The double-crystal monochromator on beam line 3.4 at the Daresbury synchrotron radiation source covers the energy region 0.8-4 keV, routinely using crystal pairs of beryl, quartz, InSb, Ge, and Si. The beam line has a unique system to cut off high-energy radiation. The whole beam line has been built on a swinging arm, which allows the glancing angle on a chromium coated premirror to be varied between 0.5-degrees and 3-degrees. This not only reduces the heat load on fragile crystals, but also reduces the higher order contributions. We have made a thorough study of the alignment problems arising in the double-crystal geometry. Crystal misalignment, Bragg angular offset, as well as heat-load effects, can cause severe beam movements. A beam-position monitor, which has recently been commissioned, is an invaluable diagnostic tool for these beam movements.
