The maximun espectral resolution (Rmax = / ) FOCES can achieve, is fixed by the given camera focal length, the echelle grating and its blaze angle. For a 2 pixel resolution element () is R=40650 with a 24 µ pixel and R=65050 with a 15 µ pixel. In practice, it ranges from 38800 in the blue part of the spectrum to 40600 in the red for a 24 µ pixel and from 62000 to 65000 for a 15 µ pixel.
This 2 pixel resolution depend directly on dispersion, that is one of the main characteristics of the spectrograph design, colimator focal length, etc ... However, resolution as the ability to separate two joined lines or as a measure of broadening of single lines, depend also on the slit width. Namely, or lines width depend on slit width . For a given slit width W(µ), the espectral resolution (R < Rmax) is calculated as R=6536457/W
A slit narrower than the diaphragm diameter imply light loss.

In order to measure FOCES spectral resolution we took 3 frames for every slit width. After bias subtraction, flat fielding and extraction, the spectra were wavelength calibrated.
For every echelle order in each frame, 5-10 line widths are measured fitting a gaussian to the line profiles.

Next tables present the empirical results for FOCES spectral resolution and its dependency on the slit width (first column). For every echelle order the wavelength (in angstroms) is given by the mean of the lines used.
Actually, in some cases the values are not accurate enought due to the semiautomatic method used to measure the big amount of lines (aprox.29000) and can be improved.