The foreign encodings available for storing WCS information in FITS headers have a number of limitations when compared with the native encoding of AST Objects (). The main ones are:
The way in which a FrameSet is translated to and from the foreign encoding also follows from this correspondence. The FrameSet's base Frame identifies the data grid coordinates of the associated FITS data. These are the same as FITS pixel coordinates, in which the first pixel (in 2 dimensions) has coordinates (1,1) at its centre. Similarly, the current Frame of the FrameSet identifies the FITS world coordinate system associated with the data.
This is mainly a consequence of the use of fixed FITS keywords by foreign encodings and the fact that you cannot, in general, have multiple FITS cards with the same keyword.
It should be understood that using foreign encodings to read and write information held in AST Objects is essentially a process of converting the data format. As such, it potentially suffers from the same problems faced by all such processes, i.e. differences between the AST data model and that of the foreign encoding may cause some information to be lost. Because the AST model is extremely flexible, however, any data loss can largely be eliminated when reading. Instead, this effect manifests itself in the form of the above encoding-dependent restrictions on the kind of AST Objects which may be written.
One of the aims of the AST library, of course, is to insulate you from
the details of these foreign encodings and the restrictions they
impose. We will see shortly, therefore, how AST provides a mechanism
for determining whether your WCS information satisfies the necessary
conditions and allows you to make an automatic choice of which
encoding to use.
AST A Library for Handling World Coordinate Systems in Astronomy