The basic FITS keywords define the structure and content of the files. These basic keywords include both the required FITS keywords and, when appropriate, certain optional reserved FITS keywords. Each line of the FITS header has the syntax keyname = value / comments, where keyname is the name of a FITS keyword conforming to the FITS keyword rules. The basic FITS keywords are itemized for each file type below. Although not shown, each FITS header must end with the required END keyword.
A project-defined keyword that needs to be mentioned is FILENAME. This keyword describes the camera image number and the type of data contained in the particular FITS header-and-data unit (HDU) and appears in every HDU containing data. For example, FILENAME would equal SWP09876.LILO in the LILO primary header and SWP09876.LFLO in the LILO extension header. In the corresponding MXLO, the FILENAME keyword appears in the binary table extension header with the value SWP09876.MXLO but does not appear in the primary header since the MXLO does not contain any primary array data.
One purpose of the FILENAME keyword is to provide users with a naming convention when separating FITS HDUs into separate disk files (e.g., when reading FITS files from tape). Since the primary header contains most or all of the information describing IUE images, it might be preferable to keep the files intact. In any event, the FILENAME keyword is useful for verifying the contents of the various data sets.
The value of the FILENAME keyword is formed by the concatenation of the following codes: