A FitsChan is a specialised form of Channel which supports I/O operations involving the use of FITS (Flexible Image Transport System) header cards. Writing an Object to a FitsChan (using AST_WRITE) will, if the Object is suitable, generate a description of that Object composed of FITS header cards, and reading from a FitsChan will create a new Object from its FITS header card description.
While a FitsChan is active, it represents a buffer which may contain zero or more 80-character
" conforming to FITS conventions. Any sequence of FITS-conforming header cards may be
stored, apart from the
" card whose existence is merely implied. The cards may be accessed in
any order by using the FitsChan
’ s integer Card attribute, which identifies a
to which subsequent operations apply. Searches based on keyword may be performed
(using AST_FINDFITS), new cards may be inserted (AST_PUTFITS, AST_PUTCARDS,
and existing ones may be deleted (AST_DELFITS) or changed
When you create a FitsChan, you have the option of specifying
" functions which
connect it to external data stores by reading and writing FITS header cards. If you provide a
source function, it is used to fill the FitsChan with header cards when it is accessed for the
first time. If you do not provide a source function, the FitsChan remains empty until you
explicitly enter data into it (e.g. using AST_PUTFITS, AST_PUTCARDS, AST_WRITE or by
using the SourceFile attribute to specifying a text file from which headers should be read).
When the FitsChan is deleted, any remaining header cards in the FitsChan can be saved in
either of two ways: 1) by specifying a value for the SinkFile attribute (the name of a text file
to which header cards should be written), or 2) by providing a sink function (used to to
deliver header cards to an external data store). If you do not provide a sink function or
a value for SinkFile, any header cards remaining when the FitsChan is deleted will be
lost, so you should arrange to extract them first if necessary (e.g. using AST_FINDFITS or
Coordinate system information may be described using FITS header cards using several different
" . When an AST Object is written to (or read from) a FitsChan, the
value of the FitsChan
’ s Encoding attribute determines how the Object is converted to (or from) a
description involving FITS header cards. In general, different encodings will result in different sets of
header cards to describe the same Object. Examples of encodings include the DSS encoding (based on
conventions used by the STScI Digitised Sky Survey data), the FITS-WCS encoding (based on a
proposed FITS standard) and the NATIVE encoding (a near loss-less way of storing AST Objects in
The available encodings differ in the range of Objects they can represent, in the number of Object descriptions that can coexist in the same FitsChan, and in their accessibility to other (external) astronomy applications (see the Encoding attribute for details). Encodings are not necessarily mutually exclusive and it may sometimes be possible to describe the same Object in several ways within a particular set of FITS header cards by using several different encodings.
The detailed behaviour of AST_READ and AST_WRITE, when used with a FitsChan, depends on the encoding in use. In general, however, all use of AST_READ is destructive, so that FITS header cards are consumed in the process of reading an Object, and are removed from the FitsChan (this deletion can be prevented for specific cards by calling the AST_RETAINFITS routine).
If the encoding in use allows only a single Object description to be stored in a FitsChan (e.g. the DSS, FITS-WCS and FITS-IRAF encodings), then write operations using AST_WRITE will over-write any existing Object description using that encoding. Otherwise (e.g. the NATIVE encoding), multiple Object descriptions are written sequentially and may later be read back in the same sequence.
If the null routine AST_NULL is supplied as the SOURCE value, the FitsChan will remain empty until cards are explicitly stored in it (e.g. using AST_PUTCARDS, AST_PUTFITS or via the SourceFile attribute).
If the null routine AST_NULL is supplied as the SINK value, and no value has been set for the SinkFile attribute, the contents of the FitsChan will be lost when it is deleted.
Note, the FITSCHAN_OPTIONS environment variable may be used to specify default options for all newly created FitsChans.
The names of the routines supplied for the SOURCE and SINK arguments should appear in EXTERNAL statements in the Fortran routine which invokes AST_FITSCHAN. However, this is not generally necessary for the null routine AST_NULL (so long as the AST_PAR include file has been used).
" card will be written via the sink routine. You should add this card yourself after the
FitsChan has been deleted.
A null Object pointer (AST__NULL) will be returned if this function is invoked with the AST error status set, or if it should fail for any reason.
Note that the null routine AST_NULL (one underscore) is different to AST__NULL (two underscores), which is the null Object pointer.