass shape.
The most complete approach to bandpass calibration is to observe a strong line-free continuum source to find the bandpass characteristics (amplitude and phase) of each antenna-receiver combination. In the observations, the bandpass calibrator was observed in the beginning and at the end of the observation session and if possible in the middle of the observations. The bandpass calibrator was also the flux calibrator.
The BP table and the bandpass corrected band shapes were examined using the AIPS task POSSM. Large oscillations across the band were sometimes found in a few antennas. These antennas were usually flagged from the entire data set using the AIPS task UVFLG.