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NEL was recently required to produce two servo-controlled goniometers for a crystal x-ray monochromator. The specification included an angular sensitivity and repeatability of 0.036 arc second and a linearity better than 0.1 arc second. Operational requirements dictated both an incremental system for operation around an experimental datum and an absolute system for fixed system datum operation, the latter enabling spatial synchronism after a switch-off. A three-track optical grating was chosen for angular pick-off, with a digitizer for coarser measurements. The finest grating track consisted of 36 000 lines at a pitch (grating wave-length) of 36 arc seconds: to approach the required sensitivity an interpolation factor of 1000 was necessary. Conventionally, moire patterns formed by a grating/index interface are detected by four photo-cells in a dc manner and the produced output is related to the light intensity, mean modulated light transmission, modulation amplitude and moire angle. Drift can be experienced to an extent dependent upon system type, and the voltage dependent interpolation methods are limited to factors of 10-40 for confident interpolation. Replacement of the dc photocells by a dynamic self-scanned photocell array enables modern signal processing techniques to be applied to the resultant ac temporal phase signal. This enhances the measurement quality, increases the interpolation possibilities by two orders, and gives immunity from all reasonable dc drift. Some measure of compensation for defects in the optical system can also be achieved. Four such ac moire detectors were fitted to the goniometers to provide a measure of spatial averaging, and single detectors to the coarser tracks for absolute build-up. The systems meet the specification and are presently undergoing field trials.
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