bandpass: used to flatten the amplitude and phase response of frequency bands in spectral line work.
A source should be chosen with constant flux over the bandpass.
primary (absolute): a point-source used to calibrate the flux density. Usually 3C286 or 3C48 is used.
secondary (phase): used to calibrate the phases of a source by taking advantage of phase closure relations.
The secondary calibrator should be near the source and observed frequently.
