RD: Read an Image from a Disk File

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.

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:

A Note About Images, Spectra, and Directories:

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 ./, the current working directory, and ignore the spectrum directory altogether. The typical exceptions are when using VISTA for quick-look data analysis at the telescope, when many users find it convenient to define the default image directory to be the incoming raw data file system.