Just a few comments on "consider a crystal bathed in a uniform
beam".
I've not fully bought into the idea that it's OK to have the
beam smaller than the crystal. I learned most of my crystallography
in a lab dedicated to precise structure determinations, and somewhere
along the line, I picked up the idea that it's better to remove
systematic errors experimentally than to correct for them computationally.
(Maybe that had something to do with the computing power and programs
available at the time?)
Anyway, I thought the reason people went to smaller beams was that
it made it possible to resolve the spots on the film or detector.
Isn't that the main reason for using small beams?
In practice, I guess the change in crystal volume actually diffracting
hasn't been a big issue and that frame-to-frame scaling deals with
the problem adequately.
I'm less convinced that frame-to-frame scaling can correct for
absorption very well. Due to our irregular-shaped protein crystals,
before the area detectors came along, we'd use an empirical correction
(one due to North comes to mind) based on rotation about the phi axis
of a four-circle goniostat. It was clearly an approximation to the
more detailed calculations of path-lengths available for crystals with
well-defined faces not surrounded by drops of mother liquor and glass
capillaries. Has anyone checked to see how frame-to-frame scaling
matches up with analytical determinations of absorption corrections?
It'd be interesting to determine the validity of the assumption that
absorption is simply a function of frame number.
Ron
