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Re: [Imgcif-l] imgCIF Standard Axis definition
- To: The Crystallographic Binary File and its imgCIF application to image data <imgcif-l@iucr.org>
- Subject: Re: [Imgcif-l] imgCIF Standard Axis definition
- From: Jon Wright <wright@esrf.fr>
- Date: Fri, 04 May 2007 00:47:39 +0200
- In-Reply-To: <Pine.BSF.4.58.0705031617010.105@epsilon.pair.com>
- References: <463A3343.4050907@mcmaster.ca><Pine.BSF.4.58.0705031617010.105@epsilon.pair.com>
Hi Herbert, > 1. No matter how the direction of the X-axis is chosen, > it is important to place the origin of the X-axis in > the sample, not in the detector. Otherwise calculations > of beam centers and detector distances become > quite difficult. A pedantic point, but the intersection of the goniometer axes would seem like a first choice of origin for ImageCIF. If there is only one axis then the intersection of that axis with the centre of the beam seems like a second choice. The finite sized sample would then be the last resort. Not sure where they go in the current dictionaries; but the Bruker/Saint practice of refining "crystal translations" during integration are useful data to be recorded. These same numbers come up in grain mapping applications, which is a growing business. These definitions really matter and are usually interesting in terms of an agreed upon laboratory co-ordinate system. I see _diffrn_orient_matrix is in mmCIF (?) We often collect images where the sample is a collection of grains, each having their own orientation and centre of mass. How should multiple crystals be dealt with now, for example with non-merohedral twins? Best, Jon > > 2. If an X-axis is chosen that is different from > the pricipal axis of the goniometer, it is important > that it be clearly documented, so that, for example > the detector axes do not get miss-identified. > > There is a draft of the current proposal prior to > David's suggestion at > > http://www.bernstein-plus-sons.com/software/CBFlib_0.7.7/doc > > Please do consider what is in the proposal and what > David has suggested as a modification, and please > send your comments and suggestions to this list. > > -- HJB > > ===================================================== > Herbert J. Bernstein, Professor of Computer Science > Dowling College, Kramer Science Center, KSC 121 > Idle Hour Blvd, Oakdale, NY, 11769 > > +1-631-244-3035 > yaya@dowling.edu > ===================================================== > > On Thu, 3 May 2007, David Brown wrote: > >> A proposal for the definition of a reference axis system in imgCIF (and >> by inference other CIF dictionaries). >> >> By I.David Brown >> >> The imgCIF dictionary recognizes that authors will require to use a >> number of different axis systems to describe, e.g., the crystal >> orientation, the reciprocal space orientation and the detector. There >> is clearly a need to be able to relate these axes to each other. >> >> For this purpose imgCIF defines a standard laboratory coordinate system >> (SLCS) based on directions that can be derived from the diffraction >> equipment being used. Two directions are needed to define the SLCS and >> in the first version of the imgCIF dictionary, these directions are the >> incident beam and the spatially fixed rotation axis of the goniometer >> that holds the specimen. X is defined as lying along the goniometer >> axis, Z as perpendicular to this and lying in the plane of X and the >> incident beam, and Y is chosen to complete a right handed rectangular >> coordinate system. The origin is placed at the sample. >> >> Problems arise if there is no goniometer as may occur, e.g., in small >> angle scattering experiments. The incident beam will always define one >> direction, but a second direction is needed to define the X axis. >> >> A recent proposal made by Bernstein is to use the principal axis of the >> detector, defined as the direction in which the detector is most rapidly >> scanned (for 1- annd 2-dimensional detectors). An alternative might be >> the direction of the fixed rotation axis of the detector if one exists. >> The possibility remains, however, that no unique detector direction can >> be defined. In this case Bernstein suggests that the Y axis be chosen >> in the direction of the gravitational field (down) or, in the case where >> the incident beam is vertical, the Y axis be chosen to point to the north. >> >> While the original definition in the current imgCIF dictionary is simple >> and covers the majority of cases, if there is no goniometer the choices >> for the second axis start to multiply and some seem quite bizarre. >> Taking directions from the diffraction equipment makes sense because the >> relationship between the goniometer and the detector is relevant to >> interpreting the results. But directions such as 'down' and 'north' are >> not related to the operation of the equipment or the interpretation of >> the measurements. Rotating the apparatus while maintaining the >> relationship between its individual components would change the SLCS but >> make no difference to the relationship between the different practical >> axis systems. >> >> The sole purpose in defining the SLCS is to allow the relationships >> between other axis systems to be expressed in a straightforward manner >> against some common coordinate system. The way in which the SLCS is >> defined is irrelevant so long as it is used consistently within a >> related set of CIFs. It is easier to interpret the transformation >> matrices used to define other axis systems if everyone chooses the same >> SLCS and it is convenient to base this SLCS on the obvious directions >> defined by the apparatus, but in those cases where the incident beam is >> the only natural direction then the choice of the SLCS X axis is >> arbitrary and there is no reason why everyone need use the same SLCS. >> Since Bernstein's proposed choice of X axis depends on whether there the >> sample is mounted on a goniometer, and what kind of detector is in use, >> whether the incident beam is vertical etc., there will no longer be a >> universal definition applicable to all experiments. >> >> PROPOSAL >> My proposal is to keep the current definition using the fixed axis of >> the sample goniometer where such a direction exists and otherwise to >> allow the X axis direction to be chosen arbitrarily by the user with the >> understanding that it must be used consistently within any set of >> related CIFs (though it is not obvious that even this restriction is >> needed since it is only the relationship between the practical units >> that is ultimately needed). It is likely that a standard SLCS would be >> adopted for instruments mounted at a major installation, even for that >> small subset of experiments that do not involve an identifiable fixed >> rotation axis for the specimen. An item should be defined in the >> dictionary where the user can explain how the X axis has been chosen. >> This proposal would have the advantage of simplicity without defeating >> the purpose of the SLCS in those rare cases where the specimen is not >> mounted on a goniometer. >> >> > _______________________________________________ > imgcif-l mailing list > imgcif-l@iucr.org > http://scripts.iucr.org/mailman/listinfo/imgcif-l _______________________________________________ imgcif-l mailing list imgcif-l@iucr.org http://scripts.iucr.org/mailman/listinfo/imgcif-l
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