FIES is a cross-dispersed high-resolution echelle spectrograph with a maximum spectral resolution of R = 67000. The entire spectral range 370-830 nm is covered without gaps in a single, fixed setting. Extended coverage up to 910 nm is available with minor inter-order wavelength gaps. To isolate it from sources of thermal and mechanical instability, FIES is mounted in a heavily insulated building separate from and adjacent to the NOT dome.

The optical fibres that connect the telescope with the spectrograph are permanently mounted with a movable 45-degree mirror near the focal plane of the telescope. The fibres run from the instrument adapter through the telescope and azimuth axis to the spectrograph enclosure. Being a permanently mounted instrument, FIES can be used at any time, also for short periods of time while other instruments are mounted.

The currently installed fiber bundle contains a high-res 1.3 arcsec fibre offering a spectral resolution of R=67000. This bundle also provides two 1.3 arcsec fibres giving R=46000 (med-res), and a fibre with a larger entrance aperture of 2.5 arcsec but a lower spectral resolution (R=25000, low-res).

FIES offers the option of simultaneous observations of wavelength reference (Thorium-Argon) spectra, for the medium- and high-resolution modes. Simultaneous sky spectra can be obtained in medium-resolution mode, restricted to wavelengths shorter than 650nm. An exposure meter is available to predict the S/N of an ongoing exposure, allowing to optimise the exposure time. A later upgrade of FIES to full polarimetric capability is being investigated.