Refracting telescope

Refracting telescopes, including Keplerian telescopes or Galilean telescopes, use lenses to produce inverted, magnified, virtual images. Here we make a Keplerian telescope from simple elements and explain its operation using a ray diagram. This page supports the multimedia tutorial Geometrical Optics.

Refracting telescope and schematic

Two converging lenses, an objective and an eyepiece, make a refacting or Keplerian telescope. Schematic at right.

In a telescope, the objective lens should have a long focal length: it is the large lens at left in the photo. A shorter focal length converging lens is at right. (Looking into the objective lens in this side view photo, we see an inverted, diminished, virtual image of the woman passing by. We also see an inverted virtual image in the eyepiece lens in the photo.) To simplify things, we have no tube to shade the lenses.

The schematic at right shows a ray diagram. We put the lens separation equal to the sum of the focal lengths. This means that the image of a very distant object produced by the objective is at the focus of both lenses. This means that the virtual image created by the eyepiece is also very distant, and so can be viewed by a relaxed eye.

Looking through the telescope

A view from the video camera without (left) and with (right) the telescope.

At left, we show a still from the camera looking across the cricket field. For the movie at right, we changed no settings on the camera, but pointed it through the telescope picture in the previous photo. In both left and right, we see the fence on the far side, and a man and a scoreboard near it. We see that the image is inverted, magnified and virtual. Note that, despite the magnification, the brightness of the image is comparable with that of the object. The reason is that the large objective lens gathers more light. We also see chromatic aberration, explained in this link. It was to avoid chromatic abberation that Newton invented the reflecting telescope.

Links and further information

  • Acoustic telescope
  • Chromatic dispersion, rainbows and Alexander's dark band
  • Dispersion and chromatic aberration
  • Lenses and images
  • Microscopes and magnifiers
  • Mirrors and images
  • Newton's prisms
  • Reflecting Newtonian telescope
  • Snells law and refraction
  • The multimedia tutorial Geometrical Optics also has a longer list of support pages
  • Creative Commons License This work is licensed under a Creative Commons License.