A parallax-free micrometer is a precise measuring instrument primarily used in astronomy and engineering to measure distances or angles without the need for parallax correction, which can cause measurement errors. This micrometer operates using a system that minimizes parallax, thereby allowing accurate measurements.

Parallax is the apparent displacement of the position of an object when it is observed from different viewing angles. This phenomenon occurs because the observer's position changes relative to the object, creating the impression that the object moves relative to the background. In astronomy, for example, parallax is used to measure distances to nearby stars. In precision instruments, parallax can cause errors if not corrected, which is why parallax-free micrometers are designed to minimize or eliminate this phenomenon, thus ensuring accurate measurements.