### Choice of Axis Ordering

When reading a FrameSetFrameSet from a FitsChanFitsChan, WCS axis 310#310 in the current FrameFrame of the resulting FrameSet corresponds to axis 310#310 in the FITS header.

When writing a FrameSet to a FitsChan, the axis ordering for the FITS header is chosen to make the CDi_j or PCi_j matrix predominately diagonal. This means that the axis numbering in the FITS header will not necessarily be the same as that in the AST Frame.