Editing Optical telescope (section)

===Inverted images===
{{Further|Star diagonal|Relay lens}}
Most telescope designs produce an inverted image at the focal plane; these are referred to as ''inverting telescopes''. In fact, the image is both turned upside down and reversed left to right, so that altogether it is rotated by 180 degrees from the object orientation. In astronomical telescopes the rotated view is normally not corrected, since it does not affect how the telescope is used. However, a mirror diagonal is often used to place the eyepiece in a more convenient viewing location, and in that case the image is erect, but still reversed left to right. In terrestrial telescopes such as [[spotting scope]]s, [[monocular]]s and [[binoculars]], prisms (e.g., [[Porro prism]]s) or a relay lens between objective and eyepiece are used to correct the image orientation. There are telescope designs that do not present an inverted image such as the [[Refracting telescope#Refracting telescope designs|Galilean refractor]] and the [[Gregorian telescope|Gregorian reflector]]. These are referred to as ''erecting telescopes''.