The Background to FITS File Conversions

The ROSAT datasets have always been distributed as collections of FITS files, however there have been three different sets of file formats within the FITS convention. The MPE and GSFC FITS files represent the first generation data format. They were developed independently, and were read by the respective national analysis packages. Datasets from the two sites contained essentially the same information, although the GSFC datasets included files that had no equivalent in the MPE datasets. UK and German PIs had data in the MPE format, US PIs had data in the GSFC format.

The fact that there were two data formats, together with the deficiencies of the individual FITS styles, led to the definition of the a format common to both MPE and GSFC. This is the `Rationalised Data Format' (RDF), which is described below. The switch between first generation FITS files and RDF FITS files took place in April 1994. In addition to current PI data, all prior data is being re-processed by the SASS and converted to RDF files.

Detailed descriptions of the files making up the datasets in the three formats are given for both the PSPC and HRI in appendix 1. This appendix also shows the correspondence between FITS files and HDS files.