Should at least add a comment here to indicate what `fits_wcs_information.shape[0]` indicates - I assume the 0'th (or FITS 1st) axis is being assumed as the spectral coordinate?  It's fairly obvious, but might not be when skimming code in the future...

Perhaps it would be worth changing the `0` to `spectral_axis` and default it to `0`?  I've found myself doing that in many of my codes long after the fact.

This is not a very important change, just a mild suggestion.