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: [log in to unmask]
> 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
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