Echelle spectrographs are named after its main optical component - an Echelle grating (from French échelle). They are used to obtain high-resolution, cross-dispersed spectra and are widely used in astronomy.
Echelle gratings have a large ruling spacing and are used in high diffraction order allowing for a high spectral resolution. Spectra of different diffraction orders are overlapping and a second component - a so called cross disperser - is used to seperate the individual diffraction orders. Compared to an echelle grating, the dispersion of the cross disperser is low and a typical choice is a low dispersion grating, a prism or a grism.
On the whole, an echelle spectrograph produces a 2D spectrum with a complex mapping between wavelength and position on the detector. It depends on the details of the spectrograph optics, mainly the echelle grating parameters and the cross disperser parameters, but also on second order effects like camera distortions and aberrations.
As an example, here we plot the echelle format of the MaroonX spectrograph. Each data point marks the center of a spectral line calculated for 50 equally spaced wavelengths per order.
