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In general, a variable repetition rate of the x‐ray bunches is needed to explore time domain problems using x‐rays. In some instances, the results of several hundreds or thousands of x‐ray pulses must be averaged requiring the sample to be in the same time‐dependent state each time the monitoring pulse strikes. The important point is that before a second x‐ray pulse hits the sample, the system must return to its initial relaxed ground state prior to another spectrum pulse in order to prepare the same excited state again. Our key design principle employs a subsonic, rotating mirror whose period is slaved to the synchrotron intrapulse period. The synchrotron x‐ray bunches will be reflected a distance of about 2 m to a narrow 0.5‐mm slit just in front of the sample. A mirror rotating at a low 7500 rpm (125 Hz) is sufficient to select a single synchrotron pulse for the Advanced Photon Source. The very precise phase stability required by this method is currently available in rotating mirror devices for laser scanning.