DATA COLLECTION STRATEGY WITH A CCD DETECTOR ON A FOUR CIRCLE GONIOMETER. Ludger Häming, George M. Sheldrick, Universität Göttingen, Germany

Most area detectors are normaly used with one or three circle goniometers. We have attached a Siemens CCD detector to our non-standard four circle goniometer (offset Huber goniometer, Stoe x-ray optics, Siemens generator and a home-made LT-device and enclosure). For diagnostic purposes the CCD system can easily be replaced by the scintillation detector within an hour. Compared with a three circle system we can vary chi, but the Eulerian cradle resticts omega to a range of about 60 degrees. We routinely collect a complete hemisphere of data. It appears to be better to collect as many equivalent reflections as possible, so we collect one structure per day rather than trying to work shifts to collect threee a day. The time for a data collection is thus independent of the unit-cell dimensions. The quality of the data is significantly improved by the higher redundancy. This enables us to obtain high-angle data for extremly weakly diffracting crystals. A semi-emphirical absorption correction can also be applied, exploiting the redundancy of the data.