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Which of the following additive constant for internal focusing techeometer ?
0 cm to 5 cm
5 cm to 10 cm
8 cm to 15 cm
15 cm to 20 cm
8 cm to 15 cm
An internal focusing telescope incorporates a concave lens fitted between the objective lens and the eyepiece to alter the focal length during focusing. In an internal focusing tacheometer, the additive constant C=f+d is significantly reduced compared to external focusing instruments, typically lying in the range of 8 cm to 15 cm (0.08 m to 0.15 m).
An internal focusing telescope incorporates a concave lens fitted between the objective lens and the eyepiece to alter the focal length during focusing. In an internal focusing tacheometer, the additive constant C=f+d is significantly reduced compared to external focusing instruments, typically lying in the range of 8 cm to 15 cm (0.08 m to 0.15 m).
D=KтЛЕS+C тАФ Tacheometric horizontal distance equation
K=ifтАЛ тАФ Multiplying constant (typically set to 100)
C=f+d тАФ Additive constant (focal length + distance from objective to instrument axis)
In external focusing telescopes, the objective glass moves, resulting in a larger additive constant C=(f+d) around 30 cm to 45 cm. Internal focusing telescopes use a movable internal double-concave lens between the objective lens and cross-hairs, which drastically reduces the physical distance d between the vertical axis and the objective lens. This design keeps the telescope length fixed and reduces the additive constant to a small range between 8 cm and 15 cm.
For an external focusing telescope, the additive constant C ranges from 30┬аcm to 45┬аcm (0.30┬аm to 0.45┬аm).
For an internal focusing telescope, the additive constant C ranges from 8┬аcm to 15┬аcm (0.08┬аm to 0.15┬аm).
When an anallatic lens is fitted in a tacheometer telescope, the additive constant C becomes exactly zero (C=0).
Fixed tube length prevents dust and moisture entry into the telescope barrel.
Small additive constant reduces instrumental error in stadia measurement calculations.
Slight loss of illumination due to reflection and absorption at the additional internal lens surfaces.
Internal mechanism makes lens cleaning and alignment adjustment more complex.
Determining horizontal distance D directly without physical chaining in rough terrain.
Preparation of topographic maps and contouring in steep or inaccessible areas.
Option A (0 cm to 5 cm) is incorrect as zero additive constant is achieved specifically using an anallatic lens, not standard internal focusing.
Option B (5 cm to 10 cm) and Option D (15 cm to 20 cm) are outside the standard empirical specification range of 8 cm to 15 cm for internal focusing tacheometers.
C is correct тАФ The additive constant for an internal focusing tacheometer lies in the standard range of 8 cm to 15 cm.
Remember for competitive exams: External focusing C=30тИТ45┬аcm, Internal focusing C=8тИТ15┬аcm, and Anallatic lens fitted C=0.
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