National Radio
Astronomy Observatory

4.5.5 Calibration Interpolation and Accumulation (accum)

The accum task is used to interpolate calibration solutions onto a different time grid, and to accumulate incremental calibrations into a cumulative calibration table. The manual accumulation of calibration is rarely required and can usually be achieved implicitly simply by running applycal with all the calibration tables given as a list in the gaintable parameter (and using gainfiled, spwmap, and interp appropriately. However, sometimes it is desirable to see the interpolated calibration prior to application, and this section describes how this can be done.

Its inputs are:

The mapping implied here is

(mathematically the cal solutions are multiplied as complex numbers as per the Measurement Equation). The tablein is optional (see below). You must specify an incrtable and a caltable.

The tablein parameter is used to specify the existing cumulative calibration table to which an incremental table is to be applied. Initially, no such table exists, and if tablein=’’ then accumulate will generate one from scratch (on-the-fly), using the timescale (in seconds) specified by the sub-parameter accumtime. These nominal solutions will be unit-amplitude, zero-phase calibration, ready to be adjusted by accumulation according to the settings of other parameters. When accumtime is negative (the default), the table name specified in tablein must exist and will be used. If tablein is specified, then the entries in that table will be used.

The incrtable parameter is used to specify the incremental table that should be applied to tablein. The calibration type of incrtable sets the type assumed in the operation, so tablein (if specified) must be of the same type. If it is not, accum will exit with an error message. (Certain combinations of types and subtypes will be supported by accum in the future.)

The caltable parameter is used to specify the name of the output table to write. If un-specified (’’), then tablein will be overwritten. Use this feature with care, since an error here will require building up the cumulative table from the most recent distinct version (if any).

The field parameter specifies those field names in tablein to which the incremental solution should be applied. The solutions for other fields will be passed to caltable unaltered. If the cumulative table was created from scratch in this run of accumulate, then the solutions for these other fields will be unit-amplitude, zero-phase, as described above.

The calfield parameter is used to specify the fields to select from incrtable to use when applying to tablein. Together, use of field and calfield permit completely flexible combinations of calibration accumulation with respect to fields. Multiple runs of accum can be used to generate a single table with many combinations. In future, a ’self’ mode will be enabled that will simplify the accumulation of field-specific solutions.

The spwmap parameter gives the mapping of the spectral windows in the incrtable onto those in tablein and caltable. The syntax is described in § 4.4.1.4.

The interp parameter controls the method used for interpolation. The options are (currently): ’nearest’, ’linear’, and ’aipslin’. These are described in § 4.4.1.4. For most purposes, the ’linear’ option should suffice.

We now describe the two uses of accum.

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