Many thanks for that most helpful and instructive set of replies Robin! And for taking the time to search out the other spectra & spectrum comparisons and construct links to them. Given the several places within the website that observations can appear, it can be difficult to keep track of developments and thus find the most relevant post.
It was interesting to learn that prism and grating spectroscopes give fundamentally different outputs and that therefore the Harlow and Hugh spectra are incompatible by definition. I don’t think this important aspect was covered in the A-Level syllabus so many years ago! Perhaps I should mention at this point that when I referred to the great differences in spectrum linearity I was not implying any fault in the instruments or technique but merely commenting on what I could see before me – apologies if anyone thought otherwise. It’s comforting to know that my deduction of a “stretched violet” for prismatic spectra was accurate though! I will now have to carefully study the information in the links Robin gave me, and the more recent posts on the Nova Cas topic, so I can get a better idea of this facinating subject.
So – keep up the good work you spectrographers, as your results are being followed not just by the “experts” but also by those with a rather less detailed knowledge of the subject!