[phenixbb] unit cell discrepancies

Jacob Keller j-keller2 at fsm.northwestern.edu
Thu Apr 21 12:26:06 PDT 2011

>  The location of the spots on the images give a wealth of information
> about the cell constants leading to very precise values, if you know
> your wave length and crystal to detector distance.  Intensities, not so much.

I think that there is a fair amount of imprecision here. For example,
what is the usual precision of the beamline-defined crystal-detector
distance, and of the wavelength? Even 1mm/100mm total distance could
have its effect on cell parameters, and I don't think refinement of
the distance based on trigonometric distortion has a very sharp
minimum, although please correct me if I am wrong (how is the detector
distance calibrated, anyway?). Even having an oblong crystal might
change the distance by ~0.5mm or so. But this is theoretical, and I
think you have much more empirical, real knowledge--what are the
levels of precision on these fronts, and how precisely do we know cell
parameters from experiment (I thought it was pretty imprecise)?


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