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Limitations of Foreign Encodings

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 (§16). The main ones are:

1. Only one class of AST ObjectObject, the FrameSetFrameSet, may be represented using a foreign FITS encoding. This should not come as a surprise, because the purpose of storing WCS information in FITS headers is to attach coordinate systems to an associated array of data. Since the FrameSet is the AST Object designed for the same purpose (§13.4), there is a natural correspondence.

   The way in which a FrameSet is translated to and from the foreign encoding also follows from this correspondence. The FrameSet's base FrameFrame 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.

2. You may store a representation of only a single FrameSet in any individual set of FITS header cards (i.e. in a single FitsChanFitsChan) at one time. If you attempt to store more than one, you may over-write the previous one or generate an invalid representation of your WCS information.

   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.

3. In general, it will not be possible to store every possible FrameSet that you might construct. Depending on the encoding, only certain FrameSets that conform to particular restrictions can be represented and, even then, some of their information may be lost. See the description of the EncodingEncoding attribute in Appendix C for more details of these limitations.

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.