a telescope with a light source and beamsplitter cube built in, with a reticle of some form to measure the misalignment , if its a quality one, it looks like a survayers instrument with the graduated mounts

A single instrument combining the functions of a telescope and a collimator to detect small angular displacements of a mirror by means of its own collimated light.

An autocollimator is an optical instrument for non-contact measurement of angles. They are typically used to align components and measure deflections in optical or mechanical systems. An autocollimator works by projecting an image onto a target mirror, and measuring the deflection of the returned image against a scale, either visually or by means of an electronic detector.