Unclear SpectralCoord warning coming from fitswcs

[dstansby]

I'm processing some FITS files that have spectral data (via. NDCube), and currently seeing the following warning a lot:

WARNING: target cannot be converted to ICRS, so will not be set on SpectralCoord [astropy.wcs.wcsapi.fitswcs]

I'm not finding this warning very helpful, for a couple of reasons:

• I don't understand why it's being raised. In particular, what is "target" and "SpectralCoord", and how are they related to world_axis_object_components? (which is the method that this warning is raised in)
• I don't understand what I can do to prevent the warning from being raised. ie., what do I need to fix in my data?

It would be good to improve the warning message to address these two points.

[pllim]

cc @astrofrog

[Cadair]

👍 this warning is very annoying.