Description

This task provides for initialization of the weight information in the MS. An overview on weights in CASA is provided in the Data Weights section of CASAdocs. For ALMA interferometry and EVLA data, it should not generally be necessary to use this task, as the weight information should have been initialized properly at fill time (when the import was performed with CASA v4.2.2 and later).

Several initialization modes are supported via the wtmode parameter. If wtmode='nyq' (the default), the SIGMA and WEIGHT will be initialized according to bandwidth and integration time. This is the theoretically correct mode for raw normalized visibilities (e.g., ALMA). For the EVLA, this is correct if switched-power and bandpass calibration will later be applied. If wtmode='sigma', WEIGHT will be initialized according to the existing SIGMA column. If wtmode='weight', WEIGHT_SPECTRUM will be initialized according to the existing WEIGHT column; dowtspec=True must be specified in this case. If wtmode='ones', SIGMA and WEIGHT will be initialized with 1.0, globally. This is a traditional means of initializing weight information, and is adequate when the integration time and bandwidth are uniform. It is not recommended for modern instruments (ALMA, EVLA), where variety in observational setups is common, and properly initialized and calibrated weights will be used for imaging sensitivity estimates.

There are two EXPERIMENTAL modes, wtmode='tsys' and 'tinttsys'. In the modes, SIGMA and WEIGHT will be initialized according to $T_{sys}$, bandwidth $\Delta\nu$, and integration time $t_{int}$ (used only in 'tinttsys'), i.e.:

  • tsys : $weight=\frac{\Delta\nu}{T_{sys}^2}$
  • tinttsys: $weight=\frac{\Delta\nu \, t_{int}}{T_{sys}^2}$

These modes use Tsys values to calculate weight as is done in Tsys calibration. Tsys values are taken from a Tsys calibration table given as tsystable. Selection of gain field (gainfield), interpolation method (interp), and spectral window mapping (spwmap) are supported, too.

Available types of interpolation are:

  • Time: 'nearest', 'linear', the variation of those with 'perobs', e.g., 'linearperobs' (enforce obsId boundaries in interpolation)
  • Freq: 'nearest', 'linear', 'cubic', 'spline', and the variation of those with 'flag', e.g., 'linearflag' (with channelized flag).

See the page of applycal for details of interpolations.

WARNING: If the weight in an MS is initialized with these modes and Tsys calibration table is applied with calwt=True after that, the weight would be contaminated by being divided by the square of Tsys twice. !!! USERS ARE ADVISED TO USE THESE EXPERIMENTAL MODES WITH CARE !!!

For the above wtmodes, if dowtsp=True (or if the WEIGHT_SPECTRUM column already exists), the WEIGHT_SPECTRUM column will be initialized (uniformly in channel in wtmode='nyq', 'sigma', 'weight', and 'ones'), in a manner consistent with the disposition of the WEIGHT column. If the WEIGHT_SPECTRUM column does not exist, dowtsp=True will force its creation. Use of the WEIGHT_SPECTRUM column is only meaningful for ALMA data which will be calibrated with channelized Tsys information, or if the weights will become channelized after calibration, e.g., via averaging over time- and channel-dependent flagging (statwt is a task for channel-dependent weight estimation from the data itself). 

In non-channelized modes (wtmode='nyq', 'sigma', 'weight', and 'ones') or when dowtsp=False, the SIGMA_SPECTRUM column will be removed from the MS. On the other hand, the SIGMA_SPECTRUM column is added and initialized in channelized modes (wtmode='tsys' and 'tinttsys') if dowtsp=True or if the WEIGHT_SPECTRUM already column exists.

Two additional modes are available for managing the spectral weight info columns; these should be used with extreme care: If wtmode='delwtsp', the WEIGHT_SPECTRUM column will be deleted (if it exists). If wtmode='delsigsp', the SIGMA_SPECTRUM column will be deleted (if it exists). Note that creation of SIGMA_SPECTRUM is not supported via this method.

NOTE: This task does not support any prior selection. Intialization of the weight information must currently be done globally or not at all. This is to maintain consistency.