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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.
Next: FITS data-types and extensions
Up: FITS Format
Previous: FITS Format
Petra Nass
1999-06-09