It is an optical instrument used to view heavenly bodies such moon, stars, planets and distant objects.
Astronomical telescope consists of two convex lenses:

  • Objective
  • Eye piece
The objective is a convex lens of large focal length and large aperture. It usually made of two convex lenses in contact with each other to reduce the chromatic and spherical aberrations.
Eye piece
The eye piece is also a convex lens. Its focal length is smaller than that of objective. It is also a combination of two lenses.
The objective is mounted on a wide metallic tube while the eye piece is mounted on a small tube . The distance b/w the eye piece and the objective can be changed by moving tubes.

The rays coming from a distant object falls on objective as parallel beam at some angle say “a” and these rays after refraction and passing through the objective converge at its focus and make an inverted & real image AB. This image acts as an object for the eye piece. The distance of the eye piece is so adjusted that the image AB lies within the focal length of the eye piece. The eye piece forms the final image .The final image is magnified ,virtual and inverted with respect to object. The final image is formed at infinity.

The magnifying power (M) of astronomical telescope is given by:
It is because the object is at infinite distance and hence the angle subtended by the object at eye may be taken as the angle subtended by the object at objective.
M = b/a …………(1)
since a and b are small angles, therefore we can take:
a = tan a……………….
b = tan b………….
In right-angled triangles DAOB & DAEB
This expression shows that in order to obtain high magnification, focal length of object must be large and that of eye piece is small.
The distance b/w objective lens and the eye piece is equal to the length of the telescope.
From figure:
OE = length of telescope =L
OE = OB + BE
OB = Fo & BE = Fe
L = focal length of objective + focal length of eye piece