Spectral resolution

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.^[1]