The current version of ASTERIX (2.2) has the capability to read and write data in both Hierarchical Data Structure (HDS) and Flexible Image Transport System (FITS) formats. However, this duality is not universal, and the FITS compatibility applies only to certain applications.
More specifically FITS I/O is available with Isys, graphical display, image processing and general utilities. It is not available with ROSAT data analysis, source searching, spectral fitting or time series analysis.