... This task operates on event data in an event list. ... In the first two cases, the celestial coordinates of the reference point are taken respectively as the values of the RA_NOM , DEC_NOM or RA_OBJ , DEC_OBJ values in the input event file header. ... Although the OM attitude information may be more accurate, it will not be present throughout part of the observation, and as such, the default source for the attitude data should be "ahf". ...