Investigation of the latest FITS standard and
the use of the CFITSIO library to upgrade FITS in GIPSY
M.G.R. Vogelaar & J.P. Terlouw
Kapteyn Institute
Groningen
The Netherlands
vogelaar@astro.rug.nl
terlouw@astro.rug.nl
Sep 1998
Short FITS primer (from the CFITSIO documentation)
A FITS file consists of one or more Header + Data Units (HDUs), where the
first HDU is called the `Primary HDU', or `Primary Array'. The primary
array contains an
N-dimensional array of pixels, such as a 1-D spectrum, a 2-D image, or
a 3-D data cube.
Five different primary datatypes are supported: Unsigned 8-bit bytes,
16 and 32-bit signed
integers, and 32 and 64-bit floating point reals. FITS also has a
convention for storing 16 and
32-bit unsigned integers (see the later section entitled `Unsigned Integers'
for more details).
The primary HDU may also consist of only a header with a null array
containing no data pixels.
Any number of additional HDUs may follow the primary array; these additional
HDUs are called FITS `extensions'. There are currently 3 types of extensions
defined by the FITS standard:
- Image Extension - a N-dimensional array of pixels, like in a primary array
- ASCII Table Extension - rows and columns of data in ASCII character format
- Binary Table Extension - rows and columns of data in binary representation
Currently GIPSY recognizes only image extensions. CFITSIO supports also
extended FITS file names like file://, ftp:// and http:// and supports
compressed files. None of these options is available in GIPSY.
Testing the library
Local experiments with the CFITSIO library require a special vesion
of the 'giplib' library
which can be found in $gip_lib/TEST (see README in that directory)
itself. A template GIPSY program is made by GIPSY program CODER.
'Compile' must be aware of the location of the special library;
this is done with setenv gip_lib $gip_lib/TEST
Experiments show that the library functions are easy to use and work as
described.
Changing compile to include the library
For a permanent solution, we probaly should change 'Compile' to allow
compilation of an external library. Kor is asked to think about a solution.
First we try to compile the CFITSIO library for different platforms and
run a test program to look for unexpected behavior. If no problems occur,
we should start writing an updated FITS reader/writer based on CFITSIO
as soon as possible.
Documentation
Documentation of the CFITSIO library need not to be converted to the GIPSY
standard dc2 documentation. At most two applications require the library.
There is a HTML version of the documentation which we can copy into our
own GIPSY documentation tree. A PostScript version of the CFITSIO
documentation is also available.
The RWFITS program
A new GIPSY program to read and write FITS files should include all
functionality found in programs RFITS and WFITS. Special care should be
given to coordinate systems. The new program can be used in two modes.
The first mode uses a GUI in which the proposed translation of
FITS keywords to GDS keywords can be manipulated. The second mode
operates without a gui. It transforms the FITS file to a GDS file
without any (graphical) user interaction.