Abstract: A laser-Doppler velocimeter, for measuring the velocity of particles passing a test location, comprises first and second lasers for providing a measurement laser beam directed along a first axis and a guide laser beam that is parallel to the measurement beam. The guide beam and the measurement beam are directed towards a beam deflecting mirror that reflects the measurement beam along a second axis and reflects the guide beam parallel to the second axis. The orientation of the second axis relative to the first axis depends on the orientation of the mirror, and the orientation of the mirror is controlled by an electrical adjustment mechanism. The measurement beam is received by a beam-dividing device that directs the measurement beam along a third axis towards the test location. Light scattered from particles at the test location focused on a collection aperture which is stationary relative to the beam-dividing device.