<p>Echelle spectrometers operate in the higher spectral orders.  (CRIES+ uses orders 10-60 for IR light, I think.  Not sure about ESPRESSO, but you can count them...)  </p>

<p>These high orders are highly overlapping, so, for example, if the input light is dispersed by the Echelle grating horizontally into a spectrum, then the light at a given point along the spectrum is has components contributed from several different orders.  But, since each contribution is a different wavelength from each order, the orders can be spread out <em>vertically</em> using a second grating, the "cross disperser", conveniently stacking them up for the camera.  You can see these components in <a href="https://www.eso.org/sci/facilities/develop/instruments/espresso.html" rel="nofollow noreferrer">this diagram of the ESPRESSO spectrograph</a>.</p>

<p>TL;DR:  Each horizontal line you see is a portion of a single spectral order, and the orders are spread vertically by a <em>cross disperser</em> grating, oriented roughly perpendicularly to the first (Echelle) grating.</p>

<p>I found <a href="http://slideplayer.com/slide/4178747/" rel="nofollow noreferrer">this presentation with more details &amp; the equations.</a></p>

CC BY-SA 3.0
2018-02-04T09:29:46.947Z
Great! Thank you for the nicely-written and clear answer!
uhoh
CC BY-SA 3.0
2018-02-04T09:28:02.600Z
2018-02-04T09:28:02.600Z
Andrew T. Smith
<p>In order to obtain a very high resolution spectrum to resolve closely spaced lines, or to measure doppler shits, spectrographs are operated with very high dispersion.</p>

<p>But photographic plates and CCDs usually have rectangular formats with aspect ratios not very far from square. </p>

<p>Echelle spectrographs can put a highly dispersed spectrum into this format, but how? This <a href="https://en.wikipedia.org/wiki/Echelle_grating" rel="nofollow noreferrer">article</a> explains to some extent, but a concise discussion here would also be helpful.</p>

<observational-astronomy><spectroscopy>
How does an Echelle spectrograph put such a long, high resolution spectrum into a square-ish format?
uhoh
# How does an Echelle spectrograph put such a long, high resolution spectrum into a square-ish format?

(Asked by: username_0 on 2018-02-04)

In order to obtain a very high resolution spectrum to resolve closely spaced lines, or to measure doppler shits, spectrographs are operated with very high dispersion.

But photographic plates and CCDs usually have rectangular formats with aspect ratios not very far from square.

Echelle spectrographs can put a highly dispersed spectrum into this format, but how? This [article](https://en.wikipedia.org/wiki/Echelle_grating) explains to some extent, but a concise discussion here would also be helpful.

**Tags:** observational-astronomy, spectroscopy


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## 1 answers

### Answer by username_1 on 2018-02-04. Score: 2 | Accepted answer

Echelle spectrometers operate in the higher spectral orders. (CRIES+ uses orders 10-60 for IR light, I think. Not sure about ESPRESSO, but you can count them...)

These high orders are highly overlapping, so, for example, if the input light is dispersed by the Echelle grating horizontally into a spectrum, then the light at a given point along the spectrum is has components contributed from several different orders. But, since each contribution is a different wavelength from each order, the orders can be spread out *vertically* using a second grating, the "cross disperser", conveniently stacking them up for the camera. You can see these components in [this diagram of the ESPRESSO spectrograph](https://www.eso.org/sci/facilities/develop/instruments/espresso.html).

TL;DR: Each horizontal line you see is a portion of a single spectral order, and the orders are spread vertically by a *cross disperser* grating, oriented roughly perpendicularly to the first (Echelle) grating.

I found [this presentation with more details & the equations.](http://slideplayer.com/slide/4178747/)

> **Comments:**
> **username_0**: Great! Thank you for the nicely-written and clear answer!  

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