5.6.1.1 Keywords and column names

  In keywords and table column names, HEASARC recommends use of the underscore (hexadecimal 5F) rather than the hyphen (hexadecimal 2D). The only cases where a hyphen is to be used in keywords are

• where use of a hyphen is established usage, as for example in the DATE-OBS keyword.
• to represent a minus (negation) sign

This set of recommendations is similar to those in the original paper for ASCII table fields, as discussed in section 3.4.2, but extends them to keywords and binary table fields. Extending the rules for keywords to column names makes it possible to replace a table column heading with a keyword with the same name if the value for that column in every row of the table is the same, as in the Green Bank convention (section 5.2.3.2).

HEASARC/OGIP requires the following additional conventions for column names in all tables it produces and recommends them for others:

• Different columns in the same table cannot have the same name and all columns must be unique in the first 16 characters.
• Embedded blanks are specifically forbidden, as they are for keywords.
• Values of the TTYPEn keyword are case-insensitive, as specified for ASCII tables in the NOST standard described in section 6.1.2.
• Column names must begin with an alphabetic character (A-Z, a-z), not a digit or underscore.
• Column names may be up to 68 characters in length.