Abstract: A method is provided for operating a drive train comprising an internal combustion engine or an electric machine as a primary drive and an electric machine as a secondary drive, wherein the electric machine is detachably coupled, together with an inverter and controller, on one of the vehicle axles. The electric machine and at least one switchable element is actuated in order to minimize drag losses of the electric machine and to provide a defined connection time for the electric machine. The electric machine is stationary and decoupled during a first speed range. The electric machine is actuated at a preset speed during a second speed range, where a defined connection time is not possible if the electric machine were stationary. The electric machine is coupled to the axle and rotates at the vehicle speed in the third range, when losses while coupled are lower than if uncoupled.

Abstract: With regard to a particularly precise measurement of a wavefront using structurally simple means, a method for measuring a curved wavefront using a wavefront sensor is specified, wherein a plurality of measurements are carried out at different positions along the wavefront using at least one wavefront sensor in order to determine a local gradient of the wavefront at the different positions, which method is characterized in that the plurality of measurements are carried out in each case with a substantially tangential alignment of a light entrance plane of the wavefront sensor(s) with the curved wavefront. A corresponding apparatus for measuring a curved wavefront using a wavefront sensor is also specified.