[ccp4bb]: Re: your mail

[Bart Hazes <bhazes@ualberta.ca>]

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On Tue, 11 Dec 2001, marius wrote:

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>
> > Can someone explain to me (in simple words) the pratical
> > and theoretical usefullness of a diffractometer with kappa
> > geometry with magic angle (54.? deg) using pre-oriented
> > and randomly oriented crystals.
>
> I have to be more specific:
> Rotation of the crystal cuts a torus out of reciprocal space.
> Once one of the reciprocal cell axes is oriented along the rotation
> axis some reflecions (or their symmetry mates) never pass
> through the Ewald sphere. Hence, completeness is severly hampered.

Setting a specific kappa angle will not help you for randomly oriented
crystals as their axis of highest symmetry could still line up with the
rotation axis. There is also no "magic" angle for cusp data. The needed
misalignment of the symmetry axis wrt the oscilation axis depends on the
resolution of the data (it should be more than "theta").

The old Siemens multi-wire detectors came with a 45 degree kappa geometry.
When mounting a crystal in a vertical position on this system, you could bring
it into a horizontal orientation by a 90 degree rotation around kappa. The
90 degree change in going from vertical to horizontal is optimal to collect
your cusp (blind region) data. But I don't know where your magic number comes
from.

Bart

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