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

$$R = {\lambda\over\Delta\lambda}$$

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 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.

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

$$R = {c\over\Delta v}$$

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