National Radio
Astronomy Observatory

4.4.1.5 Solving: solint, combine, preavg, refant, minblperant, minsnr

The parameters controlling common aspects of the solution are:

The temporal solution interval is given by solint. Optionally a frequency interval ofor each solution can be added after a comma, e.g. solint=’60s,300Hz’. Time units are in seconds unless specified differently. Frequency units can be either channels or Hz and only make sense for bandpass of frequency dependent polarization calibration. The special values ’inf’ and -1 specify an “infinite” solution interval encompassing the entire dataset, while ’int’ or zero specify a solution every integration. aYou can use time quanta in the string, e.g. solint=’1min’ and solint=’60s’ both specify solution intervals of one minute. Note that ’m’ is a unit of distance (meters); ’min’ must be used to specify minutes. The solint parameter interacts with combine to determine whether the solutions cross scan or field boundaries.

The parameter controlling the scope of the solution is combine. For the default combine=’’ solutions will break at scan, field, and spw boundaries. Specification of any of these in combine will extend the solutions over the boundaries (up to the solint). For example, combine=’spw’ will combine spectral windows together for solving, while combine=’scan’ will cross scans. Thus, to do scan-based solutions (single solution for each scan), set

while

will make a single solution for the entire dataset (for a given field and spw).

will calculate a solution for each scan, averaging over 30 channels. The solutions will span across spw boundaries.

You can specify multiple choices for combination:

for example.

The reference antenna is specified by the refant parameter. A list of antennas can be provided to this parameter and if the first antenna is not present in the data, the next antenna in the list will be used, etc. It is useful to “lock” the solutions with time, effectively rotating (after solving) the phase of the gain solutions for all antennas such that the reference antenna’s phase is constant at zero. If the selected antenna drops out, another antenna will be selected for ongoing consistency in time (at its “current” value) until the refant returns, usually at a new value (not zero), which will be kept fixed thenceforth. You can also run without a reference antenna, but in this case the solutions will formally float with time; in practice, the first antenna will be approximately constant near zero phase. It is usually prudent to select an antenna in the center of the array that is known to be particularly stable, as any gain jumps or wanders in the refant will be transferred to the other antenna solutions. Also, it is best to choose a reference antenna that never drops out.

Setting a preavg time (only needed in polcal) will let you average data over periods shorter than the solution interval first before solving on longer timescales.

The minimum signal-to-noise ratio allowed for an acceptable solution is specified in the minsnr parameter. Default is minsnr=3. The minblperant parameter sets the minimum number of baselines to other antennas that must be preset for a each antenna to be included in a solution. This enables control of the constraints that a solution will require for each antenna.

More information about CASA may be found at the CASA web page

Copyright © 2010 Associated Universities Inc., Washington, D.C.

This code is available under the terms of the GNU General Public Lincense