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All telescopic low vision aids are based upon two principal designs of distance telescopes: the Galilean, and the Keplerian telescope.
In the Galilean system the exit pupil appears within, or internal to the telescope system and in the Keplerian telescope the exit pupil is external and virtual to the system, that is to say, it appears between the eye and the eyepiece (see Figure 4).
A rubberised eyepiece cup, present on the end of a Keplerian telescope, assists patients in locating the exit pupil as it allows the patient to rest the telescope within the orbit and closer to the eye.
Caption: Figure 3 A roof prism system is placed within the barrel of the Keplerian telescope, which produces a straight optical light path
Dutch scientist Christiaan Huygens discovers Saturn's rings and its moon Titan using a Keplerian telescope with an 11-foot focal length.