PSS is written to take advantage of the many pieces of information present in an ASTERIX dataset. These include knowledge of the instrument used, the spatial extent and celestial orientation of the image, and presence of bad or missing data.
The ASTERIX binned image format (Ref 3) is a superset of the Starlink NDF structure (Ref 2). The extensions provide the astronomical content lacking in the latter which enable applications to make intelligent processing decisions.
A dataset conforming only to the NDF definitions can be searched quite successfully by PSS, with certain qualifications. The most fundamental assumption is that the image data represent a tangent plane projection of the sky. The ASTERIX standard then defines the axis values as offsets in angular units from the fiducial point of the dataset. In order to interpret the axis units correctly ASTERIX recognises all common angular units and their abbreviations. An additional assumption is made by the psf system that the spatial axes are linear with respect to pixel number. This assumption is necessary as the production of the psf is required to be independent of the binning characteristics of the dataset.
Both the CASH and GAUSSIAN statistics expect the input
data not to be background subtracted. This was not the case with
older versions of PSS, and so the program can still accept these data.
As long as a background subtracted image has
the BGND_SUBTRACTED flag set in the
MORE.ASTERIX.PROCESSING structure, then PSS will recognise the
fact and add the background model back to create the original data without
further ado.