Why aren't all Keplerian telescopes corrected by a third lens by default?

Question

The telescope that I got as a gift is a Keplerian type (all eyepieces are convex) resulting in an inverted (mirror) image of the objects I'm looking at. Is there a practical reason why these telescopes are not all by default corrected with a third lens? Apparently a prism can be purchased for that purpose to correct terrestrial images. Is it then widely accepted for celestial sights to be displayed inverted?

Answer 1

Since there is no "right-side-up" in the sky, image orientation doesn't matter in an astronomical telescope. An erecting lens or prism adds cost and complexity, and reflections off the additional glass surfaces can reduce brightness and contrast. Also, the common erecting prisms produce a diffraction spike.

Answer 2

It IS widely accepted for celestial sights to be displayed inverted. The cost of the extra lens has long been a reason, but more lenses also can cause image degradation.

By the way, inverted and mirror image are not the same. You can read a book that is upside down by turning yourself upside down to look at it, but a mirror image in not readable (easily) no matter what position you put yourself in.

Answer 3

You'll never ever see the term "Keplerian" used to refer to a refractor. It fact, "Keplerian" refers to the eyepiece: a single convex lens, and only the cheapest toy telescopes have eyepieces like that. It's rare to see any eyepiece with fewer than two (Huyghens or Ramsden) or three (Kellner) lens elements. Astronomical telescopes are never fitted with erecting lenses or prisms because of light loss, diffraction spikes, and other aberrations. There's no right way up in space.

Answer 4

Astronomical telescope normally invert the image. the addition of another lens or prism to make the image right side up will cause additional light absorption in the telescope and is not needed for astronomical images.

Answer 5

Each time light goes through a lens it loses in both brightness and focus. Since it does not matter when looking at stars if they are upside down or not, the astronomers went with quality rather than "comfort" of having up be up.