I have the same issue understanding `c1` as above, and it's not explained on the `Spectrum1DPolynomialWCS` doc page:
http://specutils.readthedocs.org/en/doc-testing/api/specutils.wcs.specwcs.Spectrum1DPolynomialWCS.html#specutils.wcs.specwcs.Spectrum1DPolynomialWCS

However, I think this indicates a documentation bug - on that page, there should be a copy of the 1D model docs, but it just says "1D polynomial model"