<p>A CN filter <a href="http://wirtanen.astro.umd.edu/46P/CN_filter_test.shtml" rel="nofollow noreferrer">shows abundances of cyanogen</a>, a molecule with the structure <span class="math-container">$(\mathrm{CN})_2$</span>. 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 <span class="math-container">$\mathrm{OH}$</span>, <span class="math-container">$\mathrm{NH}$</span> and <span class="math-container">$\mathrm{C}_2$</span>, also give away a lot of information about a comet, but the often-strong cyanogen lines are a good tool.</p>

<p>The mechanism behind cyanogen production from parent gases isn't entirely clear, although <a href="https://www2.jpl.nasa.gov/comet/news65.html" rel="nofollow noreferrer">a number of theories have been proposed</a>. For instance, cyanogen could be released by icy grains undergoing sublimation, or by the decomposition of more complex molecules.</p>

CC BY-SA 4.0
2018-12-10T15:36:52.630Z
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!
uhoh
CC BY-SA 4.0
2018-12-11T16:59:08.787Z
The common emission lines of comets are C2, C3, NH, CN and OH as shown in this [spectrum of Comet Hyakutake](https://www2.jpl.nasa.gov/comet/hyakutake/news19.html). 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)
astrosnapper
CC BY-SA 4.0
2018-12-15T11:35:18.613Z
@astrosnapper the extent of my understanding of C2 is in https://en.wikipedia.org/wiki/Swan_band
uhoh
CC BY-SA 4.0
2018-12-15T15:18:55.260Z
Yes, that's the stuff
astrosnapper
CC BY-SA 4.0
2018-12-10T15:19:15.187Z
2018-12-10T15:19:15.187Z
HDE 226868
<p>In <a href="http://wirtanen.astro.umd.edu/46P/46P_status.shtml" rel="nofollow noreferrer">this post</a> on the <a href="http://wirtanen.astro.umd.edu/" rel="nofollow noreferrer">website for the Comet Wirtanen Observing Campaign</a> at wirtanen.astro.umd.edu, there's mention of a CN filter being used to observe the comet, and this cool GIF.</p>

<p>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?</p>

<p>GIF:</p>

<p><a href="https://i.sstatic.net/vkGYB.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/vkGYB.gif" alt="enter image description here"></a></p>

<p><a href="https://i.sstatic.net/UDiW3.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/UDiW3.png" alt="http://wirtanen.astro.umd.edu/46P/46P_status.shtml#brightness_plot"></a></p>

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

(Asked by: username_0 on 2018-12-10)

In [this post](http://wirtanen.astro.umd.edu/46P/46P_status.shtml) on the [website for the Comet Wirtanen Observing Campaign](http://wirtanen.astro.umd.edu/) 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:

[![enter image description here](https://i.sstatic.net/vkGYB.gif)](https://i.sstatic.net/vkGYB.gif)

[![http://wirtanen.astro.umd.edu/46P/46P_status.shtml#brightness_plot](https://i.sstatic.net/UDiW3.png)](https://i.sstatic.net/UDiW3.png)

**Tags:** spectroscopy

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## 1 answers

### Answer by username_1 on 2018-12-10. Score: 2 | Accepted answer

A CN filter [shows abundances of cyanogen](http://wirtanen.astro.umd.edu/46P/CN_filter_test.shtml), 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](https://www2.jpl.nasa.gov/comet/news65.html). For instance, cyanogen could be released by icy grains undergoing sublimation, or by the decomposition of more complex molecules.

> **Comments:**
> **username_0**: 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!  
> **username_2**: The common emission lines of comets are C2, C3, NH, CN and OH as shown in this [spectrum of Comet Hyakutake](https://www2.jpl.nasa.gov/comet/hyakutake/news19.html). 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)  
> **username_0**: @astrosnapper the extent of my understanding of C2 is in https://en.wikipedia.org/wiki/Swan_band  
> **username_2**: Yes, that's the stuff  

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