Which of the following is NOT the advantage of the internal focusing telescope?

Question

Question

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PGCIL DT Civil 13 Aug 2021 Official Paper (NR I)

In making measurement by the stadia method, an instrument constant is almost eliminated and computations are thus simplified
The interior of the telescope is so easily accessible for field cleaning and repairs
Wear on the rack and pinion is less due to lesser movement of negative lens
Line of collimation is less likely to be affected by focusing

Answer 1

Explanation:

Internal Focusing Telescope

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

Advantages of Internal Focusing Telescope:

The 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 the telescope is not affected during focusing. As a result of it, the position of the bubble of the level tube is not much disturbed during focusing.
Since it needs the 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 the eyepiece and objective lens are not disturbed during focusing.
As the tube moving during focusing is not exposed to the atmosphere, the telescope is practically dust free and waterproof.

Disadvantages of Internal Focusing Telescope:

The additional lens between the eyepiece and the objective reduces the brightness of the image.
Maintenance, cleaning, and repair are difficult.