Electronic Autocollimator

Overview

An autocollimator measures very small angles, the tilt of a mirror, the squareness of a face, the straightness of a guideway, without touching the part. It works by projecting a pattern out to infinity, bouncing it off a flat target mirror on the part, and measuring how far the returned image has shifted. Because the shift is proportional to twice the mirror's tilt and is read on a sensitive sensor, the instrument resolves angles far finer than a protractor or level: hundredths of an arc-second, the angle a one-metre lever changes over a fraction of a hair's width.

The Optical Head projects an illuminated reticle through a collimating objective and folds the return to the detector with a beam-splitter. The Illumination Module back-lights that reticle, and the Detector Module images the returned pattern onto a sensor. The Processing Board finds where the image landed and converts that to an angle shown on the Display. The optics live in a sealed Tube Housing on a Mounting Base.

How it works

Inside the Optical Head, the Illumination Module back-lights an etched reticle at the focal plane of the collimating objective. Because the reticle sits at the focus, the objective sends out a beam in which the reticle pattern is collimated, effectively projected to infinity. That beam travels to the target mirror and reflects back into the same objective.

The geometry is the key trick: if the mirror is perfectly square to the optical axis, the return refocuses exactly onto the reticle. If the mirror tilts by an angle, the return tilts by twice that angle, and the objective refocuses the reticle image to a spot displaced from center by an amount equal to the focal length times that doubled angle. A beam-splitter diverts this returned image into the Detector Module, where a relay lens images it onto a CCD or CMOS sensor.

The Processing Board captures each frame and computes the centroid of the reticle image to a small fraction of a pixel, then divides the displacement by the objective focal length to recover pitch and yaw simultaneously. Results stream to the Display. Sub-arc-second work demands mechanical stillness, so the optics are locked in the rigid Tube Housing and the instrument is squared to the work with the fine adjuster on its mounting base.