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In AIPS++, all forms of data (i.e. visibility, images, spectra, history, header information, logs) are stored in Tables. Tables consist of information arranged in rows and columns. The columns have names (e.g. UVW, TIME, ANTENNA1, SIGMA) and the rows are numbered. Keywords may be attached to both the Table as a whole or to specific columns and are used to specify additional information. Examples of keywords are revision number of a Table, author of the Table, units in a column, coordinate information for a column and so on. Keywords may also be used to store other Tables.
A Table and its contents are stored on disk as a directory with the Table name (e.g. 3C273XC1.ms) and may contain a number of files. One technical aspect of Tables that you should be aware of is that some Tables reference other Tables. We try to avoid making copies of Tables so that if two Tables contain almost the same data except for one or two columns, then a reference Table is created that points back to the original Table for shared information. This mechanism is also used for sorts and selections. If you sort or select a Table to create another Table then the new Table points back to the original Table via a column of row numbers. The only reason you need to be aware of this is to avoid deleting the original Table.
Although there can be many different types of Tables in AIPS++, representing different types of data, there are two important types of Tables which you will commonly use: MeasurementSet (MS) (which represents data from a telescope, e.g. visibility data) and Image (which represents the distributions of intensity on the sky). An Image may have two or more dimensions, e.g. position, velocity, stokes.
A MeasurementSet(ms) is simply a Table obeying certain conventions that allow it to represent observations from many different types of radio telescopes. In addition to a number of data columns, it also contains sub-Tables stored as Table keywords (as discussed above). These keywords contain information on the antennas, feeds, fields, and spectral windows. An MS can be dynamic in that new columns can be written into it as data are processed. For example, the original measured UV data appears in the data column and subsequent processing can produce additional columns containing corrected data (for e.g. after a Selfcal) or Model data (a set of clean or other components)
An Image is also a Table. The keywords in this Table contain information on units and coordinates. The pixels are stored as a single cell in a column. Other information about the image (e.g. its co-ordinates, origin, history, characteristics) are stored under separate keywords.