Jorge,
You said,
I remember one former good (small molecule ?) crystallography book
with words a kind of this "the crystals should be completely bathed by
the x-ray beam during the whole data collection" and also some other
concerns about beam homogeneity in its cross section. How serious is
this nowadays ? Can processing programs easily overcome, in a certain
mounting, the fact that not all crystal orientations have the same
number of unit cells exposed to x-rays ? What about inhomogeneities at
the beam ? I understand that technical difficulties may lead you to
exposed your crystal partially to the beam, etc..., but how hard should
we care about this (how much effort to avoid this) ?
The original motive for bathing the whole crystal was to assure that the
relative intensity of the data on each successive film pack was very
nearly constant. This was possible (one might say "necessary") in the old
days because the laboratory sources were very stable and the intensity was
low enough that there wasn't a lot of x-ray damage to the crystals.
There were a couple of other good reasons to pay attention to details like
this. One was that methods for scaling images together were not quite as
good as now, and another was that film data were relatively very much less
accurate than what is achievable now with excellent detectors and brighter
sources. To combat all of that, we tried to do everything possible to
make things better.
These days scaling algorithms are good, the detectors are excellent, and
very often it pays to employ a beam smaller than the x-tal. This, the
non-uniformity of many synchrotron beams, and the systematic damage
to crystals that we observe now with synchrotron sources cause serious
systematic errors. We're forced to depend on good scaling and good
detectors to get accurate measurements. Making the measurements in many
different crystal orientations (redundancy) helps to smooth out these
systematic errors.
Nonetheless, it will always pay you to watch for EACH of these sources of
error and to minimize them as best you can.
Bob
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Robert M. Sweet E-Dress: [EMAIL PROTECTED]
Group Leader, PXRR: Macromolecular ^ (that's L
Crystallography Research Resource at NSLS not 1)
http://px.nsls.bnl.gov/
Biology Dept
Brookhaven Nat'l Lab. Phones:
Upton, NY 11973 631 344 3401 (Office)
U.S.A. 631 344 2741 (Facsimile)
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