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If external focusing telescope is used in tacheometer when that extra lens provides is known as___
Convex glass
Concave glass
Anallatic lens
In A or C
In A or C
An extra lens provided in an external focusing telescope of a tacheometer is an anallatic lens, which is physically a convex glass lens. It is fitted between the object glass and eyepiece to eliminate the additive constant C=(f+d), making C=0. Hence, both Option A (Convex glass) and Option C (Anallatic lens) describe this lens, making Option D the correct choice.
An extra lens provided in an external focusing telescope of a tacheometer is an anallatic lens, which is physically a convex glass lens. It is fitted between the object glass and eyepiece to eliminate the additive constant C=(f+d), making C=0. Hence, both Option A (Convex glass) and Option C (Anallatic lens) describe this lens, making Option D the correct choice.
D=K⋅s+C — General tacheometric distance equation
K=if=100 — Multiplying constant
C=f+d=0 — Additive constant (with anallatic convex lens)
In an ordinary external focusing telescope, the tacheometric distance equation is D=K⋅s+C, where C=(f+d) is the additive constant. By inserting a special convex lens (anallatic lens) between the object glass and the eyepiece at a fixed distance, the apex of the measuring triangle is shifted directly to the instrument's vertical axis. This forces the additive constant C to become zero, simplifying field calculations to a direct linear proportion D=K⋅s.
An anallatic lens is a convex glass lens fitted inside an external focusing telescope between the objective glass and the eyepiece.
Its primary objective is to make the additive constant C=(f+d) equal to zero.
It simplifies field calculations, enabling direct distance calculation as D=100×s where s is the staff intercept.
Internal focusing telescopes inherently have an additive constant close to zero, so an additional anallatic lens is generally not required.
Simplifies distance computations in tacheometric surveying by eliminating the additive term.
Direct calculation saves time and reduces manual computation errors during field mapping.
Reduces the brightness and clarity of the image due to light absorption through the extra glass lens.
Increases the internal complexity and cost of the telescope assembly.
Topographic mapping and detail surveying in rugged, inaccessible terrains.
Reconnaissance surveys for highways, railways, and hydrographic projects.
Option A (Convex glass): Correct regarding the optical type and material of the lens.
Option B (Concave glass): Incorrect, as a concave lens would diverge light rays instead of converging them to shift the analytical center.
Option C (Anallatic lens): Correct functional name of the additional lens used in tacheometry.
Option D (In A or C): Correct choice, as the extra lens is functionally called an 'Anallatic lens' and structurally made of 'Convex glass'.
D is correct — The additional lens provided in an external focusing telescope is functionally called an anallatic lens, which is structurally a convex glass lens.
Remember that an anallatic lens is used specifically in external focusing telescopes to make C=0; internal focusing telescopes inherently have an additive constant close to zero.