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Introduction

A FITS file, in general, is a sequence of header-data-units. The file as a whole and the structure of each header-data-unit is subject to strict formatting rules. The header portion of a header-data-unit consists of ASCII text that describes the structure and content of the data portion, which immediately follows. This header portion consists of an arbitrary number of eighty byte sequences of keyword name, data value and comment in the form ``keyword = value / comment''. Keywords are divided into standard and user-defined; standard keywords are also either mandatory or optional. Mandatory keywords identify the type of header-data-unit, the data type, and the data structure and size. Optional keywords include scaling factors, units, and keywords to aid in the physical interpretation of the data. Supported data structures include multi-dimensional arrays, random groups, ASCII tables, binary tables, and the image extension.

This document gives an overview of a set of C++ classes designed to aid in the construction of programs to read and write FITS files. They also aim to strictly adhere to the FITS standards. On input, errors or failures to adhere to FITS standards are flagged but reasonable attempts are made to correct these deficiencies and continue processing. On output, every attempt to enforce FITS standards is made; the classes will not knowingly write a file that does not adhere to FITS standards.

This set of classes assumes a sequential processing model in processing the FITS file, whether on input or output. This is consistent with the FITS standards, which define a linear sequence of header-data-units followed by an optional sequence of ``special'' records. In other words, no assumptions are made which require the processed file to be on random access media. The header-data-units are processed in the linear order in which they occur within the file. The primary purpose of this set of classes is to allow the user to write programs that import data into or export data from some internal format.