 | Acta Crystallographica Section D Acta Crystallographica Section D BIOLOGICAL CRYSTALLOGRAPHY |
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![[Figure 3]](dz5376fig3mag.jpg)
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Figure 3
Schematic illustration of conditions causing an apparent shift in 2θ. A mosaic vector ![[{\bf d}_{hkl}^*]](teximages/dz5376fi1.gif) (0) is slightly off the Bragg condition for an X-ray beam from the centre of the focus F (blue line) and makes an angle of 90° − η with the X-ray beam. However, the Bragg condition tends to be fulfilled with rays emerging from the lower part of the focus (red line; the arc corresponds to an angle of 90° − θ) or for the upper part of the crystal (green line). Therefore, the reflected beam appears at a larger 2θ angle. Owing to averaging of many rays this effect is not, or is hardly, visible for rotation images, but it is for still images or ultrafine-sliced images. As the shift in apparent 2θ is always in the direction of 2η, we can account for this by the `flex' parameter in PEAKREF. |
![[Figure 3]](dz5376fig3.jpg)
| BIOLOGICAL CRYSTALLOGRAPHY |
ISSN: 1399-0047
