Multiple image files and image sections

Some data formats, notably FITS and NDF, can include more than one image in a single file. If you encounter such a file then you need to specify which image is to be displayed. In a FITS file each image occurs in a separate `extension', which is identified by a sequential integer number. To display a FITS extension image, either open the disk file and choose the extension from the `HDU' selector window that appears (HDU or `Header and Data Unit' is FITS jargon for an image and its associated auxiliary information), or add the extension number to the disk-file name:

% gaia mef_file.fits'[2]'

(the quotation marks embedded in the file-name in this example are to prevent the square brackets from being interpreted by the Unix shell). The first image in the FITS file is called the `primary array' and is numbered 1.

A similar mechanism exists for NDFs stored in container files at other than the top-level (`container file' is NDF jargon for a file which contains one or more NDFs):

% gaia hdscontainer.ndf_1

In this case any other NDFs stored at the same level in the container file will also be shown in a selector window. It is also possible to access an `NDF slice', that is, a portion of an image rather than the whole thing (this facility can be useful for very large images). For example type:

% gaia hdscontainer.ndf_1'(200:500,100:700)'

See SUN/33[19] for further details of specifying NDF slices. This notation can also be applied to FITS files and other `foreign' formats:

% gaia file.fits'(300:700,300:700)'

Note, however, that here the FITS file will now be accessed as a foreign format; that is, it will automatically be converted to an NDF prior to being read.