12.2 FITS Header Format

The main source of information regarding the format and nature of IUE data contained in each FITS file is stored in the primary header. Each primary header includes the following sections:

• Basic FITS keywords,
• Core Data Items (CDIs),
• Original IUE VICAR label,
• NEWSIPS Image Processing History.

All of these items are contained solely in the primary header of each Final Archive file; the extension headers do not duplicate this information and contain only the basic FITS keywords needed to read the data stored in that extension (with the exception of the FILENAME keyword described below).

It should be noted that the structure of the FITS header is such that some information may appear in more than one form. For example, specific information may appear in multiple places in the original IUE label as well as in a CDI FITS keyword and/or the processing history. In the instances where these entries disagree, the CDIs should always be considered the most reliable source. The contents of each of these sections is described below. Examples of complete low- and high-dispersion primary headers are also given at the conclusion of this section.