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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.

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