### Alternate Axis Descriptions

When reading a FrameSetFrameSet from a FitsChanFitsChan which contains alternate axis descriptions, each complete set of axis descriptions results in a single FrameFrame being added to the final FrameSet, connected via an appropriate MappingMapping to the base pixel Frame. The IdentIdent attribute of the Frame is set to hold the single alphabetical character which is used to identify the set of axis descriptions within the FITS header (a single space is used for the primary axis descriptions).

When writing a FrameSet to a FitsChan, it is assumed that the base Frame represents pixel coordinates, and the current Frame represents the primary axis descriptions. If there are any other Frames present in the FrameSet, an attempt is made to create a complete set of “alternate” set of keywords describing each additional Frame. The first character in the Ident attribute of the Frame is used as the single character descriptor to be appended to the keyword, with the proviso that a given character can only be used once. If a second Frame is found with an Ident attribute which has already been used, its Ident attribute is ignored and the next free character is used instead. Note, failure to write a set of alternate axis descriptions does not result in failure of the entire write operation: the primary axis descriptions are still written, together with any other alternate axis descriptions which can be produced successfully.