Refracting Telescope

The refracting telescopes are of two types:

  1. Astronomical telescopes are used to observe heavenly or astronomical bodies.
  2. Terrestrial telescopes are used to see distant objects on the earth.

Astronomical Telescope

An astronomical telescope produces a virtual and erect image. As heavenly bodies are round, the inverted image does not affect the observation. This telescope consists of a two lens system. The lens facing the object has a large aperture and large focal length (fo). It is called the objective. The other lens, which is towards the eye, is called the eye lens. It has a small aperture and short focal length (fe). The objective and eye piece are mounted co-axially in two metallic tubes.

The objective forms a real and inverted image of the distant object in its focal plane. The position of the lens is so adjusted that the final image is formed at infinity. (This adjustment is called normal adjustment.) The position of the eyepiece can also be adjusted so that the final image is formed at the least distance of distinct vision.

Case 1: When the final image is formed at infinity (Normal adjustment), the paraxial rays coming from a heavenly object are parallel to each other and they make an angle α with the principal axis. These rays after passing through the objective, form a real and inverted image in the focal plane of objective. In this case, the position of the eyepiece is so adjusted that the final image is formed at infinity.

Magnifying power of a telescope is defined as the ratio of the angle subtended by the image at the eye as seen through the telescope to the angle subtended by the object at objective when both the object and the image lie at infinity. It is also called angular magnification and is denoted by M.

M = β/α

Since αand βare small, they can be replaced by their tangents.

M = (tan β)/(tan α)

M = fo/fe

The magnifying power of a telescope in normal adjustment will be large if the objective is of large focal length and the eyepiece is of short focal length. The length of telescope in normal adjustment is (fo + fe).

Case 2: When the final image is formed at the least distance of distinct vision, the paraxial rays coming from a heavenly object make an angle αwith the principal axis. After passing through the objective, they meet on the other side of it and form a real and inverted image AB. The position of the eyepiece is so adjusted that it finally forms the image at the least distance of distinct vision.

Magnifying Power

M = β/α = (tan β)/(tan α)

M = - (fo/fe)(1+fe/D)

The negative sign of magnifying power of the telescope suggests that the final image is inverted and real.