Reinhard, Thank you for your contribution and that of others. We all need to be reminded of what we are doing and why we are doing it (and the compromises along the way).
What we measure is a mere reflection of the sample; use a different machine or different settings and you will get a different pattern (and I am not talking about scale/ the effect of count-time) – this is regardless of how well or otherwise your diffractometer is aligned (separate but important issue), or what your detector is actually counting (also important but neglected) . Certainly there is not much we can do to keep the sample on the focussing circle. What the world needs is better control of antiscatter. Mis-set anti-scatter or absent anti-scatter slits make life a misery if one has any intention of modelling a diffraction pattern. I have great hopes for automatic knife edges but I can see the great scope for it all going dreadfully wrong. I have yet to see any model for how such a thing should work (as in how far the slit should be above the sample as a function of diffractometer radius or divergence slit width). Manual knife edges are great (properly aligned at about 10um tolerance in height, and about 0.1 deg in chi [that’s hard to do]) – but then only useful for a restricted (to very restricted) 2theta range otherwise “asymmetrically limiting the incident/diffracted beam bundle“. I have great hopes for automatic variable divergence slits also – just do or don’t think of the anti-scatter issues (as in requires a programmable a-scatter on both the incident and diffracted beam side [diffracted side is the problem as it depends on detector parameters for a 1D detector]) . regards, Tony Raftery Tony Raftery | Projects Manager – X-ray & Particles Laboratory Central Analytical Research Facility (CARF) | Queensland University of Technology (QUT) [cid:image005.jpg@01D123A7.3CFF2600] [http://web.utk.edu/~prssa/images/flickr.png] <https://www.flickr.com/photos/ifequt/sets/72157638595671243> [http://www.marbus.com.ar/wp-content/uploads/2014/06/Twitter.png] <https://twitter.com/IFE_QUT> Address: Level 6, P Block, Gardens Point campus, 2 George St Brisbane QLD 4000 Tel: +61 7 3138 5122 | Mob: 0481 008 595| Email: a.raft...@qut.edu.au<mailto:a.raft...@qut.edu.au> | Web: www.qut.edu.au/ife/carf<http://www.qut.edu.au/ife/carf> P Please consider the environment before printing this email CRICOS No: 00213J From: rietveld_l-requ...@ill.fr [mailto:rietveld_l-requ...@ill.fr] On Behalf Of Reinhard Kleeberg Sent: Tuesday, 3 May 2016 7:18 PM To: Matthew Rowles <rowle...@gmail.com> Cc: rietveld_l@ill.fr Subject: Re: Parafocussing definition? Am 03/05/2016 um 03:00 schrieb Matthew Rowles: 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 Yes. And as ACB is not constant, a "focusing sample" should change their curvature (bent radius) following the focusing circle versus 2theta, getting more bent with higher diffraction angle, what is hard to do... That's why Bragg-Brentano geometry (flat sample) is the working compromise. Btw, understanding the geometric principles seems to be a bit out of fashion, as we nowadays can find "strange" instrumental configurations for common powder diffraction work in many laboratories, for example: - parallel primary beam optics in combination with narrow point detectors, - overmuch long linear detectors combined with a Bragg-Brentano setup, - small "antiscatter" slits in front of big linear detectors, - "knife edge" antiscatter slits above the sample, asymmetrically limiting the incident/diffracted beam bundle, etc. pp.... Reinhard 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<mailto: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> [mailto: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<mailto: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<mailto:jcl...@nist.gov> > (301) 975 5793<tel:%28301%29%20975%205793> > FAX (301) 975 5334<tel:%28301%29%20975%205334> > > From: rietveld_l-requ...@ill.fr<mailto:rietveld_l-requ...@ill.fr> > [mailto: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<mailto: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<http://openwebmail.org/>) ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Please do NOT attach files to the whole list <alan.he...@neutronoptics.com><mailto:alan.he...@neutronoptics.com> Send commands to <lists...@ill.fr><mailto:lists...@ill.fr> eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -- TU Bergakademie Freiberg Dr. R. 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