Optics for Magnification Change: Galilean Telescope
The Galilean telescope used in the reversed configuration, and positioned in front of an objective lens, is a traditional way for adding wide field of view capability to a narrow field of view system. The Galilean, an afocal unit, introduces no change in focus to the following optics.
The “Galilean” comprises a negative “eye lens” and positive objective lens as shown in the thin-lens diagram above. Magnification is the ratio of the focal lengths. The image is erect.
The resultant system magnification when the Galilean is placed in front of a telescope is the product of the Galilean and telescope magnifications. When located in front of an objective lens the resultant system equivalent focal length is the product of the Galilean magnification and the focal length of the objective lens.
The reversed Galilean configuration permits the system entrance pupil to be near the Galilean Eyelens. This results in a relatively small ray bundle envelope cross-section at the entrance to the Galilean.This in turn maintains a correspondingly small size for a head window, mirror or prism which would be located in front of the optics.
In a submarine periscope the head prism size must be minimized in order to have a small overall periscope head profile. The illustration below of a detailed lens design illustrates these points.
The Galilean can also be implemented in a normal, unreversed configuration to increase the system magnification. This can be accomplished by its placement in front of an objective lens or its placement within a collimated light space of relay optics.
See “Tri-Power Periscope” for an example where two Galileans are used in tandem to achieve three different magnifications.