POLANAL, and its first axis is parallel to the reference direction. This Frame will be updated by any POLPACK application which produces a change in the orientation of the reference direction. This will happen for instance if POLKA introduces a rotation into the images during the alignment process6. If you process your images using applications from another package, then be aware that the POLANAL Frame may be lost or invalidated. Basically, you will probably be safe if you only use packages which correctly propagate WCS information in form of Starlink AST FrameSets (see SUN/210). KAPPA, for instance, is such a package.
You specify the reference direction when you run POLIMP or POLEXT by
giving a value for the ANGROT item (the anti-clockwise angle in degrees
from the first image axis to the reference direction). In versions of
POLPACK prior to V2.0, ANGROT was stored explicitly as a floating point
value within the POLPACK extension. As of V2.0, ANGROT is no longer
stored in the POLPACK extension. Instead, its value is implied by the
orientation of the
POLANAL Frame within the WCS information.
However, you can consider this an ``implementation detail'' - as far as
the general user is concerned, ANGROT can be treated like any of the
genuine extension items. In particular, it can be set or examined using
POLIMP, POLEXT and POLEXP just like any other POLPACK header item.