If the phase stability is bad during the observation (the atmospheric conditions are poor), the phase of the antenna gains can vary rapidly. A change of tens of degrees or more during the calibrator scan is not uncommon. However the software that determines the antenna gains assumes that the gains are constant during a solution interval. So during periods of poor phase stability, it is often desirable to make the solution interval of the calibration software quite short. While the resultant gains probably track the phase during the calibrator scan, what we are really interested in are the antenna gains for the program source. If you solved for a number of time intervals during the calibrator scan, the best guess at the antenna gains for the program source is derived by interpolating between some average of the calibrator scan gains. Thus after determining the gains at a fine time step in the calibrator scan, you should average these gains together to get some representative gain for the whole calibrator scan. This is probably the best guess you can make (at least as far as correcting the program source is concerned), although in times of truly awful phase stability, its a pretty poor guess (self-calibration will be needed in this case).
There are two ways to average your gains, either the ``vector'' or ``scalar'' averages. Which is the most appropriate will depend on whether you want good estimates of the gains or good estimates of the resultant images.
The task gpaver can be used to perform averaging of antenna gains. Its pretty straightforward.Typical inputs are
|vis=vela.5500||Average the gain table for the program source|
|interval=2||Averaging interval (calibrator scan length)|
|options=scalar||Scalar average if self-calibrating later,|
|options=vector||vector average otherwise|