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Three non-standard data types have been suggested for accommodation within the standard MS definition, namely: (i) lag data; (ii) triple-product data, and (iii) FLOAT_DATA with an arbitrary axis other than frequency.
The need to store lag data is motivated by the output format of XF correlators and the requirements for data loading and inspection in this case. Storage of triple-product data is required for optical interferometry processing but is also useful for the implementation of incoherent VLBI fringe-fitting algorithms. The requirement for non-standard FLOAT_DATA axes is concerned with more flexible single-dish data formats.
Lag data can be incorporated by allowing an optional MAIN column:
LAG_DATA Complex(NUM_CORR,NUM_LAG) (Lag function) (Opt.)
A LAG_ID index (via DATA_DESC_ID) points to a LAG table containing the details of the lag spectrum, including range. Lag data are not expected to be calibrated directly and would be supported primarily as an interim format in an anticipated data reformatting sequence. The routine storing of both lag and transformed spectral data in one MS would often lead to prohibitive disk space requirements, and is not anticipated here. Further information regarding the joint storing of lag functions and frequency spectra in the same MS can be found in the MAIN table description in Section 5.
Triple-product data can be incorporated in the MS definition by adding additional antenna labels such as:
ANTENNA3 Int (Third antenna) (Opt.)
FEED3 Int (Feed on ANTENNA3) (Opt.)
The UVW column is similarly affected. The triple-product data would be expected to be formed for matching time intervals, so other columns such as SOURCE_ID would be held to be common. The triple-product data would be stored in the DATA column and the presence of a valid ANTENNA3 column would label the data to be of this type. It is stressed that triple-product and lag data will not have nearly equal standing in the calibration system, as compared to conventional frequency spectra.
Support for an arbitrary axis in the FLOAT_DATA column, by generalizing the SPECTRAL_WINDOW_ID index, is more difficult to accommodate, and is best approached in the same manner as the lag data, by enumerating special cases with separate indices such as LAG_ID. The lag and triple-product data formats are still fundamentally visibility data and may also share at least some of the broader synthesis infrastructure. Their impact on the sub-table formats is also limited. This is not true for arbitrary data types in the MS such as may result if the axis type in the FLOAT_DATA column was allowed to vary arbitrarily.
Accommodating non-standard columns (such as LAG_DATA) in this way is contingent on the requirement that they do not permit the representation of identical data in multiple equivalent formats. This would represent a significant application overhead and would affect the global structure of the MS as a whole. Most importantly, this would limit joint processing of data from separate instruments (eg. single-dish and interferometry data).