Data Selection

Data accessor layer provides a capability to pre-select the data based on some criteria. As all synthesis applications see only data returned by the accessor, it can be used to restrict Cimager (see cimager) and Ccalibrator (see ccalibrator) to use a given subset of the data. All synthesis applications understand the keywords given in the following table (must have Ccalibrator. or Cimager. prefix), i.e. Cimager.MinUV = 100). All these parameters are optional. If present, a corresponding data selection is done and only those data will appear in the iteration using accessor layer.

Parameter	Type	Default	Description
Feed	uint32	None	Select a subset of the data corresponding to a particular feed (should be called synthetic beam in the ASKAP terminology). If defined, only data corresponding to the given beam will be read from the dataset and processed by either Cimager or Ccalibrator. The value is 0-based number of the beam.
Beam	uint32	None	Synonym of Feed. Should probably be used instead to avoid confusion. Note, only one of the two recognised keywords is allowed to be present in the same parset.
Antenna	uint32	None	Select all baselines to the given antenna, irrespective of the order.
Baseline	vector<uint32>	None	Select a subset of the data corresponding to a particular baseline. If defined, only data from the given baseline will be read from the dataset and processed by either Cimager or Ccalibrator. The vector should have exactly two elements, which are treated as zero-based antenna indices.
Channels	vector<uint32>	None	Select a subset of spectral channels. If defined, only a slice of the visibility cube is passed to either Cimager or Ccalibrator. Currently should always be a 2-element vector with the number of channels requested and the first channel (0-based) of the slice. The optional 3rd element of the vector is intended for on-the-fly averaging in spectral channels (it is a number of adjacent spectral channels to average). It is not yet implemented by the accessor, although understood at the parset level.
SpectralWindow	uint32	None	Select data from a given spectral window only. If defined, only data corresponding to this spectral window will be read. The value is 0-based index of the spectral window
Polarisations	string	None	Intended for accessor-based selection of polarisation. Not yet implemented.
TimeRange	vector<double>	None	Select a subset of data based on time. If defined, only data within the given timerange are read. Time is defined in UTC seconds since MJD 0.
Tiles	vector<uint32>	None	Select a subset of data based on tile number. This is only allowed for data stored in tables using tiling. Either specify a 2 elements vector giving the number of tiles and the starting tile (“%w” type substutions are performed) or the single string “auto”, which will distribute tiles over ranks automatically. The casacore command showtableinfo can be used to check the tiling of a dataset.
CorrelationType	string	None	Select certain type of correlations. Allowed values are: auto, cross and all. Defining this parameter allows to select, e.g. just cross-correlations.
ScanNumber	uint32	None	Select a single scan number. If defined, only data corresponding to the given scan number will be read from the dataset. The value is 0-based.
MinUV	double	None	Select data based on the uv-distance (in metres). Only visibility points corresponding to uv-distance greater than the given value (defined in metres) are read from the measurement set. Note, the selection is done without taking frequency information into account.
MinNonZeroUV	double	None	Select data based on the uv-distance (in metres). Only visibility points corresponding to uv-distance grater than the given value (defined in metres) or zero vectors are read from the measurement set. Special treatment of samples with exactly zero u,v and w is the difference from the MinUV option (i.e. selection criterion is only applied if uvw vector has non-zero length).
MaxUV	double	None	Select data based on the uv-distance (in metres). Only visibility points corresponding to uv-distance smaller than the given value (defined in metres) are read from the measurement set. Note, the selection is done without taking frequency information into account.