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''
(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 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
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