The present study concerns the determination of the optimal focal curve of the reflected beam from a spherical mirror. This study has been used in a stigmatic high resolution UV-A spectrograph operating in UV-A region. As a light source we have used a low pressure air discharged lamp while the light detector was a cooled CCD camera with a resolution of 1600×1200 pixels. Locating it on the optimal position along the useful spectral range allows us to exploit the full resolution provided by the spectrograph. An analytical expression of the focal curve has been derived as a function of the main geometrical parameters of the spectrograph and the focusing spherical mirror. A set of experimental validation tests have been performed recording several band heads and the associated molecular rotational lines of nitrogen provided by the lamp. The obtained result has been used in high resolution spectroscopy relevant to air fluorescence yield determination in Ultra High Energy Cosmic Ray research.