A 4x Galilean telescope used in the reverse configuration in front of the objective lens has traditionally served as the means to achieve wide field of view in periscopes. It converts a periscope with 6x magnification, 8 degree field of view into one with 11/2x , 32 degrees field of view. The objective and eye lenses of the reversed Galilean are mounted for insertion and removal from the optical path to achieve magnification change.
A 12x, 4 degree field for highest resolution is accomplished by introduction of a 2x Galilean telescope into the periscope optics. Since there is no space available in the periscope head for the addition of more optics, the 2x Galilean is located in the collimated light region of the mast relay lenses. The objective and eye lenses are necessarily larger diameter components and are inserted and removed from the optical path, like the reversed 4x Galilean in the head, to achieve magnification change.
An alternate approach to achieving these magnifications and fields of view is offered by the Tri-Power Periscope concept, illustrated in the thin-lens diagram below
Two compact reversible Galilean telescopes, one with 2x magnification, the other with √2x magnification, are located one after the other in front of a fixed objective lens. Since the Galileans are afocal, parfocality between modes is maintained. A mast relay lens train and eyepiece (not shown) follow the objective lens. System optical data are shown for the three modes assuming a 71.7 mm focal length eyepiece.
Transition from 12x to 6x occurs with reversal of the √2x Galilean and from 6x to 1 ½x with additional reversal of the 2x Galilean. In the Tri-Power concept all optics are in the optical path at all times unlike the reversed Galilean concept. This offers certain optical image quality and mechanical packaging and operation advantages as explained in U.S. Patent 4,017,148. The figure below from the patent illustrates detailed design implementation of the concept.