Spectrograph: instrument for separating and recording a wave spectrum

Main parts and basic operation

Most optical spectrographs share a common sequence of elements that take incoming radiation and form a measurable spectrum. A typical layout includes:

• Entrance aperture or slit: defines the portion of the source accepted and helps set spectral resolution.
• Collimator: converts light from the slit into a parallel beam for dispersion.
• Dispersive element: a prism or diffraction grating that separates wavelengths spatially.
• Camera or focusing optics: images the dispersed light onto a recorder.
• Detector: historically a photographic plate, now typically a CCD or other electronic sensor.

Spectral resolving power is often expressed as R = λ/Δλ, where Δλ is the smallest distinguishable wavelength difference. For non‑optical waves (audio or radio) the equivalent components perform filtering and time–frequency analysis to produce spectrograms.

History and development

The idea of analyzing light by wavelength dates to early studies of optical dispersion; the term "spectrograph" was first recorded in the late 19th century. Photographic spectrographs became essential tools in astronomy and chemistry as photographic plates allowed permanent records of spectra. In the 20th and 21st centuries, electronic detectors, diffraction gratings, fiber feeds and cross‑dispersed echelle designs have greatly increased sensitivity and resolution.

Types and notable variants

• Long‑slit spectrograph: used to record variation along an extended source such as a galaxy or nebula.
• Echelle spectrograph: a high‑resolution design that uses multiple orders and cross dispersion.
• Fiber‑fed spectrograph: commonly used in modern observatories to bring light from a telescope focus to a stable bench instrument.
• Polychromator: a related device that separates light into several discrete bands, often directing each band to a separate detector.

Uses and examples

Spectrographs are central to many scientific and practical fields. In astronomy they determine chemical composition, radial velocities (via Doppler shifts) and physical conditions of stars and galaxies. In laboratory and industrial settings they support chemical analysis, material identification and process monitoring. Acoustic spectrography and radio spectrography are used in signal analysis, speech research and telecommunications. For further technical reading see related references.

Distinctions and notable facts

Terms such as spectrograph and spectrometer are sometimes used interchangeably: broadly, a spectrograph records a spectrum (often an image), while a spectrometer emphasizes quantitative measurement and may output numerical data. Modern instruments blur this distinction because electronic detectors both record and enable direct numerical analysis. Advances in detector technology and dispersion techniques continue to expand the range of phenomena that can be investigated spectroscopically.