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The Native FITS Encoding

As it turns out, we are not the first to have thought of storing WCS information in FITS headers. In fact, the original FITS standard (1981 vintage) defined a set of header keywords for this purpose which have been widely used, although they have proved too limited for many practical purposes.

At the time of writing, a number of different ways of using FITS headers for storing WCS information are in use, most (although not all) based on the original standard. We will refer to these alternative ways of storing the information as FITS encodings but will defer a discussion of their advantages and limitations until the next section (§17).

Here, we will examine how to store AST Objects directly in FITS headers. In effect, this defines a new encoding, which we will term the native encoding. This is a special kind of encoding, because not only does it allow us to associate conventional WCS calibration information with FITS data, but it also allows any other information that can be expressed in terms of AST Objects to be stored as well. In fact, the native encoding provides us with facilities roughly analogous to those of the ChannelChannel (§15)—i.e. a lossless way of transferring AST Objects from program to program—but based on FITS headers instead of free-format text.