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 explains to some extent, but a concise discussion here would also be helpful.


1 Answer 1


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.

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.

  • $\begingroup$ Great! Thank you for the nicely-written and clear answer! $\endgroup$
    – uhoh
    Feb 4, 2018 at 9:29

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