Galileo’s Telescope

This was the telescope invented by Galileo. He used this to conduct groundbreaking research. His telescope is similar to the refractive telescope, in fact it was an earlier model. The difference is that the eye piece is a concave lens instead of a convex lens.

• 2 different lens
  • eye piece - concave lens
  • object lens - convex lens
• The first lens, the objective lens is used to gather light from a distant object, this then forms a real image inside the telescope tube, which acts as an object for the second lens, the eyepiece lens. The eyepiece lens is adjusted such that the object (the real image) is close to the lens, and hence it produces a much more magnified virtual image in the eyes of the observer.

Refractive Telescope

• This is also known as Kepler’s Telescope
• The refractive telescope is made up of two lenses. Both are convex.
• There is the objective lens(the focal length is $f_o$) and the eye piece(the focal length is $f_e$).

Chromatic Aberration

The image will be a combination of various different colored images. Hence the image will be blurry. As the light enters the lens the light will split in color.

Reflecting Telescope

• Objective mirror is a concave mirror.
• Secondary mirror is a convex mirror. Placed between focus of objective mirror and the eyepiece.
• The parallel rays are incidented on the objective mirror. Then these rays will reflect and go on a path to converge at the focus. But they will hit and reflect off of the secondary mirror first, where it will then reflect into the eyepiece.
• Reflecting telescope is free from chromatic and spherical aberration.

spherical aberration: