12.2.2 Core Data Items

The CDIs are defined to be the minimum set of parameters needed for image processing and scientific analysis. They include both input CDIs, which are verified before processing and used by NEWSIPS to determine the type of processing to be performed, and output CDIs, which are generated by NEWSIPS and verified during quality control after pipeline processing. The CDIs appear in the FITS header of each file, as well as in the IUE Final Observing Log. Each CDI is assigned a unique FITS keyword, although some CDIs may have multiple values and, therefore, require more than one FITS keyword. For example, low-dispersion double-aperture image files will contain the FITS keywords LEXPTIME and SEXPTIME to store the large- and small-aperture effective exposure times. For those cases in which the CDI is either unknown or undefined for a particular image, the related keyword will not be included in the header. This follows the standard convention for optional FITS keywords. A complete description of the CDIs is included in Chapter 14.

The portion of the header containing the CDIs may be divided into three sections:

• Common set (includes aperture-independent parameters),
• Large-aperture set,
• Small-aperture set.

Each set will be preceded by three COMMENT lines as indicated in the header examples given at the end of this section. Low-dispersion files corresponding to a single aperture exposure and all high-dispersion files will contain only the corresponding (large or small) CDI set.