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→Practical considerations
(→An estimate for the overall quality of an experimental setup: fix typo and change <math> formatting to simpler e.g. <sup>) |
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== Practical considerations == | == Practical considerations == | ||
In practice, both crystal quality and beamline quality limit the value of | In practice, both crystal quality and beamline quality limit the value of I/Sigma(I)<sup>asymptotic</sup>. A good crystal (even with elevated mosaicity and medium resolution) should give a high value on a good beamline. | ||
I have seen values around 15-20 for good crystals that still allowed my to solve a MAD structure, but that required high multiplicity of observations. Values around 30 allowed me to solve a sulfur-SAD structure at medium resolution (diffraction to 2.3 A, anomalous signal to 3 A). | I have seen values around 15-20 for good crystals that still allowed my to solve a MAD structure, but that required high multiplicity of observations. Values around 30 allowed me to solve a sulfur-SAD structure at medium resolution (diffraction to 2.3 A, anomalous signal to 3 A). | ||
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On the other hand, I have sometimes obtained values less than 10 with good test crystals, clearly indicating strong systematic errors. It is always good to discuss this with the people who are responsible for the beamline. They might know what is broken, or might be able to find out what went wrong. | On the other hand, I have sometimes obtained values less than 10 with good test crystals, clearly indicating strong systematic errors. It is always good to discuss this with the people who are responsible for the beamline. They might know what is broken, or might be able to find out what went wrong. | ||
A low | A low I/Sigma(I)<sup>asymptotic</sup> may be compensated by high multiplicity, at the expense of radiation damage. Conversely, high multiplicity is not needed to solve a structure, if the data have a high I/Sigma(I)<sup>asymptotic</sup>. | ||
For molecular replacement and refinement, a high value of | For molecular replacement and refinement, a high value of I/Sigma(I)<sup>asymptotic</sup> is not strictly needed (but the maps are better with better data, and the model R values lower!). | ||