Re: [ccp4bb]: Problems with low mosaicity crystals

[Pete <Peter.Burkhard@unibas.ch>]

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To me this sounds like a contradiction: Suppose you have indeed 
vibration of your loop, as an effect you would simply get an increased
mosaicity. The fact, that you have low mosaicity should be an indicator
that you don't have vibration.

Cheers, Pete

PS.
I once had the case when I saw vibration of the loop (even by eye). The
mosaicity in this data collection was (not surprisingly) very high.




> Jim Pflugrath wrote:
> 
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> 
> > People at APS beamline 19 pointed out the vibration of the loop (hence the
> >...
> >
> > Too much description of the problem, here is the question: how the vibration
> > of the loop causes this problem? Strictly speaking, all the loops we are
> > using are vibrating in the cold stream, and, compared to the cell constants
> >...
> 
> Consider that each Bragg reflection has a 3D dimensional volume in
> reciprocal space.  For the sake of argument, let's assume it is
> ellipsoidal.  When the loop vibrates, a refln near the surface of the
> Ewald sphere (in other words near the Laue condition; in other words near
> the diffraction condition) will move in and out of the diffraction
> condition.  If this movement is large relative to the reciprocal space
> volume of the refln, then you will have errors in measuring the integrated
> intensity within that volume.  Because of the vibration, sometimes the
> spot will diffract when it some, sometimes it will vibrate out of the
> diffraction condition.  Sometimes when it should be out of the diffraction
> condition, it will vibrate into the diffraction condition.  If you collect
> images with a narrow rotation angle increment, the errors can be enhanced
> because of shutter jitter and goniometer errors.
> 
> Let me try to make the idea simpler.  Take a tennis ball (that's your
> Bragg refln) and move it through the smooth ocean surface (that's the
> surface of the Ewald sphere).  You want to get the tennis ball wet exactly
> once (that is, integrate the refln once).  So the fuzz on the surface of
> the tennis ball should get wet exactly once.  Anytime you re-wet the fuzz
> or hair, that contributes to error.  So if your hand is not steady because
> it vibrates, or if the ocean surface is not steady, you create error.
> 
> Can you see that if your mosaicity is larger, then you have a American
> football (not a tennis ball) and you will have less relative error when
> wetting the surface of the football?

-- 
________________________________________________________________________

PD Dr. Peter Burkhard
M.E. Müller Institut (MSB)
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