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A number of ARQUERY commands enable an archive user to search and select from the Observations Catalogue, and to combine different selection tables. The idea is that by intelligently using these functions of the query software, a final selection is composed of catalogue entries of interest, which then may be submitted to the archive manager as a request for the release of the data.
Although it is possible to select the observations by using only one, possibly quite complicated, SELECT command (see section ) a step-by-step procedure is probably more convenient for most users. For instance, as a first step a coarse search could be made of the catalogue, followed by subsequent narrowing down of the selection. Alternatively, one might build-up a final selection by combining a number of small individual selections at particular sky positions or dates, which can be done very efficiently because of the use of catalogue indexes (section ).
Each step of such a selection process will create a new output table from the input table(s). If a search is made on the Observations Catalogue, input table `0', and subsequent `query' tables will be named `1', `2', `3', etc. , unless the user specifies a different name with the /OUTPUT_TABLE qualifier (see section for table nomenclature and section for the default table name conventions). If the /INPUT_TABLE qualifier is not used, selections are made from the default input table (`0', or the latest output table), which is mentioned with every ARCQUERY prompt. Non-default table names must be used when, at any point in the procedure, repeatedly selections are made from the same (input) table by different criteria. Such selections may be combined by using the UNION, INTERSECTION, and DIFFERENCE commands (see section ).
The EDIT command (section ) can be used to select in edit mode from any of the tables created. The CALSELECT command (section ) serves to find, for a given selection of observations, the accompanying calibration exposures in the catalogue; this function makes use of the astronomical codes defined at the Data Acquisition stage or by the archive manager; see section for details.
Once the user is satisfied with the selection a REQUEST (section ) can be made to retrieve the data from the archive and to send them to the user on a FITS tape.