ASTERIX files are based on the Starlink standard structures defined in SGP38; however this defines only certain basic constructions, and is free of astronomical content and instrumental detail. It has therefore been necessary to extend it considerably by defining new structures. The purpose of this document is to define these standard ASTERIX structures so that they can be consistently utilised by programmers, and also to specify certain conventions relating to the interpretation and manipulation of data.
There are two main types of datasets within ASTERIX: binned datasets and event datasets. The former are used to represent binned data of any dimensionality (up to 7) and are based on the Starlink NDF construction (see SGP38) in order that they should be accessible by standard Starlink software. Event datasets represent the various properties of a set of individual (unbinned) photon events; this is done by a set of parallel LIST structures, one for each property represented. Each LIST is in fact an NDF in its own right (though it lacks any axis information) and so can be processed by many analysis applications, which can be useful.