VISTA (starting with v4.2) has adopted the FITS format as the default image
format for input and output. FITS images are organized (in their simplest
form) into header and data records. The FITS format has the great
advantage that is it defined independently of your machine type and is a
nearly universally accepted and well-documented standard for data
interchange in the astronomical community. Consequently, it is possible to
exchange images between different machines and/or operating systems without
having to use special translation software. For more information on the
FITS format, see the FITS Support Office Home Page at the NASA Goddard
Space Flight Center
(http://fits.gsfc.nasa.gov/).
If the specified image cannot be found, the program will attempt to find
a ``gzip''ed version of the file by appending a .gz to the file name. If
RD finds a file name with a .gz extension, it will copy the file to the
/tmp directory, uncompress it, read it, and then delete the temporary
uncompressed verions.
The optional keywords WFPC, DOM, and SDAS may be used to specify that the
file to be read is a WF/PC team format file (in DEC byte order). If WFPC is
specified, the names are taken to be filename.IMG and filename.HDR; for
SDAS, the filenames are filename.HHD and filename.HHH; for DOM, the file is
specified by a number, W00000n.IMG and W00000n.HDR. If any of these
extensions are explicitly mentioned in the filename, the WFPC switch is
automatically turned on.
If the directory is not explicitly given in the filename, the program tries
to read from the image directory (or from the spectrum directory if the
word SPEC is given, although this usage is now considered archaic, see the
notes below). If a filename extension is not given in the filename, the
program puts on a default extension from the list of extensions. You can
show the default directories and extensions with the command PRINT
DIRECTORIES.
In the examples below, assume that the default directories
/vista/ccd/ for images and /vista/spectra/ for spectra:
<#1769#>A Note About Images, Spectra, and Directories:<#1769#>
We wish to point out that in general there is no requirement within VISTA
to treat images and spectra separately, as both are treated equally as 2-D
or 1-D FITS format images, respectively. The old segregation was an
artifact of earlier versions of VISTA in which 2-D images and 1-D spectra
had completely separate data formats (images were .ccd files, spectra .spc
files), and separate commands associated with them (e.g., WD for images, WS
for spectra, separate image and spectral arithmetic routines, etc.). With
Version 3, this illogical separation was eliminated, but the old SPEC
syntax was retained for backward compatibility. It often confuses new
users, and in general we suggest that new users ignore the SPEC keyword and
V_SPECDIR environment variables altogether. Old users with bad habits are
on their own.
The use of special image and/or spectrum directories is of course an
artifact of the old days when computer systems had large scratch disks
(``large'' at that time meant 300Mb units the size of a large-capacity
washing machine) and very small user disk areas, and hence it was necessary
to farm out large images to centralized data disks assigned by the system
administrator. In this day and age of multi-gigabyte disks that fit in the
palm of your hand, this no longer makes any sense, but it has been retained
to ensure some measure of backwards compatibility (note that IRAF carries
the convention out as well). In general, we find that many VISTA users
define their default image directory to be
This command reads images from the disk into a buffer. Images with only one
dimension (i.e. spectra) are converted to two-dimensional images when read
from the disk.