echelle

Echelle, derived from the French échelle meaning ladder or scale, is a term used in optics and astronomy to describe a diffraction grating and the spectrographs that employ it. In this context, an echelle element is designed to operate at high diffraction orders to achieve high spectral resolution.

An echelle grating has relatively coarse groove spacing and is used at large angles of incidence. Diffraction into many high orders produces overlapping spectral bands that, without further dispersion, would clutter the spectrum. A cross-disperser—typically a prism or a secondary grating—separates these orders in a direction perpendicular to the main dispersion, yielding a two-dimensional, multi-order spectrum on the detector.

Echelle spectrographs provide high resolving powers, often from tens of thousands up to 100,000 or more, while covering wide wavelength ranges by stitching together multiple orders. They are widely used in astronomy for precise radial velocity work, abundance analyses, and stellar atmosphere studies. Notable instruments include examples such as HARPS, ESPaDOnS, UVES, and HIRES, among others.

Outside optics, the word échelle in French means ladder or scale, and the concept of an echelle order arrangement is referenced in descriptions of ladder-like spectral formats.