cbpcalibrator

The ccalibrator program performs calibration in a parallel/distributed environment or on a single computer system. The software leverages MPI, however can be run on a simple laptop or a large supercomputer.

The cbpcalibrator program is a specialised tool for bandpass calibration. It is closely related to ccalibrator (see ccalibrator) and performs one of its functions, bandpass calibration, in an optimised fashion. The program can be run in a parallel/distributed environment or on a single computer system. It automatically distributes the work between multiple MPI ranks, if used on a supercomputer, with no parameter change required for the user. Unlike ccalibrator, this tool

• solves for bandpass only
• works only with preaveraging calibration approach
• does not support multiple chunks in time (i.e. only one solution is made for the whole dataset)
• does not support data distribution except per beam
• does not support a distributed model (e.g. with individual workers dealing with individual Taylor terms)
• does not require exact match between number of workers and number of channel chunks, data are dealt with serially by each worker with multiple iterations over data, if required.
• solves normal equations at the worker level in the parallel case

This specialised tool matches closely BETA needs and will be used for BETA initially (at least until we converge on the best approach to do bandpass calibration). The lifetime of this tool is uncertain at present. In many instances the code is quick and dirty, just to suit our immediate needs.

Running the program

It can be run with the following command, where “config.in” is a file containing the configuration parameters described in the next section.

$ <MPI wrapper> cbpcalibrator -c config.in

Parallel/Distributed Execution

The program is distributed and used a master/worker pattern to distribute and manage work. Unlike cimager and ccalibrator, this program manages distribution of workload implicitly (c.f. with the section entitled Parallel/Distributed Execution in the cimager, where the user is responsible for explicit workload distribution). If executed in a parallel environment (i.e. two or more MPI ranks are available), the program will automatically distribute beams and channels across all available workers is uniformly as possible (i.e. it pays to have nbeam x nchan / integer_number + 1 MPI ranks available) and the master will be responsible for collating and storing the results. The dataset(s) will be read multiple times. It is possible to give a separate dataset for each beam. In this case, each dataset will be read nchan times.

Configuration Parameters

Parset parameters understood by cbpcalibrator are given in the following table (all parameters must have Cbpcalibrator prefix, i.e. Cbpcalibrator.dataset). Although the substitution rules (e.g. see %w and %n in ccalibrator) are supported, their use is not intended.

The results are stored via calibaccess interface (see Access to calibrator solutions), however only casa table option currently supports bandpass products.

A number of other parameters allowing to narrow down the data selection are understood. They are given in [Data Selection] and should also have the Cbpcalibrator prefix.

Parameter	Type	Default	Description
imagetype	string	“casa”	Type of the image handler (determines the format of the images read from the disk). The default is to read casa format and this is the only option implemented so far.
nAnt	uint	36	Number of antennas to solve bandpasses for. The code will fail if it is requested to solve for more antennas than it has the data for
nBeam	uint	1	Number of beams to solve for. The code will fail if it is requested to solve for more beams than it has the data for
dataset	string or vector<string>	None	Data set file name to produce. If the parameter is given as a vector of strings, it is interpreted as one dataset per beam. Therefore, the length of the string should be either 1 or the number of beams.
datacolumn	string	“DATA”	The name of the data column in the measurement set which will be the source of visibilities.This can be useful to process real telescope data which were passed through casapy at some stage (e.g. to run on calibrated data which are stored in the CORRECTED_DATA column). In the measurement set convention, the DATA column which is used by default contains raw uncalibrated data as received directly from the telescope. Calibration tasks in casapy make a copy when calibration is applied creating a new data column.
nUVWMachines	int32	1	Size of uvw-machines cache. uvw-machines are used to convert uvw from a given phase centre to a common tangent point. To reduce the cost to set the machine up (calculation of the transformation matrix), a number of these machines is cached. The key to the cache is a pair of two directions: the current phase centre and the tangent centre. If the required pair is within the tolerances of that used to setup one of the machines in the cache, this machine is reused. If none of the cache items matches the least accessed one is replaced by the new machine which is set up with the new pair of directions. The code would work faster if this parameter is set to the number of phase centres encountered during imaging. In non-faceting case, the optimal setting would be the number of synthetic beams times the number of fields. For faceting (btw, the performance gain is quite significant in this case), it should be further multiplied by the number of facets. Direction tolerances are given as a separate parameter.
uvwMachineDirTolerance	quantity string	“1e-6rad”	Direction tolerance for the management of the uvw-machine cache (see nUVWMachines for details). The value should be an angular quantity. The default value corresponds roughly to 0.2 arcsec and seems sufficient for all practical applications within the scope of ASKAPsoft.
refantenna	int32	-1	If not negative, this is assumed to be the index of the reference antenna. All phases in the resulting bandpass are rotated so the chosen antenna has zero phase for all beams and all channels
sources.definition	string	None	Optional parameter. If defined, the sky model (i.e. source info given with sources.something parameters) is read from a separate parset file (name is given by this parameter). If this parameter is not defined, source description should be given in the main parset file. Usual substitution rules apply. The parameters to define sky model are described in csimulator (with Cbpcalibrator prefix instead of Csimulator)
gridder	string	None	Name of the gridder, further parameters are given by gridder.something. See Gridders for details.
rankstoringcf	int	1	In the parallel mode, only this rank will attempt to export convolution functions if this operation is requested (see tablename option in the Gridders). This option is ignored in the serial mode.
visweights	string	“”	If this parameter is set to “MFS” gridders are setup to degrid with the weight required for the models given as Taylor series (i.e. multi-frequency synthesis models). At the moment, this parameter is decoupled from the setup of the model parameters. The user has to set it separately and in a consistent way with the model setup (the nterms parameter in the model definition (see csimulator for more details) should be set to something greater than 1 and there should be an appropriate number of models defined).
visweights.MFS.reffreq	double	1.405e9	Reference frequency in Hz for MFS-model simulation (see above)
ncycles	int32	1	Number of solving iterations (and iterations over the dataset, which can be called major cycles, although we don’t do any minor cycles for calibration)
freqframe	string	topo	Frequency frame to work in (the frame is converted when the dataset is read). Either lsrk or topo is supported.
calibaccess	string	“parset”	The destination for the calibration solution. Note, the default parset option does not work for this application. Therefore, table option must be used and calibaccess.table.<params> parameters should be defined. For more details see Access to calibrator solutions.

The resulting parameters are stored into a solution source (or sink to be exact) as described in Access to calibrator solutions

Example

Cbpcalibrator.dataset                   = calibration_data.ms
Cbpcalibrator.nAnt                      = 6
Cbpcalibrator.nChan                     = 304
Cbpcalibrator.nBeam                     = 9
Cbpcalibrator.refantenna                = 1
Cbpcalibrator.calibaccess               = table
Cbpcalibrator.calibaccess.table.maxbeam = 9
Cbpcalibrator.calibaccess.table.maxant  = 6
Cbpcalibrator.calibaccess.table.maxchan = 304

Cbpcalibrator.sources.names             =       [src1]
Cbpcalibrator.sources.src1.components   = [cal]
Cbpcalibrator.sources.cal.calibrator    = 1934-638

Cbpcalibrator.gridder                   = SphFunc
Cbpcalibrator.ncycles                   = 5