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 location of an image within a single cell of a binary table may also be specified, as described below.
The absolute position of the extension is specified either by enclosed the number in square brackets (e.g., `' = 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.
FITS images are most commonly stored in the primary array or an image extension, but images can also be stored as a vector in a single cell of a binary table (i.e. each row of the vector column contains a different image). Such an image can be opened with CFITSIO by specifying the desired column name and the row number after the binary table HDU specifier as shown in the following examples. The column name is separated from the HDU specifier by a semicolon and the row number is enclosed in parentheses. In this case CFITSIO copies the image from the table cell into a temporary primary array before it is opened. The application program then just sees the image in the primary array, without any extensions. The particular row to be opened may be specified either by giving an absolute integer row number (starting with 1 for the first row), or by specifying a boolean expression that evaluates to TRUE for the desired row. The first row that satisfies the expression will be used. The row selection expression has the same syntax as described in the Row Filter Specifier section, below.
myfile.fits - 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' myfile.fits[3; images(17)] - opens the image in row 17 of the 'images' column in the 3rd extension of the file. myfile.fits[3; images(exposure > 100)] - as above, but opens the image in the first row that has an 'exposure' column value greater than 100.