Array File Specifications

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SAOtng can load array files containing "raw" images. In order to do so, the program needs to know the following information about the file:

xdim -- the width of the file in pixels
ydim -- the height of the file in pixels
bitpix -- the data type of each pixel
skip -- the size of the header to skip (in bytes)
endian -- 'b' or 'l' for big (Sun) or little (PC) endian data

This information is added to an array filename using the IRAF bracket specification:


  foo.array[xdim=xdim,ydim=ydim,bitpix=bitpix,skip=skip]

The keywords can be placed in any order. The skip keyword is optional and defaults to 0. The bitpix keyword takes on the same value as would be used in a FITS file:

8 -- unsigned byte
16 -- 2-byte signed integer
32 -- 4-byte signed integer
-32 -- 4-byte float
-64 -- 8-byte float

In addition you can specify unsigned short ints using:

-16 -- 2-byte unsigned integer

The contents of such an array should be unsigned shorts, not signed ints that have been shifted (as is done in conventional FITS images). Of course, this use of -16 for bitpix is not legal in FITS images, but is a convenient convention for use in arrays.

Thus, one can load a 700 x 600 short int FITS image that has a 2880-byte header using:

  foo.fits[xdim=700,ydim=600,bitpix=16,skip=2880]

Of course, the FITS header will be ignored in this case (i.e., no WCS display).

The array specification can be shortened by leaving out the brackets and using positional values instead. The meaning of the numbers entered after the filename depends on the number of values entered:

xdim -- assume that xdim and ydim are equal. Assume no header to skip. From this, calculate the data type based on the file size.
xdim bitpix -- assume that xdim and ydim are equal. Assume no header to skip.
xdim ydim bitpix -- use values as assigned, assume no header to skip
xdim ydim bitpix skip -- use values as assigned and skip a header

Thus, a 512 x 512 array of any type without a header can be specified as:


  foo.arr 512

while a 600 x 700 double array with a 1024-byte header can be specified as:

  foo.arr 600 700 -64 1024

For convenience, the above syntax can be input in a shortened form:

  foo.arr[aa=]

where spec is of the form:

  {type}{xdim}[.{ydim}][:{skip}][{endian}]

Type is one of the following:

b -- 8-bit unsigned char
s -- 16-bit short int
u -- 16-bit unsigned short int
i -- 32-bit int
r,f -- 32-bit float
d -- 64-bit float

Note that xdim is required, but ydim is optional and defaults to xdim. Skip is optional. Endian is optional but can be 'l' or 'b'.

Example:

r512 -- bitpix=-32 xdim=512 ydim=512
r512.400 -- bitpix=-32 xdim=512 ydim=400
r512.400 -- bitpix=-32 xdim=512 ydim=400
r512.400:2880 -- itpix=-32 xdim=512 ydim=400 skip=2880
r512l -- bitpix=-32 xdim=512 ydim=512 endian=little
If an array specification is sent to SAOtng from the Unix shell, the file name and specification must be enclosed in quotes. Quotes are not required from the Load a New Image menu option.
Note that the bracket notation is required on the SAOtng command line in order to avoid putting white space into the filename (which the script will strip off and misinterpret).

Last Updated June 14, 1997

Array File Specifications

Last Updated June 14, 1997