WD writes the image contained in buffer 'buf' to the specified file.
Unless you specify otherwise, the image will be written in FITS format to
the default directory for images with the current default file extension.
By default, WD writes the images in FITS format using 16-bit signed
integers (BITPIX=16). You can alternatively specify 32-bit signed integers
(BITPIX=32) by using the FITS=32 keyword, or 32-bit floating point values
(BITPIX=-32) using the FULL keyword. Because VISTA stores all image data
internally as 32-bit floating-point arrays, the FULL option is often the
fastest as it requires no data conversion/scaling computations to convert
the data arrays into the specified integer formats. Also note that
conversion of data to 16-bit integers might incur precision degradation (or
loss of dynamic range), especially for heavily processed data comprised of
a combination of many individual images.
The optional keywords WFPC, DOM, and SDAS may be used to specify that the
file is to be written in proprietary formats used by the WF/PC-1 team (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. General users are
advised to ignore these options and write all data in FITS format, as FITS
data are generally portable and should be understood by all other data
analysis programs.
The SPEC word tells WD to write to the spectrum directory and to
append the default file extension for spectra [NB: This is an archaic usage
that is artifact of an earlier version of VISTA where images and spectra
were treated as separate entities with separate commands, and is no longer
required. You can treat images and spectra identically in the current
version of VISTA. See the notes at the end of the RD command for a further
discussion].
The command PRINT DIRECTORIES shows the current directories and
extensions. If you do not give a filename, the program will ask you for it.
FULL is used to write the pixels in 32-bit floating point format. This
preserves the full precision of the pixels in the buffers. Note that the
WF/PC standard is currently defined to be 16 bit integers, so FULL cannot
be used with WF/PC files. If the data are to be written as integers
(either 16- or 32-bit formats), WD will first compute the maximum and
minimum pixel values, and then scale the pixels so that the maximum and
minimum values will be packed into the full allowed data range for that bit
precision (+/-32767 for 16-bit integers, and +/-2147483647 for 32-bit
integers). The scaling parameters are printed on the terminal, and stored
in the FITS header as the BZERO and BSCALE keywords, which will be used by
subsequent FITS file readers to de-scale the data according to the formula:
The command RD will use these parameters to restore the pixel values when
the image is read from the disk. This is similar to the default option of
the WT command.
NOTE: This scaling convention conforms to FITS standard. In versions 3
and earlier of VISTA, a backwards convention was used such that if for
some reason you encounter old FITS images written with VISTA before about
1985, you could find data scaling problems. If you invert the BSCALE
variable in the header and make BZERO negative, it should come out OK.
In the examples below, assume the default image directory and extension are
/vista/ccd/ and .fits, and the default spectrum directory and extension are
/vista/spectra and .fits.
In earlier versions of VISTA, the 32-bit per pixel format was the default.
The FULL keyword is new to version 3.
See the notes at the end of the RD command for an explanation of the
archaic custom of separating images and spectra.
The scaling parameters can be overridden by the
ZERO=z and SCALE=s keywords, following the formula:
but playing with data scaling is ill-advised unless you are absolutely
certain of what you are doing.