# Resolution

## Resolution estimation

iSpec reports an estimate of the resolution based on the FWHM of the telluric lines.

This estimation can be found in the velocity determination function relative to telluric lines and it is obtained by the following equation:

$$R = \frac{c}{ (FWHM_{telluric} - FWHM_{theoretical}) }$$

where $c$ is the speed of light in the vacuum (km/s), $FWHM_{telluric}$ (km/s) correspond to the telluric lines observed in the spectrum and $FWHM_{theoretical}$ (km/s) to the theoretical telluric lines.

The resolution is also reported when iSpec determines the velocity profile relative to the atomic data, however this is a worse estimator since it uses directly the FWHM measured and there are other effects (apart from the spectral resolution) that contribute to the broadening of the absorption lines (i.e. star's rotation).

In any case, it is preferable to know the real resolution of the spectra and not rely on these estimation since the error margin can be quite large.

$$FWHM = \frac{wavelength}{ final\_resolution }$$