Re: [ccp4bb]: Problems with low mosaicity crystals

[Jim Pflugrath <jwp@RigakuMSC.com>]

***  For details on how to be removed from this list visit the  ***
***          CCP4 home page http://www.ccp4.ac.uk         ***

> 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?