Spectral resolution

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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 \Delta \lambda , and is closely related to the resolving power of the spectrograph, defined as

R={\lambda \over \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.[1]

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={c \over \Delta v}

where 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 or frequency difference between two lines in a spectrum that can be distinguished.[2] Resolving power, R, is given by the transition wavenumber, wavelength or frequency, divided by the resolution.[3]

See also

References

  1. ↑ - CRIRES Instrument page at ESO
  2. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "resolution in optical spectroscopy".
  3. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "resolving power, R, in optical spectroscopy".

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)
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