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In x-ray crystallography why is it necessary to oscillate the crystal by a few degrees to get proper spot intensities?

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    $\begingroup$ Are you referring to the oscillation method? $\endgroup$ – John Rennie Jun 14 '16 at 16:57
  • $\begingroup$ In the full diffraction analysis the crystal is being turned very precisely around two axes in a goniometer: helmholtz-berlin.de/forschung/oe/em/mikro/methoden/eta_en.html $\endgroup$ – CuriousOne Jun 14 '16 at 18:40
  • $\begingroup$ Successful data recovery from oscillation photographs containing strong polycrystalline diffraction rings from an unknown small-molecule contaminant: preliminary structure solution of Salmonella typhimurium pyridoxal kinase (PdxK) G. Deka,a J. N. Kalyani,b J. F. Benazir,a H. S. Savithri,b,* and M. R. N. Murthya,*...... Acta Crystallogr F Struct Biol Commun. 2014 Apr 1; 70(Pt 4): 526–529. Published online 2014 Mar 25. doi: 10.1107/S2053230X14005342so there seems to get some advantage of 'oscillations' $\endgroup$ – drvrm Jun 14 '16 at 19:34
  • $\begingroup$ Why the down vote, the question is ok? No not the oscillation method. Of course, I understand that to get sufficient reflections for structure determination it is necessary to move the crystal so as to get reflections from different lattice planes into the Ewald sphere. As the structure is determined by the spot intensities and not their positions, it is necessary to get intensities accurately recorded on the detector. When using small crystals (such as from proteins at a few microns across) I understood that a small oscillation was required so that the proper intensity could be recorded. $\endgroup$ – porphyrin Jun 15 '16 at 17:34
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An interesting question. The reason is due to the mosacity in the crystals, the Bragg peaks spread and so their intensity is not measured properly, usually 0.1 degree rotation is needed to get 'full' intensity. Without measuring full intensity it is really difficult to index the space group. And you need a lot of data and use Monte Carlo methods to make a best guess

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