Basic Information on qvack

Task: qvack
Purpose: Flag after change to observing setup
Categories: calibration

        Qvack is a Miriad task to flag data for a number of integrations
        near an observing setup change. For example, this is used to
        flag data that is suspect because the instrument had not settled
        correctly after the change in the setup. The visibility data are
        should to be in time order.

Key: vis
        Input visibility data file. No default.

Key: select
        Standard uv data selection. Default is to select all data.

Key: interval
        This gives the time interval, in minutes, for which the data are
        flagged after a setup change (or before a setup change, if its
        value is negative).  The default is 0.1, which flags for
        the first 6 seconds after a switch.

Key: force
        The time interval, in minutes, which indicates the amount of time
        that should ellapse before forcing a (virtual) setup change.
        Thus you can QVACK on data that has distinct scans but no actual
        change in observing setup.  Note that this means the first
        INTERVAL amount of data is always flagged. The MODE option is
        not needed, nor is it used if you set FORCE
        Defaults to no forced setup change.

Key: options
        Extra processing options. Currently only one exists:
          anticipate   Normally the interval given by the "interval" keyword
                       is relative to the time of the first data found with
                       a new setup. options=anticipate makes it relative
                       to the time of the last data before a setup change
                       occurs.

Key: mode
        This determines what changes in setup cause flagging. Several
        values can be given, separated by commas, and minimum match is used.
        Possible values are:
          frequency    A change in the frequency setup. Do not use this
                       option if Doppler tracking is used.
          source       A change in the source observed.
          mosaic       A change in the observing centre of a mosaiced
                       observation.