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.
AST A Library for Handling World Coordinate Systems in Astronomy