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Frequency Handling

Both selfcal
and gpscal
assume that the gains are independent
of frequency. This allows them to use data from all channels in forming
gain solutions. However, as mentioned above, they do not apply bandpass
corrections. Consequently, if you have multi-channel data and models, you will
probably want to form a bandpass-corrected visibility dataset before
doing self-calibration.

The assumption that the gains are independent of frequency should be
treated with some caution. If all the visibilities for a given baseline
go through one signal path, then the instrumental phase for the different
visibilities should be equal (assuming the signal path is well equalised).
However, atmospheric phase should vary linearly with frequency. Provided the
fractional bandwidth is small, it is still a reasonable approximation
that the gains are independent of frequency. Indeed this is the normal
approximation used in deriving calibration solutions from a `channel 0'
dataset.

If the visibilities for a given baseline go through multiple signal paths
(*e.g.* the 2 IF system for the ATCA), then the instrumental phases for the
different paths could be significantly different. Tentative
results for the 2 IF system of the ATCA show that, apart from an offset,
the phase of the two signal paths track each other as the ratio of the
frequencies (*i.e.* the path length difference is constant). The offset
between them can be eliminated either by adjusting the phases
of the two paths to zero at the start of the observation, or by the primary
calibration process. Provided the offset has been eliminated, and provided
the frequencies are not too different, it
may be useful to approximate the gains as independent of frequency.

If the phases vary significantly between the frequencies, then the different
frequencies need to be self-calibrated individually. Ideally there would be a
self-calibration task which knows that the phase errors at different
frequencies tracked as the ratio of the frequencies.

When self-calibrating multi-channel visibility datasets, there are two
distinct modes of calculating the variation of the model with
frequency, depending on whether the model contains multiple planes,
or whether multi-frequency techniques were used. We discuss
these two possibilities in turn.

**Subsections**
*Miriad manager *

2016-06-21