K&A (and also Brentano, upon rereading) say that in order to be focussing, the sample also needs to be toroidally curved; curved along ACB as well as be rotated about the axis AB; so it is a little more than the traditional flat-sample effect
So summarising everyone here, and the papers I've been reading, parafocussing means "sort of focussing given the constraints of actually making a physical diffractometer". Also, we have a finite source and detector size, and that with the dimensions of beam footprint and goniometer radii that we use mean that it all pretty much works out in the end. Matthew On 2 May 2016 at 20:07, Kern, Arnt <arnt.k...@bruker.com> wrote: > Matthew, > > > > I think Klug & Alexander (1974) give a good explanation about the origin > of the term "parafocusing": 2nd edition, section "parafocusing methods", > page 222 ff. > > > > Cheers, > > > > Arnt > > > > *From:* rietveld_l-requ...@ill.fr [mailto:rietveld_l-requ...@ill.fr] *On > Behalf Of *Eduard E. Levin > *Sent:* Montag, 2. Mai 2016 13:21 > *To:* Cline, James Dr. (Fed); Matthew Rowles > *Cc:* rietveld_l@ill.fr > *Subject:* RE: Parafocussing definition? > > > > Dear James, > > For me too, please, if it would not be much of a trouble. > Thank you in advance! > > Eduard > > *On Mon, 2 May 2016 11:09:31 +0000, Cline, James Dr. (Fed) wrote* > > Bob Cheary and I developed and presented a workshop several times in the > 1990's that included a discussion of this issue. I can send you the notes > for it if you would like them. > > > > Jim > > > > > > James P. Cline > > Materials Measurement Science Division > > National Institute of Standards and Technology > > 100 Bureau Dr. stop 8520 [ B113 / Bldg 217 ] > > Gaithersburg, MD 20899-8523 USA > > jcl...@nist.gov > > (301) 975 5793 > > FAX (301) 975 5334 > > > > *From:* rietveld_l-requ...@ill.fr [mailto:rietveld_l-requ...@ill.fr] *On > Behalf Of *Matthew Rowles > > *Sent:* Monday, May 02, 2016 1:53 AM > > *To:* RIETVELD_L Distribution List <rietveld_l@ill.fr> > > *Subject:* Parafocussing definition? > > > > > > Hi all > > > > > > > > I've been trying to find a good explanation of what parafocussing (wrt > Bragg-Brentato geometry) actually is, but haven't been able to find one. > > > > > > > > Klug and Alexander just reference Brentano's papers. > > > > > > > > "The Basics of Crystallography and Diffraction" 2nd ed say that B-B > geometry is "semi-focussing" because the sample is flat, and not curved to > follow the focussing circle (this doesn't sound right to me) > > > > > > > > Brentano, J Appl. Phys. 17, 420 (1946) says that a ray reflecting off > the arc defined by ACB where A is the source, C is the centre of the gonio, > and B is the detector (ie the focussing circle) is automatically > parafocussing, because you only can establish the location of the > crystallites, not their orientation, but then goes on to say that you can > actually find the orientation, as the lattice plane normal bisects the > angle ACB. > > > > > > > > I also haven't been able to find a use of the word "parafocus" outside > of the diffraction literature, so I can't see how the word is used > elsewhere. > > > > > > > > Any ideas? > > > > > > > > > > > > Matthew > > > -- > Open WebMail Project (http://openwebmail.org) >
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