How telescope forms magnified image

mgibson (66)in #science • 9 years ago

Object lying at a large distance from an observer appears very small, however it may be quite large. Telescopes are used in order to make a distant object brighter and bigger, so that the observer can have a clear vision of such a distant object.

The optical system of a telescope, just like the compound microscope, has two important components that are the objective and the eye-piece. The objective lens creates a real, inverted and diminished image of the distant object while the eye-piece forms a virtual and magnified image of the real image produced by the objective lens.

Telescope may be divided in three classes:

Refracting Telescope
Reflecting Telescope
Catadioptric Telescope, which consists of lenses and mirrors to gather light

Refracting Telescope

Refracting Telescope was discovered by Hans Lippershey of Middleburg in 1608. The instrument was used by the soldiers first time and Galileo was the first person who used it in space research. The designs of refracting telescope made by both Lippershey and Gallieo had a combination of concave and convex lenses. In 1611, Kepler used two convex lenses in his design to make the image upside-down. His design is still considered as the major design of refractive telescope. They consist of the following parts:

A long tube made of metal, wood, or plastic
An objective lens at the front end
An eye-piece

Image credit Refracting Telescope

The lenses are placed in the tube at the right distance from one another. The tube prevents moisture, dust and light from entering inside the telescope that may obstruct a good image to be formed. The objective lens collects and refracts the light to a meeting point which is close to the back of the tube. The eye-piece brings the magnified image to your eye. The focal lengths of these eyepieces are generally much shorter than objective lenses.

Reflecting Telescope

Reflecting telescope was discovered by Isaac Newton in 1680 in order to stop the chromatic aberration problem that gave much trouble to the refractors. Instead of a lens, a curved metal mirror was used as primary mirror to collect and reflect the light to a focal point. Alike the lenses, mirrors don’t produce the chromatic aberration problems. The curved metal mirror is fixed in the back of the tube.

Image credit Reflecting Telescope

Newton placed a small flat mirror in the focal path of the primary mirror to change the direction of the reflected light from the curved metal mirror or primary mirror to the eye-piece. Despite the secondary mirror being placed in the way of the incoming light towards the primary mirror, the image would not be blocked by the secondary mirror, because its size is much smaller than the primary mirror.

Catadioptric Telescope

Catadioptric telescopes consist of both reflector and refractor elements. Bernhard Schmidt, a German astronomer, invented the catadioptric telescope first in 1930. In the Schmidt telescope, a primary mirror was placed at the back of the tube, and a glass corrector was placed in the front of the tube so that spherical aberration can be removed. These types of telescope were mainly made for photography. Instead of secondary mirror or eyepiece, Catadioptric telescope had a photographic film which was placed at the focal point of the primary mirror. Today the design of the Schmidt-Cassegrain, which was later discovered in 1960, is very popular telescope. It has a secondary mirror that bounces the reflected light from the primary mirror through a hole to an eyepiece.