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Image of telescope and separation between rear lens and real image of object

Notes about Keplerian ‘expanded field’ telescopes.

An invited blog contribution by Alan Johnston, OD, FAAO

Dr. Alan Johnston is an Australian optometrist and a 1985 low vision diplomate of the American Academy of Optometry.  He practices in Melbourne, Australia.

Some optical facts:

Keplerian telescopes provide additional benefits for bioptic use over Galilean types.  Originally designed for astronomical use, Keplerians provide a wider field of view than Galileans of comparable magnification (Mx). 

Keplerian telescopes for astronomy provide a reversed and upside-down image, but for patients the image must be upright and translated left-to-right.  Image inversion/translation was achieved at first using Porro prisms, named after their Italian inventor.  Porro prism binoculars have the characteristic, dog-leg shape where the front lenses are more widely spaced than the lenses close to the eye.  A more compact prism design is the lightweight Pechan-Schmidt combination, known generally as roof prisms, which have become popular in higher quality binoculars.  Prisms offer the additional benefit of ‘folding’ the optical path hence shortening the physical length of the telescope.   

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