Section 9.1.6.8. The Weissenberg camera

To avoid the overlap of reflections on adjacent lunes and allow much larger rotation ranges per image, up to 5–10°, the Weissenberg camera was reintroduced (Sakabe, 1991). This minimized the number of exposures for a data set, which fitted well with some imaging-plate detectors with large size and slow read-out. In the Weissenberg method, the detector is translated along the axis of rotation at a rate directly coupled to the rate of rotation. The method required a finely collimated and parallel SR beam so that the spot size on the detector was small. Rows of spots in a particular lune then lay between those from the previous one. Data could be recorded in a very short time on a series of rapidly exchanged imaging plates, which were subsequently read out off-line. Complete data could thus be recorded in a mattter of minutes.

This was an application of screenless Weissenberg geometry, quite different from that originally used for small molecules, with the imaging-plate translation being small, sufficient only to offset the spots from adjacent lunes. The speed of the system was especially useful for looking at short-lived states, with a lifetime of minutes to hours. However, there are severe limitations, the first of which is that the background is relatively high, as it is recorded over the whole of the large rotation range. This substantially degrades the signal-to-noise ratio for the integrated intensities. In addition, the prediction of crystal orientation and hence reflection position, and of optimum rotation ranges, is less straightforward than for the rotation method. Finally, the handling of the imaging plates off-line leads to limitations in the subsequent processing and analysis, already a problem in the initial orientation and evaluation of the sample.

Recent developments at the ESRF involve the use of a robot in changing and reading the plates (Wakatsuki et al., 1998), but this system has not been in operation long enough to lead to a sound judgement of its impact. In general, the Weissenberg method is at present not as widely used as the simpler rotation geometry.