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Select observations: SELECT (SE)

The syntax of the SELECT command (see also section ) is similar to a logical expression in Fortran:

SEL[/qualifiers] keyword.ro.value[.operator.keyword.ro.value[......]]

where .ro. stands for a relational operator, and .operator. stands for a logical operator. Table shows the allowed operators and their order of evaluation in an expression. The relational operators are used in the keyword construction; wildcards, value-sets and value-ranges can often be used; see section for details. Preceding the keyword by NO negates the relational operator.

Logical operators combine keyword constructions into logical expressions; the order of precedence may be altered by appropriate use of parentheses.

The versatility of the ARCQUERY command interpreter often makes it possible to formulate the same selection in different ways. For example, the selection:

SEL (OBJE <> (3C*,M*) & DATE >= 3MAR86) | OBJE = M81

is equivalent to:

SEL (OBJE.NE.3C*.AND.OBJE.NE.M*.AND.DATE.GE.3MAR86).OR.OBJE.EQ.M81

is equivalent to:

SEL NOOBJE= (3C*,M*), NODATE < 3MAR86 .OR.OBJE=M81

Blanks are allowed at any place, but are very seldom mandatory. The user is referred to tables and for the qualifiers applicable to the SELECT command.

Keywords

All entries of the Observations Catalogue can be used as keywords for selection. Some `derived' keywords are allowed as well; these are quantities calculated from the actual catalogue information, but which are themselves not stored in the data base. The available catalogue keywords are listed in Table . Values of a number of keywords have been coded in the Observations Catalogue (see footnote 4) for the sake of compactness; these keys can be specified either with the code number or with the decoded (more meaningful) value; however, wildcards are then not allowed. The object-type code (OBJ_TYPE) and the seeing and sky-quality codes are exceptions to this rule, and can only be addressed with the code number.

Note that information about the original data tapes from which the observations were loaded does not figure in the observations catalogue, but is stored separately. Apart from the Observations Catalogue keywords, two more keys: FITS_TAPE and FITS_FILE can be used to select data. A search for a particular tape, however, is best done in combination with a DATE window, because testing of all catalogue entries for the tapename is extremely time consuming, while the usage of a date range ensures the application of the date index, which works very much quicker.

Indexes

The query software maintains indexes of two items: POSITION and DATE. Searches on those keywords are indexed searched, and therefore very fast. The position index is obtained by deviding the sky into a large number of individual cells (6450) of approximately equal size (7 square degrees), and indexing the cell numbers (table ). Observations in cells found from the index are tested against the original search specification(s). The CELL number is a catalogue keyword, on which data can be selected. The query software decides when to use an index, based on the structure of the select string. Sometimes, if a very complex date or position specification is made, the index will not be used, and the whole catalogue is tested instead.

On entry ARCQUERY lists the items for which indexes are maintained; it is possible to have other catalogue items, apart from position and date, indexed too at some time in the future.