There's a simple rule of thumb I have heard for predicting if spot
overlap is likely to be a problem for a given beam divergence: 500 A
of unit cell corresponds to 1 mr of divergence. So, for example, 250
A would be a unit cell limit for a 2 mr beam. Has anyone heard of
this rule before? I heard it from someone who heard it from someone,
so I have no original reference and don't know how valid it is.
There are a number of factors which influence spot overlap: beam
divergence, mosaic spread of crystal, point spread function of
detector, and resolution range of interest. I would love to find some
references to simple estimates based on these parameters. Best I have
seen so far is a paper by Sarvestani et. al. (J. Appl. Cryst. (1998)
31 899-909, but it is a detailed simulation rather than a single
formula.
Richard Gillilan
MacCHESS
