HDU Location Specification

The optional HDU location specifier defines which HDU (Header-Data Unit, also known as an `extension') within the FITS file to initially open. It must immediately follow the base file name (or the output file name if present). If it is not specified then the first HDU (the primary array) is opened. The HDU location specifier is required if the colFilter, rowFilter, or binSpec specifiers are present, because the primary array is not a valid HDU for these operations. The HDU may be specified either by absolute position number, starting with 0 for the primary array, or by reference to the HDU name, and optionally, the version number and the HDU type of the desired extension. The absolute position is specified either by enclosed the number in square brackets (e.g., `[1]' = the first extension following the primary array) or by preceded the number with a plus sign (`+1'). To specify the HDU by name, give the name of the desired HDU (the value of the EXTNAME or HDUNAME keyword) and optionally the extension version number (value of the EXTVER keyword) and the extension type (value of the XTENSION keyword: IMAGE, ASCII or TABLE, or BINTABLE), separated by commas and all enclosed in square brackets. If the value of EXTVER and XTENSION are not specified, then the first extension with the correct value of EXTNAME is opened. The extension name and type are not case sensitive, and the extension type may be abbreviated to a single letter (e.g., I = IMAGE extension or primary array, A or T = ASCII table extension, and B = binary table BINTABLE extension). If the HDU location specifier is equal to `[PRIMARY]' or `[P]', then the primary array (the first HDU) will be opened. Examples:

   myfile.fits[3] - open the 3rd HDU following the primary array
   myfile.fits+3  - same as above, but using the FTOOLS-style notation
   myfile.fits[EVENTS] - open the extension that has EXTNAME = 'EVENTS'
   myfile.fits[EVENTS, 2]  - same as above, but also requires EXTVER = 2
   myfile.fits[events,2,b] - same, but also requires XTENSION = 'BINTABLE'