The simplest Kepler telescopes (named after astronomer Johannes Kepler, 1571 – 1630) consist of a collecting lens for the objective, and a collecting lens for the eyepiece. The objective lens projects an upside-down, reversed image in the intermediate image plane. Cross-line grids can be added here to estimate distance, or data, e.g. from a compass or range finder, can be injected. Because the image is upside down and reversed, a Kepler telescope requires an erecting system (prisms or lens elements) if it is to be used for earth observation.
All modern binoculars and riflescopes are Kepler telescopes.