Reference Directions

The reference direction within each image is recorded by adding a new co-ordinate Frame to the World Co-ordinate System (WCS) information stored with the image (see here). The new Frame has the Domain name 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.