One additional point to add not raised by Bob is that crystals are
different. So you can shoot at one end of the crystal and say have a
mosaicity of 0.2 degrees but somewhere else it might be 1.4 or even
worse. In such cases e.g. rod like needles it pays off to have a smaller
than crystal beam and walk over you crystal for the best spot to collect
your dataset.
Jürgen
Robert Sweet wrote:
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
=========================================================================
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)
=========================================================================
--
Jürgen Bosch
University of Washington
Dept. of Biochemistry, K-426
1705 NE Pacific Street
Seattle, WA 98195
Box 357742
Phone: +1-206-616-4510
FAX: +1-206-685-7002
Web: http://faculty.washington.edu/jbosch
