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The optical system used in the typical contemporary submarine periscope is unique in that, like the borescope, the length-to-diameter ratio of the optical system may approach a value of 100:1. Figure 9.1 illustrates a modern submarine, and shows the relationship of the periscope to the rest of the boat. Note that the periscope length is chosen such that when in the down position the periscope is completely housed within the boat. When in the up position, the periscope penetrates the surface, allowing the viewer to observe items on and above the surface, while the boat hovers at a keel depth of some 60 feet. Structural design considerations dictate a maximum periscope diameter of 7.5 inches. This in turn leads to a limitation on the diameter of the internal optics of about 100 mm (4 inches).
The submarine periscope is typically made up of three distinct optical modules: the head, the mast, and the eyepiece box. The head module contains the head window, the head prism, the objective lens, and the telemeter. The mast module contains six relay lenses and two collector/field lenses. The eyepiece box module contains a folding mirror or prism, a stanching window, and the eyepiece.
The head window will generally be racetrack shaped, with a clear aperture of about 50 x 150 mm. The window will have tapered sides and will be installed such that the undersea pressure that will be encountered will assist in forming a reliable seal. The head prism is a right-angle prism which deflects the incoming line of sight, directing it downward into the boat. The head prism will be designed to rotate about a horizontal axis to provide elevation scan capability (Fig. 9.1). Horizontal, or azimuth, scanning of the line of sight is accomplished when the viewer rotates the entire periscope, and himself, about the vertical axis through an unlimited (n x 360-deg) scan angle.
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