FITS Files and Data Object Locators (DOLs)

ISDC adopted the FITS format to handle and store the INTEGRAL data at each level of the data processing. The detailed information on FITS formats is available from the FITS Support Office at NASA/GSFC: http://fits.gsfc.nasa.gov

A FITS file consists of one or more HDU - an ASCII Header Unit followed by an optional Data Unit. The first HDU is called the `Primary HDU'. At ISDC, data analyses system primary HDU consists of only a header. In the following, we also use the term data structure along with HDU.

Each Header Unit consists of any number of 80-character keyword records which have the general form:

KEYNAME = value / comment string

A keyword may also have a null or undefined value if there is no specified value. Any number of additional HDUs may follow the primary array; these additional HDUs are called FITS `extensions'. There are currently 3 types of extensions defined by the FITS standard:

• Image Extension - a N -dimensional array of pixels
• ASCII Table Extension - rows and columns of data in ASCII character format
• Binary Table Extension - rows and columns of data in binary representation

To specify which data structure should be opened at run time the HDU name or number should be specified after the root file name. For example, `myfile.fits[4]' opens the 4th extension, which corresponds to the 5th HDU in the file (note that the HDU numbering starts with 0), and `myfile.fits[EVENTS]' opens the data structure with the name `EVENTS' (as defined by the EXTNAME or HDUNAME keywords). If no HDU is specified, then the first extension is opened by default (i.e., this is equivalent to specifying 'myfile.fits[1]').

Such an ability to have an access to the desired data structure leads to the possibility of arranging a group with its members residing in separate FITS files. This allows data objects to be distributed across multiple data files, which has the following consequences:

• different data objects may simultaneously share the same data elements (e.g., two science windows may share the same orbit data),
• the construction of data object structures (e.g., science windows, observations) is not limited by the content of the individual data files,
• data objects may span multiple file systems and archives across the Internet,
• applications do not have to be concerned about what or how many data files are being accessed when they open a data object.

The particular existing data object is determined by specifying a Data Object Locator, or DOL. When an analysis script asks you to specify the DOL of the catalog you want to use in your analysis you should specify the path of the corresponding FITS file and the corresponding name or number (do not forget that numbering starts with 0!) of the data structure with the catalog in square brackets, e.g.

DOL of Reference Catalog []: ../../cat/hec/gnrl\_ref\_cat\_007.fits[1]

or

DOL of Reference Catalog []: ../../cat/hec/gnrl\_ref\_cat\_007.fits[GNRL-REFR-CAT]

omission of the extension is equivalent to `[1]'.