Spectrum of Nova Cas 2021

[Steve Holmes]

I have recently been looking at the various spectra produced for Nova Cas 2021 and shown as part of the topic on this nova in the Variable Stars forum. While these spectra are excellent pieces of work and most interesting, I have found that they do not seem to be consistent between themselves nor with diagrams of the Balmer spectrum.

I firstly looked at the image produced by Mike Harlow (19th March). This is a great illustration of both emission lines and Fraunhofer lines, shown even more clearly in the version in the Observations section of the website. However, when I try to align it with a standard Balmer spectrum it becomes clear that if one matches up the H-alpha and H-beta lines the gap between the H-beta and the H-gamma  and -delta lines in the Harlow spectrum is almost twice as wide as it should be. Similarly, if one tries to align the spectrum produced by Allan Hugh (23rd Mar) with a standard Balmer spectrum then the H-gamma and -delta lines again do not match up. The gaps are now not wide enough, albeit by quite a small margin, and consequently the Harlow and Hugh spectra do not align with each other either. Conversely, the line spectrum produced by Robin Leadbeater (23rd Mar) precisely aligns with a standard Balmer spectrum (but, of course, not with either of the spectra mentioned above).

How is one to reconcile these findings? Might it be that Mike’s spectroscope gives a very non-linear output, particularly at the violet end, and that Allan’s is similar but to a lesser extent and in the opposite sense? There is clearly no doubt that the lines being shown are indeed the Balmer lines, but surely the lines shown in all spectra should line up both with each other and also with a standard line spectrum?

[Robin Leadbeater]

All spectrographs are non linear to at least some degree and have to be individually calibrated in wavelength. Mike’s spectrograph is fundamentally different to mine for example. His uses a prism and the dispersion is due to refraction whereas mine (and Hugh’s) uses a diffraction grating and the dispersion is due to diffraction. As a result the relationship between wavelength and distance along the spectrum are completely different and in fact run in opposite directions !  (with diffraction the short wavelengths are least deflected whereas in refraction the short wavelengths are refracted the most)

Cheers

Robin

[Robin Leadbeater]

I just checked mine and Hugh’s spectra from 23rd March in the Nova Cas 2021 thread. The graphed spectra are calibrated in wavelength with a linear X axis and the Balmer (and other line) wavelengths are correct and consistent between the two spectra

https://britastro.org/comment/10272#comment-10272

https://britastro.org/comment/10277#comment-10277

Mike’s spectrum image is not calibrated in wavelength.

https://britastro.org/comment/10242#comment-10242

Prism spectra are very non linear with the short wavelength end significantly stretched and the longer wavelengths compressed

[Robin Leadbeater]

In fact I see further down the thread Hugh actually wavelength calibrated  Mike’s prism spectrum to produce a graph of the spectrum with a linear wavelength axis.

https://britastro.org/comment/10463#comment-10463

You can see there that the wavelengths then agree in Mike’s Hugh’s and my spectra (and indeed with those by David Boyd and John Coffin in the same thread)

[Robin Leadbeater]

Here are 6 spectra around that date from the BAA spectroscopy database (From David Boyd (2), Kevin Gurney, Hugh Allen, John Coffin and me)

https://britastro.org/specdb/data_graph.php?obs_id=9120%2C9031%2C9159%2C9035%2C9118%2C9039&multi=yes&mix_flux=yes&plot=Plot+up+to+20+Spectra

They all line up in wavelength precisely

Cheers

Robin

[Steve Holmes]

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!

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