I see (this shows that documentation is getting more and more important ;-) ): so the `_wcs_attributes` show what attributes could be possible. If your current WCS unit can not be converted to the attribute's unit then the attribute is not usable. For example, you make a WCS with unit = 'km/s' and do not specify an equivalency that makes it convertible to wavelength then if you do `spec1d.wavelength` you get an `AttributeError` (this reminds me that I should add velocity now to the attributes).