Re: [ccp4bb]: ...Replies: Data Reduction for mosaic crystals...

[Bart Hazes <bhazes@ualberta.ca>]

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Just a few more tidbits on this...

If your mosaicity + rotation width is larger than the theoretical allowed
value (e.g. the arctan(Res/EMPCA) that Tassos mentioned) then you will still
collect a lot of perfectly fine data. Even if lunes (intersections of planes
of reciprocal space with the Ewald sphere) overlap, the spots in those lunes
often do not, depending a bit on how the crystal is aligned. The problem is
also not global. It will only show up for the highest resolution reflections
and when the longest axis really is close to parallel to the X-ray beam.

In situations where spot overlap really limits your choice of rotation range,
try to mount the crystal with the longest unit cell parallel to the rotation
axis. This will reduce the problem, especially for elongated unit cell shapes.

If your apparent mosaicity is M (crystal mosaicity plus beam di/convergence)
and your oscilation width is x*M then on average each reflection will
accumulate (x+1)*M degrees of background. So when your oscillation width is
the same as the mosaicity (x=1) you accumulate 2*M of background on average.
Reducing x to 0.5 (doubling the number of images) gives 1.5*M of background, a
25% improvement. Doubling the number of images once more to get an x of 0.25
adds only 12.5% to this number. Clearly the rewards of slicing finer become
smaller as you push it further especially since there are also practical
drawbacks to fine-slicing. I would say an x of 0.5 to 2.0 is reasonable for
standard data collection.

Bart

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