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 :doc:`cimager`) and **Ccalibrator** (see :doc:`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|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|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|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|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. | +----------------------+--------------+--------------+------------------------------------------------------------+