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Spectral resolution

Resolving power of a spectrograph

Resolving power of a spectrograph

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 \Delta\lambda, and is closely related to the resolving power of the spectrograph, defined as R = \frac{\lambda}{\Delta\lambda}, where \Delta\lambda is the smallest difference in wavelengths that can be distinguished at a wavelength of \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 \Delta v that can be distinguished through the Doppler effect. Then, the resolution is \Delta v and the resolving power is R = \frac{c}{\Delta v}, where c is the speed of light. The STIS example above then has a spectral resolution of 51.

IUPAC definition

IUPAC defines resolution in optical spectroscopy as the minimum wavenumber, wavelength or 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.

References

References

  1. [http://www.eso.org/instruments/crires/ - CRIRES Instrument page at ESO]
  2. "resolution in optical spectroscopy".
  3. "resolving power, R, in optical spectroscopy".
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