Structure of FITS files

A FITS file contains a sequence of logical header/data units (HDU) which all start with a set of header records describing the following data records. The logical record length of a FITS file is always 2880 bytes of 8 bits. Both header and data sections start in a new logical record. FITS headers are encoded in ASCII as 80 character card images each starting with an 8 character keyword defining the type of information contained on the card. The card images follow each other directly without any end-of-line character which means that many standard text processing tools may have problems. Values of parameters are decoded using standard FORTRAN-77 rules. They describe in detail the data following the header records. Since a single FITS file may have many HDU's each corresponding to a data set (e.g. an image or a table), the translation of it may produce several result frames. After the last HDU in the file additional records may exist.

The basic FITS paper [1] specified both a logical and physical record length of 2880 bytes. The increasing volume of data and higher recording densities made this physical record size inefficient. To increase storage efficiency and make use of new recording media such as optical disks and helical scan devices, the FITS standard was extended to allow physical blocking factors different from one [3]. The allowed range of blocking factors is explicitly defined for a given media. For most tape media, factors between 1 and 10 are allowed giving a maximum physical block length of 28800 bytes. Each file terminates with a tape-mark, and the last file on tape terminates with a double tape-mark i.e. end of information.