Something else you could try is using a kappa goniometer. There were a couple of presentations at the last CCP4 meeting where they discussed that alignment along the appropriate axis using a Kappa goniometer often worked much better than the inverse beam mode. By aligning the crystal with the beam in such a way that one of the cell axes lies perpendicular to the beam, you can obtain diffraction images that have bilateral symmetry and where both parts of a Friedel pair are visible on the same image. This way the differences between each pair are easily measured and compared and, because they are measured at the same time, the differences cannot be due to X-ray damage, scaling or anisotropy that result when the two parts of a Friedel pair are measured from different images taken from different positions on the crystal at different times.
At the Diamond Light Source they have started to implement these goniometers on some of the macromolecular beam lines. You simply take 2 to 4 test images between 0 and 90 degrees and the software automatically spits out the theta and kappa angles you need to orient the goniometer. After you align the crystal and collect the data set you should collect a second data set where you misalign all the crystal axes. This is useful since the first data set will have no data for the aligned axis and it will help determine the correct space group if there are systematic absences along that plane. By integrating both data sets and then merging and scaling them together you will also obtain much better completeness. Tony ------------------------------------------------------ Dr. Antonio Ariza University of Oxford Sir William Dunn School of Pathology South Parks Road Oxford OX1 3RE
