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[Imgcif-l] Beam convergence vs divergence

I am working with the NanED group (https://naned.eu) on extensions toCIF for 3D electron diffraction. For some techniques they use aconvergent electron beam. A typical example of their data collectionreport is:
Beam Convergence     Parallel beam, convergence <0.1mrad
Note in this example that an upper limit is specified.
imgCIF has data names
_diffrn_radiation.div_x_source_diffrn_radiation.div_y_source_diffrn_radiation.div_x_y_source
for beam /divergence/. A typical definition (for *_x_source) is:
    Beam crossfire in degrees parallel to the laboratory X axis     (see AXIS  category).
     This is a characteristic of the X-ray beam as it illuminates     the sample (or specimen) after all monochromation and     collimation.
     This is the standard uncertainty (estimated standard     deviation, e.s.d.)  of the directions of photons in     the XZ plane around the mean source beam direction.
     Note that for some synchrotrons this value is specified     in milliradians, in which case a conversion is needed.     To convert a value in milliradians to a value in degrees,     multiply by 0.180 and divide by \p.
The terminology is perhaps specific to synchrotron X-ray practice. Isthis CIF item appropriate for recording an electron-beam convergence?My first thought was that a negative value could be taken to indicateconvergence instead of divergence. But on closer reading, as you aredefining this as a s.u. of scattered directions, perhaps the resultwill always be positive (in which case should that be defined withappropriate _item_range.*/_enumeration.range attributes?).
Opinions welcome.
ThanksBrian_________________________________________________________________________
Brian McMahonInternational Union of Crystallography            e-mail:  bm@iucr.org5 Abbey Square, Chester CH1 2HU, England_______________________________________________imgcif-l mailing listimgcif-l@iucr.orghttp://mailman.iucr.org/cgi-bin/mailman/listinfo/imgcif-l

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