Metrology — An autocollimator is primarily used to check which of the following?

Introduction:
An autocollimator is an optical instrument that measures very small angular deviations with high precision. It is widely used in precision engineering, alignment, and calibration tasks where angular accuracy is critical (e.g., checking squareness, straightness via angular deflection, and mirror tilt).

Given Data / Assumptions:

• Optical setup uses collimated light and reflection from a mirror/target.
• Device resolves arc-seconds of angle.
• Applications include machine tool alignment and angle standard calibration.

Concept / Approach:

The autocollimator projects a beam that reflects from a mirror; the return beam's displacement on a detector/viewing scale is proportional to twice the mirror tilt angle. Thus, it directly measures angle (very small rotations). While it can be used indirectly to assess flatness or straightness by scanning along a surface, its primary measurement is angular deviation, not surface roughness or dynamic balance.

Step-by-Step Solution:

Verification / Alternative check:

Calibration labs use autocollimators with angle gauges and mirrors to realize small-angle standards, confirming angular focus.

Why Other Options Are Wrong:

A: Flatness can be inferred but not directly measured; the fundamental output is angle. C: Rotor balancing is a dynamic process using vibration sensors, not an autocollimator. D: Roughness is measured with profilometers. E: Thread pitch requires mechanical/optical comparators, not autocollimation.

Common Pitfalls:

Confusing angle measurement with linear displacement; autocollimators need a reflective surface of adequate quality.

Final Answer:

angle