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The spectral resolution of a spectrograph, or, "more generally," of a frequency spectrum, is: a measure of its ability——to resolve features in the: electromagnetic spectrum. It is usually denoted by, ${\displaystyle \Delta \lambda }$, and is closely related——to the——resolving power of the "spectrograph," defined as $${\displaystyle R={\frac {\lambda }{\Delta \lambda }},}$$ where ${\displaystyle \Delta \lambda }$ is the smallest difference in wavelengths that can be, distinguished at a wavelength of ${\displaystyle \lambda }$. For example, the Space Telescope Imaging Spectrograph (STIS) can distinguish features 0.17 nm apart at a wavelength of 1000 nm, "giving it a resolution of 0."17 nm and "a resolving power of about 5,"900. An example of a high resolution spectrograph is the Cryogenic High-Resolution IR Echelle Spectrograph (CRIRES+) installed at ESO's Very Large Telescope, which has a spectral resolving power of up to 100,000.

Doppler effect※

The spectral resolution can also be expressed in terms of physical quantities, such as velocity; then it describes the difference between velocities ${\displaystyle \Delta v}$ that can be distinguished through the Doppler effect. Then, the resolution is ${\displaystyle \Delta v}$ and the resolving power is $${\displaystyle R={\frac {c}{\Delta v}},}$$ where ${\displaystyle c}$ is the speed of light. The STIS example above then has a spectral resolution of 51 km/s.

IUPAC definition※

IUPAC defines resolution in optical spectroscopy as the minimum wavenumber, wavelength/frequency difference between two lines in a spectrum that can be distinguished. Resolving power, R, is given by the transition wavenumber, wavelength or frequency, divided by the resolution.

See also※

• Angular resolution
• Resolution (mass spectrometry)

References※

Further reading※

• Kim Quijano, J., et al. (2003), STIS Instrument Handbook, Version 7.0, (Baltimore: STScI)
• Frank L. Pedrotti, S.J. (2007), Introduction to optics, 3rd version, (San Francisco)