Alternative term: Focuser (G.B.)
An optical device used for focusing (2-2-155), for example, a short telescope with cross-wire, fixed to the frame of the optic.
Note: In France and Germany, the type of focusing device mainly used projects, by means of lenses and prisms, an image of the luminous element of the light source on a screen fitted with a cross-wire and usually also with concentric rings.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
centre of gravity
Centre of buoyancy
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM