The present invention relates to an apparatus (10) for generating X-rays. It is described to produce (210) with at least one power supply (40) a voltage between a cathode (20) and an anode (30). The cathode is positioned relative to the anode, and the cathode and anode are operable such that electrons emitted from the cathode interact with the anode with energies corresponding to the voltage, and wherein the electrons interact with the anode at a focal spot to generate X-rays. The at least one power supply provides (220) the cathode with a cathode current. An electron detector (50) is positioned (230) relative to the anode, and a backscatter electron signal is measured (240) from the anode. The measured backscatter electron signal is provided (250) to a processing unit (60). The processing unit determines (260) a cathode current correction and/or a correction to the voltage between the cathode and the anode, wherein the determination comprises utilization of the measured backscatter electron signal and a correlation between anode surface roughness and backscatter electron emission. The cathode current correction and/or the correction to the voltage between the cathode and the anode is provided (270) to the at least one power supply.