Combining Bandpasses

Under construction

Sometimes the bandpass calibrator at the required frequency is contaminated with line absorption or emission. In particular, this is nuisance for observations close to the rest frequency of HI where Galactic HI is observed at almost any sightline.

A technique to still obtain a useable bandpass solution is to observe frequencies offset to the main frequency. These offset frequencies should be chosen clean from any line contamination. Ideally it is advisable to observe 2 offset frequencies, symmetric to the main observing frequency.

In the following we explain how to combine these offset frequencies for a common bandpass and how to apply it to the central frequency.

Thee steps are needed:

1) *gaincal:* the offset frequencies typically also have different phases. Since the bandpass is obtained by 'vector averaging' the data, one needs to correct for the phase offsets. The best way to do this is to derive and apply a complex gain solution.

2) *bandpass:* now we combine the two offset frequency observations into a single bandpass solution by averaging.

3) *applycal:* The combined bandpass will, by definition, be based on the frequency of the first input spw. The third step is map this solution to the central spw.