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2022
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Scantable Description
Introduction
The Scantable is the ASAP single-dish data container. It was designed based on the data abstraction of the PKSreader API.
ASAP data handling works on objects called scantables. A scantable holds your data, and also provides functions to operate upon it.
The building block of a scantable is an integration, which is a single row of a scantable. Each row contains just one spectrum for each beam, IF and polarisation. For example Parkes OH-multibeam data would normally contain 13 beams, 1 IF and 2 polarisations, Parkes methanol-multibeam data would contain 7 beams, 2 IFs and 2 polarisations while the Mopra 8-GHz MOPS filterbank will produce one beam, many IFs, and 2-4 polarisations.
All of the combinations of Beams/IFs an Polarisations are contained in separate rows. These rows are grouped in cycles (same time stamp).
A collection of cycles for one source is termed a scan (and each scan has a unique numeric identifier, the SCANNO). A scantable is then a collection of one or more scans. If you have scan-averaged your data in time, i.e. you have averaged all cycles within a scan, then each scan would hold just one (averaged) integration.
Schema
The Scantable consist of a main (casacore) Table and several sub-tables which are referenced via ID. The following descriptions use the following syntax
COLUMN/KEYWORD NAME - DATA_TYPE(N)=DEFAULT_VALUE
where DEFAULT_VALUE is optional and (N) indicates a Vector of that type
Main Table
The main table consists of global data (keywords) and row-based data (columns)
Keywords
VERSION - uInt
the version of the scantable for compatibility tests
POLTYPE - String=linear
the polarisation type, i.e. stokes, linear or circular
DIRECTIONREF - String=J2000
the reference frame of the direction (coordinate) [redundant?!]
OBSMODE - String=""
no idea...
UTC - String
most likely the starting time of the observation (file)
AntennaName - String
the name of the antenna
AntennaPosition - Double(3)
the WGS position of the antenna
FluxUnit - String
the unit of the channel values i.e. Jy or K
Observer - String
the name of the observer
Obstype - String
probably telescope specific name/encoding
Project - String
e.g. proposal project number, survey name
Bandwidth - Double
self-explanatory
nIF, nBeam, nPol, nChan - Int
the number of IF, Beam, Polarisation and Channel values in the scantable This is currently FIXED
FreqRefFrame - String
redundant info - also in FREQUENCIES sub-table
FreqRefVal - Double
no idea...
Columns
SPECTRA - Float(nchannel)
the spectral channel data.
FLAGTRA - uChar(nchannel)
the corresponding flags
TSYS - Float(nchannel)
the channel-based system temperature values
FLAGROW - uInt=-1
spectrum based flags
TIME - MEpoch
The mjd time when the observation took place
INTERVAL - Double
the integration time in seconds
DIRECTION - MDirection
the associated direction on the sky
AZIMUTH, ELEVATION - Float
the azimuth/elevation when the spectrum was collected
SRCNAME - string
the name of the source observered
SRCTYPE - Int=-1
the tyep of the source, i.e. indicating if it is an on source scan or off source. This will be used for calibration
SCANNO, BEAMNO, POLNO, IFNO, CYCLENO - uInt
These columns index (0-based) the respective values.
- SCANNO: the number odf the scan. A scan is usually multiple integrations
- (cycles)
- CYCLENO: the integration number within a scan (sub-scan?)
- IFNO: the index of the IF (spectral window)
- BEAMNO: the index of the beam (in a multibeam system)
- POLNO: the index of the polarisation, e.g. XX=0, YY=1, Real(XY)=2,
- Imag(XY)=3
REFBEAMNO - Int=-1
optional index of the reference beam in a multibeam obervation
FREQ_ID, MOLECULE_ID, TCAL_ID, FOCUS_ID, WEATHER_ID, FIT_ID - Int
the reference indeces to the sub-tables
FIELDNAME - string
the name of the field the source belongs to
OPACITY - Float
maybe redundant - not used anywhere
SRCPROPERMOTION, SRCVELOCITY, SRCDIRECTION
see livedata docs - not used in ASAP
SCANRATE
see livedata docs - not used in ASAP
Sub-Tables
FREQUENCY
Keywords
- BASEFRAME - String=TOPO
- FRAME - String=TOPO
- EQUINOX - String=J2000
- UNIT - String=""
- DOPPLER - String=RADIO
Columns
- REFPIX - Double
- REFVAL - Double
- INCREMENT - Double
MOLECULES
Keywords
- UNIT - String=Hz
Columns
- RESTFREQUENCY - Double()
- NAME - String()
- FORMATTEDNAME - String()
FOCUS
Keywords
PARALLACTIFY - Bool=False
indicates whether the observation was parallactified i.e. observed in such a way that the sum of the phases is zero.
Columns
PARANGLE - Float
the parallacticle angle of the observation
ROTATION - Float
feed rotation
AXIS - Float
? ask Mark Calabretta
TAN - Float
? ask Mark Calabretta
HAND - Float
the sign of the polarisation i.e. 1 for XY and '-1' for YX
USERANGLE - Float
A user definable phase correction term
MOUNT - Float
? ask Jim Caswell
XYOFFSET - Float
? ask Jim Caswell
XYPHASEOFFSET - Float
? ask Jim Caswell
.. todo:: add all sub-table descriptions
Mapping to other formats
MS
MeasurementSets are specifically designed for synthesis data. As such one of the main dfferences between the Scantable and a MS is how spectra are stored. An MS stores those in an n_pol x n_channel complex matrix (DATA column) and/or a n_pol x n_channel float matrix (FLOAT_DATA column) whereas the Scantable stores each polarisation seperately. In case of polarimetry data the (complex) cross-polarisation is serialised into real and imaginary parts, e.g. a total of four rows would describe one polarisation.
