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Re: [ccp4bb] To bathe or not to bathe. |
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CCP4bb navigationCCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999Subject: Re: To bathe or not to bathe. From: Juergen Bosch jbosch {- at -} U {- dot -} WASHINGTON {- dot -} EDU Date: 2007-11-24 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: sweet@bnl.gov > 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 CCP4bb navigationCCP4bb <-- 1999 <-- November 1999 <-- 30 November 1999 |
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