Spectral resolution

The spectral resolution or resolving power of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its power to resolve features in the electromagnetic spectrum. It is usually defined by

$R\; =\; \{lambdaoverDeltalambda\}$

where $Deltalambda$ 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 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 resolution of up to 100,000 [ [http://www.eso.org/instruments/crires/ - CRIRES Instrument page at ESO] ] .

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 definition is

$R\; =\; \{coverDelta\; v\}$

where $c$ is the speed of light. The STIS example above then has a spectral resolution of 51 km/s.

References

# Kim Quijano, J., et al. (2003), "STIS Instrument Handbook", Version 7.0, (Baltimore: STScI)