So two comments to that. I thought about adding an `npix` or equivalent to the WCS class - but decided against it because it "felt" cleaner to keep NAXIS1 and WCS separate. Maybe an interesting compromise might be to tell wcs about its parent and thus it could access this information. 

The second design decision was to give the first pixel the index 0. I know, that IRAF doesn't do it that way (that's why CRPIX1=1 for the first pixel).