A measurement system for use in radiation therapy, for example for measuring radiation sources, in particular for a water phantom. The water phantom has a water container and a measurement system, on which at least one holder for a radiation detector is arranged. The holder can be moved within the water container along at least one movement axis, with a control unit being present, which accepts and executes commands for controlling the movement axis. A virtual coordinate system which is aligned to the water surface is defined so that the movement device does not have to be aligned with great complexity. A conversion unit transfers control commands from the virtual coordinate system into the real coordinate system of the movement device so movements of the holder are always parallel and/or perpendicular to the water surface.