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:
- Overall length of the tube is less and hence it is lighter.
- It can be easily rotated about its axis.
- 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.
- Since it needs lesser movement of focusing lens wear on the rack and pinion is less.
- The line of collimation is less likely to be affected since the positions of eyepiece and object lens are not disturbed during focusing.
- As the tube moving during focusing is not exposed to atmosphere, the telescope is practically dust free and waterproof.
Disadvantages of Internal Focusing Telescope:
- The additional lens between the eyepiece and objective reduces the brightness of the image.
- Maintenance and repair is difficult.