MeasurementSets

Visibility data in AIPS++ are stored in an AIPS++ table known as a MeasurementSet (MS). An MS consists of a main table containing the visibility data and associated sub-tables containing auxiliary or secondary information. The full MS data format is described in AIPS++ Note 229. Tables 1 & 2 identify the commonly accessed components of the MAIN table and list data selection parameters used during typical data reduction sessions.

All AIPS++ data files, including MeasurementSets, are written into the current working directory by default, with each AIPS++ Table represented as a separate sub-directory. MS names therefore need only comply with UNIX file or directory naming conventions, and can be referred to from within AIPS++ directly, or via full pathnames.

Each row of the DATA column in the MS contains a matrix of observed complex visibilities at a single time stamp, for a single baseline in a single spectral window. The shape of the data matrix is given by the number of channels and the number of correlations (voltage-products) formed by the backend instrument of the telescope (e.g. a correlator for an array, or a spectrometer for a single dish).

**Table:** Commonly accessed MAIN Table components (Data, Flags, & Weights *only*)
Name	Format	Comments
DATA	Complex(N_c, N_f)	Complex visibility matrix
		Observed DATA for synthesis arrays
WEIGHT	Float(N_c)	Weight for whole data matrix
FLAG	Bool(N_c, N_f^*)	Cumulative data flags
CORRECTED_DATA	Complex(N_c, N_f)	Corrected data created by calibrater or imager tools
MODEL_DATA	Complex(N_c, N_f)	Model data created by calibrater or imager tools
IMAGING_WEIGHT	Float(N_c)	Imaging weight, e.g. uniform, natural, robust
		Created by calibrater or imager tools

$\dagger$ Additional data attributes are described in AIPS++ Note 229
$\dagger$ $\dagger$ N_c= number of correlators, N_f= number of frequency channels.
$\dagger$ $\dagger$ $\dagger$ The MODEL_DATA column is created with each value having AMP = 1 & phase = 0^o.

**Table:** Common Data Selection Parameters $\dagger$
Parameter	Contents
ANTENNA1	First antenna in baseline
ANTENNA2	Second antenna in baseline
FIELD_ID	Field (source no.) identification
SPECTRAL_WINDOW_ID	Spectral window number (IF no.)
ARRAY_ID	Subarray number
OBSERVATION_ID	Observation identification
POLARIZATION_ID	Polarization identification
SCAN_NUMBER	Scan number
TIME	Integration midpoint time
UVW	UVW coordinates

$\dagger$ Selection of data from an MS currently appears in different guises in the various synthesis tools due to independent development of these tools. An effort to reconcile these differences is underway.