Bandpass

The baseline bandpasses can be measured after calibration by averaging the complex visibilities over time. We reduce them to station-based values with full complex fits.

Note that there is a degeneracy between some calibration parameters and the bandpass. Because the calibration is performed after applying a guess for the bandpass, phases, delays and curvatures can end up partly in the calibration parameters and partly in the bandpass.

There is an additional complication: We do everything per baseline. The complete solution should 'close' perfectly, which means it can be reduced to station-based parameters. But this may not be true for calibration and bandbass individually. For this reason we apply the remaining closure errors from the calibration parameters to the baseline bandpasses before reducing them to station-based bandpasses.

Symbolically, we can write (neglecting complex conjugation):

(true signal) = (baseline-cal) * (baseline-bandpass)
              = (station-cal1 * station-cal2) * (station-bandpass1 * station-bandpass2)
              
                                          (baseline-bandpass) * (baseline-cal)        
(station-bandpass1 * station-bandpass2) = ------------------------------------
                                             (station-cal1 * station-cal2)