Placing a normal eyepiece straight into the focuser of a Newtonian reflector, refractor, Schmidt-Cassegrain (SCT) or Maksutov-Cassegrain telescope produces an upside-down image.
However, all but Newtonian reflectors are normally used with a star diagonal to make the viewing angle more comfortable and this will correct the image vertically but flip it side to side!
If you absolutely must have a correctly orientated view or you want to use your astronomical telescope for terrestrial observing then you can use a special diagonal containing an Amici prism, which will present a correctly orientated view in refractors, SCTs and Maksutov-Cassegrains.
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