Off-axis zone plate monochromator for high-power undulator radiation

We report the design and construction of an off-axis zone-plate monochromator for diffraction-imaging experiments at beam line 9.0.1 at the Advanced Light Source (ALS) synchrotron-radiation facility at Berkeley USA. The device is based on an off-axis zone plate which can be conveniently inserted into or retracted from the beam. We discuss design issues such as the efficiency and spectral purity of the system and the technique for designing heat-tolerant windows for soft x-ray undulator beams. The monochromator functions successfully and good-quality diffractions patterns are being made with the beam it delivers.