In this post on the website for the Comet Wirtanen Observing Campaign at wirtanen.astro.umd.edu, there's mention of a CN filter being used to observe the comet, and this cool GIF.

What exactly is a CN filter? What wavelengths does it pass? Where can I read more about it? Why is it showing a spiral shape of stuff being emitted from comet 46P? Is this comet dust, gas, something else?


enter image description here



1 Answer 1


A CN filter shows abundances of cyanogen, a molecule with the structure $(\mathrm{CN})_2$. Cyanogen emission lines at 387 nm allow us to study gas in the coma of a comet, telling us more about its structure, properties and outgassing. Other gases, like $\mathrm{OH}$, $\mathrm{NH}$ and $\mathrm{C}_2$, also give away a lot of information about a comet, but the often-strong cyanogen lines are a good tool.

The mechanism behind cyanogen production from parent gases isn't entirely clear, although a number of theories have been proposed. For instance, cyanogen could be released by icy grains undergoing sublimation, or by the decomposition of more complex molecules.

  • $\begingroup$ I was surprised to find out that comets were (often) green, then surprised that that was because of C₂ which I'd never heard of, now I'm surprised to hear about cyanogen, never heard of the stuff either! $\endgroup$
    – uhoh
    Dec 10, 2018 at 15:36
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    $\begingroup$ The common emission lines of comets are C2, C3, NH, CN and OH as shown in this spectrum of Comet Hyakutake. We use narrowband filters centered on these wavelengths, plus a continuum filter in the red to remove the background, to isolate the gas emission from the dust (which is mostly what you see in wider filters) $\endgroup$ Dec 11, 2018 at 16:59
  • $\begingroup$ @astrosnapper the extent of my understanding of C2 is in en.wikipedia.org/wiki/Swan_band $\endgroup$
    – uhoh
    Dec 15, 2018 at 11:35
  • $\begingroup$ Yes, that's the stuff $\endgroup$ Dec 15, 2018 at 15:18

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