XMM-Newton Data Files Handbook

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3.1 FITS format

All of the ODF/SDF component files, with the exception of the summary files, reconstructed orbit file, and raw attitude file, are FITS files and conform to the FITS standard. A description of the FITS format can be found in [5], which is accessible also at the URL
The calibration files and the bulk of the PPS products also conform to the FITS standard. Wherever possible and desirable the calibration files and the PPS products follow the conventions of the OGIP (Office of Guest Investigator Programs) FITS working group. The HEASARC FITS Working Group activities are described at the following URL:
For FITS files where OGIP FITS standards are not applicable or available, new standards closely following the OGIP approach are used.

The FITS format is primarily designed to store scientific data sets consisting of multidimensional arrays (1-D spectra, 2-D images or 3-D data cubes) and 2-dimensional tables containing rows and columns of data. A FITS data file is composed of a sequence of Header + Data Units (HDUs).

The general structure of a FITS file is as follows:

Each extension consists of an extension header and a data section. Extensions are named and can appear in any order. Only the following FITS extensions are used:

The header consists of keyword=value statements, which describe the organisation of the data in the HDU and the format of the contents. It may also provide additional information, for example, about instrument status or the history of the data. The following block contains the data, which are structured as specified in the header. The data section of the HDU may contain a digital image, a table or a multidimensional matrix that is not an image. An HDU need not contain data.

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European Space Agency - XMM-Newton Science Operations Centre