A very common use for a CmpFrameCmpFrame within astronomy is to represent a
“spectral cube”. This is a 3-dimensional FrameFrame in which one of the axes
represents position within a spectrum, and the other two axes represent
position on the sky (or some other spatial domain such as the focal plane
of a telescope). As an example, we create such a CmpFrame in which axes
1 and 2 represent Right Ascension and Declination (ICRS), and axis 3
represents wavelength (these are the default coordinate Systems
represented by a SkyFrameSkyFrame and a SpecFrameSpecFrame respectively):
If it was desired to make RA and Dec correspond to axes 1 and 3, with
axis 2 being the spectral axis, then the axes of the CmpFrame created
above would need to be permuted as follows: