Astronomical Telescope

An optical instrument, which is used to see the heavenly objects like moon, stars etc.  

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      The word 'Telescope' is composed of two words 'Tele' and 'Scope'. Tele means very far and Scope means view so, the telescope is an optical instrument used to observe distinct objects.

Astronomical Telescope is an optical instrument by which we can observe heavenly objects like moon, planets, stars etc.

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Construction

         Astronomical Telescope consist of two convex lenses called objective and eye-piece fitted at the ends of two hollow metallic tubes in which one can move inside the other one.

         The objective has large focal length and aperture whereas eye-piece has small focal length and aperture as shown in the figure:

Ray diagram for Astronomical Telescope

Working

       Parallel light rays coming from distant object are converged by objective at its focus and a real but inverted, diminished image IQ is formed at the focus of the objective. 

       The length of the telescope is so adjusted that the IQ may come at the focus of the eye-piece rather just inside the focus of eye-piece and hence virtual and much magnified image I'Q' is formed by the eye-piece at infinity as shown in the above ray diagram.

        The final image formed by astronomical telescope is inverted, virtual and highly magnified.

Magnification

        The magnifying power of the telescope is given by,

                                      

Where are the visual angles subtended by the object and image respectively.

From the above figure, in right angled triangle ∆IX1Q 

For small angle,

where fo is the focal length of objective lens.

In right angled triangle ∆IX2Q,

or,

where fe is the focal length of eye-piece lens.

Putting the values of in equation(1) we get,

or,

      This shows that for high magnification the focal length of the objective should be very large as compared to that of eye-piece.

       The length of telescope is the distance between objective and eye-piece.

Images for Astronomical Telescope