The internal coordinate system used by arfgen is DET, so if the input coordinate system is EQPOS, POS or TEL, a conversion to DET is performed. If the input system is POS, it is firstly converted to EQPOS. To perform this intermediate conversion, the nominal pointing position is required. This is taken from the global attributes REFXCRVL and REFYCRVL of the spectrum dataset.
The conversion from EQPOS to DET in general requires knowledge of the spacecraft attitude. By default the task uses the mean pointing position contained within the spectrum header in the RA_PNT, DEC_PNT, PA_PNT keywords. This is expected to be accurate enough in most cases. An alternative approach is to use the attitude from the OAL corresponding to the start of the observation by specifying useodfatt=true. In this case, as the OAL is used by attcalc and for the coordinate conversion, the environment variable SAS_ODF, or the task parameter odf need to be set to the appropriate ODF directory in order for the conversion to be correct.
It is recommended to use DET coordinates if the mean attitude over the exposure period differs significantly from the start attitude, say by more than 30 arcseconds.
An intermediate stage of the conversion from equatorial to detector coordinates is via the telescope position (theta, phi). The source position may be input directly in terms of theta, phi by setting sourcecoords=TEL, sourcex=theta and sourcey=phi. In this case, sourcex=0 sourcey=0 refers to the optical axis.