spectral resolution

Spectral resolution is a measure of a spectrograph's ability to distinguish features in the spectrum, typically noted as Δλ, meaning the smallest division in wavelengths that the spectrograph can distinguish, i.e., a distance, e.g., in nm. The (spectral) resolving power (R) is defined as:

    λ
R = ——
    Δλ

(Note that what is termed resolving power is very often labeled as the spectral resolution.) Examples:

• STIS (HST): distinguishes 0.17 nm resolution in a 1000 nm wave (yielding 5900 as the resolving power).
• CRIRES (ESO VLT): a resolving power up to 100,000.

There is a tradeoff between spectral resolution and throughput, i.e., how much light is necessary to produce the spectrum date, which is (apparent) brightness of the source times the time necessary to produce the spectral energy distribution (SED). For example, for extremely distant galaxies that are dim to us, spectrography may be impossible, and approaching such distances, higher-resolution SEDs may be impractical or impossible.

Radial velocity (RV) measurements using spectral lines depend upon spectral resolution, but far more precise RVs are now inferred than that implied by spectral resolution of the spectrograph: astrophysicists and engineers constantly develop the techniques contributing to such precise RVs, but a basic technique is to analyze the SEDs for a number of lines and/or from a number of observations and statistically work out the most likely RV-pattern to produce all of them, presuming the relatively-rough measured wavelengths of the spectral line are probabilistically scattered according to an appropriate probability density function, e.g., the normal distribution.

The term resolving power can also logically refer to that of angular resolution and is occasionally used as such.