What might a CN filter be in the context of comet watching? Is it showing dust, or gas, or something else? 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?

GIF:
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.