## Creating a CmpFrame

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):

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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:

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