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A spectrograph is an instrument that splits light into its component wavelengths and records the resulting spectrum. This page explains what a spectrograph is, how its parts work in sequence, the main variants and how the word differs from spectrometer and spectroscope. It is written for lab and core-facility staff, students and anyone reading an instrument specification.
Who uses it: astronomers and observatory instrument teams, and laboratory and core-facility staff who work with optical spectroscopy instruments.
What is a spectrograph?
The CSIRO Australia Telescope National Facility describes a spectrograph as an instrument used to obtain and record an astronomical spectrum. The purpose is to separate light into its component wavelengths so that the intensity at each wavelength can be examined. NASA describes the same function for the Hubble Space Telescope Imaging Spectrograph (STIS): separating light into component wavelengths reveals the chemical content, temperature and motion of the source.
How a spectrograph is built
The CSIRO tutorial walks through the light path in the order below. Each element controls a different property of the recorded spectrum.
- Slit. Limits the incoming light to a narrow aperture, which isolates a point source of light from a larger image. A long slit also allows spectra to be taken across regions of an extended source.
- Dispersing element. Usually a diffraction grating: thousands of narrow lines ruled onto a glass surface. A grating reflects rather than refracts light, loses fewer photons than a prism and works in the ultraviolet.
- Camera optics. A camera mirror focuses the dispersed light onto the detector. Rotating the grating moves which portion of the spectrum lands on the detector.
- Detector. Photographic plates were used historically. CCDs can reach a quantum efficiency of about 90%, against roughly 1% for photographic emulsions, according to the same source.
Types and variants
| Variant | What differs | Source statement |
|---|---|---|
| Slit spectrograph | Records one object or region at a time | CSIRO ATNF: slit systems observe one object or region at a time |
| Multi-fibre spectrograph | Records many objects at once through fibres | CSIRO ATNF: the 2dF system records 400 spectra at once over a 2 degree field; OzPoz on the VLT records 132 simultaneously |
| Grating dispersion | Reflective ruled surface | CSIRO ATNF: fewer photons lost than a prism and usable in the ultraviolet |
| CCD detector | Electronic detector, about 90% quantum efficiency | CSIRO ATNF: compared with about 1% for photographic emulsions |
Worked example: Hubble STIS
NASA’s Goddard fact sheet for STIS gives real specifications for one spectrograph, which makes the abstract parts above concrete.
| Specification | STIS value | Source |
|---|---|---|
| Wavelength range | 1150 to 10300 angstroms | NASA Goddard STIS fact sheet |
| Resolving power R | 500 to 1.1 × 105 depending on mode | NASA Goddard STIS fact sheet |
| Detectors | One CCD (optical and near-infrared) and two MAMA detectors (near and far ultraviolet) | NASA Science, STIS page |
| Long-slit output | A spectrum along each of 1,024 pixel rows | NASA Science, STIS page |
| History | Installed 1997 (servicing mission 2), failed August 2004, repaired 2009 (servicing mission 4) | NASA Science, STIS page |
Resolving power is conventionally written R = λ/Δλ, the wavelength divided by the smallest wavelength difference that can be separated, so a higher R means finer spectral detail.
Spectrograph vs spectrometer vs spectroscope
Usage overlaps and sources define the words slightly differently. A common distinction treats a spectroscope as an instrument for viewing a spectrum, a spectrograph as one that records it, and a spectrometer as one that measures wavelengths and intensities. Because many instruments are sold under either name, read the specification (wavelength range, resolving power, detector) rather than relying on the name. For laboratory instruments built around the same principle, see Raman spectrometers, LIBS spectroscopy and fluorescence spectroscopy. For mass analysis, which is a different measurement, see mass spectrometers; for radio-frequency signals, see spectrum analyzers.
Trade-offs stated in the sources
- A grating loses fewer photons than a prism and reaches into the ultraviolet (CSIRO ATNF).
- Slit systems observe one target at a time; multi-fibre systems record hundreds at once but need fibre positioning across a wide field (CSIRO ATNF examples above).
- A long slit yields spatial information along the slit as well as wavelength (NASA STIS).
Frequently asked questions
What is a spectrograph?
An instrument that separates light into its component wavelengths and records the spectrum, typically with a slit, a grating, camera optics and a detector.
What is the difference between a spectrograph and a spectrometer?
Usage overlaps. The stricter distinction is that a spectrograph records a spectrum while a spectrometer measures wavelengths and intensities, but instrument names do not follow it consistently.
What does a spectrograph tell you?
According to NASA, spectra reveal the chemical content, temperature and motion of the light source.
Sources
- CSIRO Australia Telescope National Facility, Obtaining Astronomical Spectra: Spectrographs, read 2026-11-07.
- NASA Science, Hubble Space Telescope Imaging Spectrograph, read 2026-11-07.
- NASA Goddard, STIS fact sheet, read 2026-11-07.
