spectrographes

Spectrographs, sometimes called spectrographes in French, are optical instruments that disperse light into its constituent wavelengths to produce a spectrum for quantitative analysis. Unlike a simple spectroscope, a spectrograph records the spectrum on a photosensitive detector, enabling precise measurements of wavelength, intensity, and, in many cases, spatial information. They are used in astronomy, chemistry, physics, and atmospheric science.

Principle of operation: light from a source enters through an entrance slit, is collimated by a lens or mirror, and is dispersed by a dispersive element—most commonly a diffraction grating, though prisms are used for certain applications. The dispersed light is focused by a camera onto a detector such as a charge‑coupled device (CCD), a complementary metal‑oxide‑semiconductor (CMOS) array, or an infrared detector. The detector records spectra that can be one- or two-dimensional, depending on the instrument design. Wavelength calibration is performed using known emission lines from calibration lamps, and flat-fielding corrects detector nonuniformities.

Types and capabilities: slit-based spectrographs obtain one-dimensional spectra along a slit; echelle spectrographs use high-order gratings and cross-dispersers to achieve high resolving power; integral-field spectrographs (IFU) capture spectra across a two-dimensional field; multi-object spectrographs (MOS) use fibers or slitlets to acquire spectra of many objects simultaneously. Modern instruments rely on high-efficiency gratings, cooled infrared detectors, and often fiber optics for flexibility and multiplexing.

Applications: in astronomy, spectrographs determine chemical abundances, stellar velocities, redshifts, and atmospheric signatures of exoplanets; in chemistry and physics, they identify substances and study molecular structures; in remote sensing and industry, they enable material analysis and quality control.

History: spectrographs emerged in the 19th century alongside the discovery of spectral lines, progressing from photographic plates to electronic detectors and space-based systems, enabling increasingly precise measurements of light properties.