Alignment requirements in PRIMA BES diagnostics

This document reports the requirements in the alignment precision of the lines of sight of BES diagnostics in both SPIDER and MITICA, for hydrogen and deuterium filling. The accuracy in the mounting of the optical heads, that are responsible of the position and direction of the LOSs, affects the performances of the whole BES diagnostic. For this reason tolerances in the mounting must be set in order to not spoil the measurements and to drive the design of the mounting interfaces between the diagnostics (optics) and the vessel (observation ports) To complicate the picture, it should be considered that the direction and position of SPIDER and MITICA beams with respect to the vessel can differ from their nominal values. Since normally the optical head mechanical alignment is made referenced to the vessel, but what really counts is their alignment referenced to the beam, special solutions in the alignment procedure may be required. The increase of the optical head lens diameter is also discussed as a possible solution.