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 astWrite) 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
" header cards
" 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
’ s integer Card attribute, which identifies a
" card, to
which subsequent operations apply. Searches based on keyword may be performed
(using astFindFits), new cards may be inserted (astPutFits, astPutCards,
and existing ones may be deleted (astDelFits) 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 astPutFits, astPutCards, astWrite 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 astFindFits or
Coordinate system information may be described using FITS header cards using
several different conventions, termed
" . 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 FITS
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 astRead and astWrite, when used with a FitsChan, depends on the encoding in use. In general, however, all use of astRead 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 astRetainFits function).
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 astWrite 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.
" is NULL, the FitsChan will remain empty until cards are explicitly stored
in it (e.g. using astPutCards, astPutFits or via the SourceFile attribute).
" is NULL, and no value has been set for the SinkFile attribute, the contents
of the FitsChan will be lost when it is deleted.
"format specifiers identified by
"symbols in the normal way.
"format specifiers, then an optional list of additional arguments may follow it in order to supply values to be substituted for these specifiers. The rules for supplying these are identical to those for the astSet function (and for the C
Note, the FITSCHAN_OPTIONS environment variable may be used to specify default options for all newly created FitsChans.
" card will be written via the sink function. 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.