A number of basic ANSI C data structures have been defined to correspond to the FITS HDUs mentioned above. The most fundamental structure consists of a two-dimensional array of an arbitrary size and various types (short, float, etc.), together with an array of header lines to represent collections of keywords associated with the data. This basic structure is then specialized to represent specific types of header plus two-dimensional array, viz., Science, Error, Data Quality, Samples, and Integration Times HDUs.
In addition, the basic structures above are grouped into single and multiple groups of HDUs. A single STIS group consists of a global header together with a Science, Error, and Data Quality HDU. A single NICMOS group consists of a global header together with a Science, Error, Data Quality, Samples and Integration Times HDU. A multiple STIS group consists of a global header together with N groups of Science, Error, and Data Quality HDUs. Likewise, a multiple NICMOS group consists of a global header together with N groups of Science, Error, Data Quality, Samples and Integration Times HDUs.