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An internal focussing type of telescope is focussed by the movement of:-
1. Convex lens
2. Concave lens
3. Plano-convex lens
4. Objective glass

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Correct Answer - Option 2 : Concave lens

Concepts:

In an Internal focusing telescope, eyepiece and objective lens are fixed and focusing is achieved by movement of double concave lens mounted in a short tube.

Advantages of Internal Focusing Telescope:

  1. Overall length of the tube is less and hence it is lighter.
  2. It can be easily rotated about its axis.
  3. The sliding part is light and is located in the middle. Hence the balance of telescope is not affected during focusing. As a result of it, position of bubble of level tube is not much disturbed during focusing.
  4. Since it needs lesser movement of focusing lens wear on the rack and pinion is less.
  5. The line of collimation is less likely to be affected since the positions of eyepiece and object lens are not disturbed during focusing.
  6.  As the tube moving during focusing is not exposed to atmosphere, the telescope is practically dust free and waterproof.

Disadvantages of Internal Focusing Telescope:

  1. The additional lens between the eyepiece and objective reduces the brightness of the image.
  2. Maintenance and repair is difficult.  

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