F The POLPACK NDF Extension

Figure 13 lists the items which may be stored in the POLPACK extension within a data file. The POLEXP and POLIMP applications may be used to transfer these items to and from a set of corresponding FITS keywords. POLEXT can be used to set them to explicit values (see section 5.4). The contents of the POLPACK extension within an NDF may be examined using POLEXT or the hdstrace command.

Versions of POLPACK prior to V2.0 included an extra item called ANGROT. This gave the anti-clockwise angle from the first image axis to the reference direction. POLPACK will still read these ANGROT values if they exist, but will no longer write them. As of V2.0 the orientation of the reference direction is specified instead by the POLANAL Frame added to the NDF WCS component by POLIMP or POLEXT. If data written by POLPACK V2.0 or later is read by an earlier version of POLPACK, a default value of zero will be adopted for the missing ANGROT extension item (i.e. it will be assumed that the reference direction is parallel to the first image axis).

Figure 13: The allowed items within the POLPACK extension.