spectrographs

A spectrograph is an optical instrument that disperses incoming light into its component wavelengths and records the resulting spectrum for analysis. It is used in fields such as astronomy, chemistry, and materials science to determine composition, physical conditions, and motion of light-emitting or absorbing sources.

The basic configuration includes an input aperture or slit, a collimator to produce a parallel beam, a dispersive element (such as a prism or diffraction grating) to separate wavelengths, and a camera to focus the spectrum onto a detector, typically a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) array. Calibration hardware, such as comparison lamps, and fore-optics or fiber feeds may also be included. In more advanced systems, a echelle grating with cross-dispersion is used to achieve very high spectral resolution.

Dispersive elements determine the instrument’s performance. Prism spectrographs rely on refractive dispersion, while grating spectrographs use diffraction from densely spaced grooves. Echelle spectrographs operate in high orders and require an additional cross-disperser to separate overlapping spectra. The grating equation mλ = d(sin α + sin β) governs the dispersion, where m is the diffraction order, λ the wavelength, d the groove spacing, and α and β are incident and diffracted angles. The spectral resolving power R ≈ λ/Δλ depends on slit width, illuminated groove count, and order.

Applications include measuring chemical compositions, atmospheric constituents, stellar abundances and velocities, and industrial material analysis. Data are typically one- or two-dimensional spectra that require calibration, wavelength reference data, and flux correction to extract quantitative information.