Re: [ccp4bb]: Problems with low mosaicity crystals

[ALAN <ASIMPSON@bragg.bio.purdue.edu>]

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Hello,
   I think that this is likely to be a spindle error problem, with
the size of the error presumably larger than the mosaicity of the
"low mosaicity" crystals but maybe smaller than that of the worse
crystals. I have also seen the "vibrating crystal" syndrome and this
could be similar in principle, since the crystal also rotates a little
as it vibrates, but I think that the actual size of the rotation
that this causes is usually small, even if it looks bad on screen.
  Also, the mosaicity of some protein crystals can be very small 
and even if the "total" effective mosaicity (= "real mosaicity"
+ "assorted errors") is , lets say 0.1 degrees, this 
still leaves room for the "errors" part to be significant. It also wouldn't
matter if the crystal vibration or whatever occurrs constantly and on a 
faster time scale than the exposure, since it would get averaged out, 
implying that it is really a spindle/goniometer problem. Depending on
what the nature of the unknown problem is, and your level of desperation,
you might be able to extract something from the data by treating each 
partial separately, rather than adding them. Or better yet, discard the
partials, but this probably isn't an option.
The crystal orientation and mosaicity for each image would have to be
correct if you scale them up separately...

Alan Simpson.