[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Reply to: [list | sender only]
Re: [Imgcif-l] imgCIF Standard Axis definition
- To: The Crystallographic Binary File and its imgCIF application to image data <imgcif-l@iucr.org>, s.parsons@ed.ac.uk
- Subject: Re: [Imgcif-l] imgCIF Standard Axis definition
- From: Jon Wright <wright@esrf.fr>
- Date: Fri, 04 May 2007 10:22:38 +0200
- In-Reply-To: <a06230900c2603136c2d3@[192.168.10.211]>
- References: <463A3343.4050907@mcmaster.ca> <Pine.BSF.4.58.0705031617010.105@epsilon.pair.com> <463A668B.7010502@esrf.fr><a06230900c2603136c2d3@[192.168.10.211]>
Dear Herbert, There already seems to be some work on using multiple crystals in more conventional crystallography, so I wonder if there is way to re-use those labels? Perhaps I am trying to put a square peg in a round hole here... Currently there is _diffrn_id in the _diffrn_orient_matrix category, which identifies the diffraction experiment. I would suggest adding a pointer to the category '_exptl_crystal_id' which might also be called '_diffrn.crystal_id' in mmCIF? eg, just add: '_diffrn_orient_matrix.crystal_id' ...and allow a loop giving a list of orientation matrices all pointing at different crystal_id's. This seems to be the 'many crystal' cif category, but I don't quite see how to list several crystals in the same cif file for now (would I have to give all the crystal colors for grains buried inside a piece of rock?). After integration the same data for _exptl_crystal_id should then show up in the integrated reflections as: _refln_crystal_id and/or: _diffrn_refln_crystal_id Somehow it would be useful to distinguish the cases of having several crystals and several structure refinements, or several crystals giving a single merged dataset and a single refinement. This would be closely related to the existing practices dealing with twins in small molecule crystallography. People make a dataset from single twin component in order to solve the strucuture (shelx hklf 4) and then refine it using all the reflections (shelx hklf 5 I think). I've copied this mail to Simon Parsons in case he knows how this is done in cif already. Then for crystal translations mmCIF already has: '_diffrn.crystal_id' '_diffrn.crystal_support' '_diffrn.crystal_treatment' How about then adding a category: '_diffrn_crystal_translation' containing: '_diffrn_crystal_translation.x' '_diffrn_crystal_translation.y' '_diffrn_crystal_translation.z' '_diffrn_crystal_translation.diffrn_id' '_diffrn_crystal_translation.crystal_id' These are x, y, z translations of the crystal with respect to the laboratory axes with all goniometer angles set to zero. Units are mm. The diffrn_id refers to the diffraction experiment and there must also be a matching _diffrn_orient_matrix.diffrn_id entry and a matching crystal_id. The axis x/y/z definition here will match that of the orientation matrix (and therefore the laboratory co-ordinate system) and if the category is absent then these values are assumed to be zero. This category is allowed to _loop over all the crystals in the sample. If the orientation_matrix category contains a UB matrix, one should expect to be able to match the unit cell parameters for each crystal to the ones that can be derived from UB (eg (UB)^T.UB is the metric tensor, which in turn gives the unit cell). Since there are many cif files which implicitly assume a single crystal, it seems the default value for crystal_id in any category must be assumed as the first and only entry in _exptl_crystal_id. Otherwise almost all derived data will end up being littered with crystal_id entries. I wonder if orientation matrix should have been: '_diffrn_crystal_orient_matrix.UB[0][0]' etc where I've added crystal in the label. The orientation matrix is a property of the crystal in the diffraction experiment and you can't normally find one without mounting your crystal! Is it a bug in mmCIF to have this independent of the crystal? What are your thoughts? Perhaps a new category would be better if this is a mis-use of the intentions of crystal_id Best, Jon PS: Apologies if this mail was sent twice. Herbert J. Bernstein wrote: > Dear Jon, > > Your point is a significant one, not a pedantic one -- it comes down > to what should be refined from the images: the axis definitions or > offsets relative to fixed axis definitions. Let us start with the > central issue: where should the origin of the axis system be? You are > right -- we have a better chance of a stable axis system against which > to measure offsets if we use the intersection of two mechanically stable > axes to set the origin. Hopefully, the sample will have been mounted > so that that mechanical origin is bathed by the center of the beam and > so that that origin is within the sample, so I would proposed the > following approach to axis definition: > > 1. The origin of the axis system should, if possible, be defined > in terms of mechanically stable axes to be in the beam. If the > sample goniometer or other sample positioner has two axes the > intersection which defines a unique point at which the sample should > be mounted to be bathed by the beam, that will be the origin of the > axis system. If no such point is defined, then the midpoint of the > line of intersection between the sample and the center of the beam > will define the origin. For this definition the sample positioning > system will be set at its initial reference position for the experiment. > > 2. It the sample positioning system has a principal axis that > intersects the origin and which forms an angle of more than 22.5 > degrees with the beam axis, the X-axis will point from the origin > specified above along the principal axis of the goniometer, > > ... and then continue with the definitions as before using either > David's simpler approach or the more detailed one suggested on the > web site. > > You are also right about the need for multiple orientation matrices. > I think we need to add a GRAIN or SUBSAMPLE category to list > the grains and and then an extra _diffrn_orient_matrix.grain_id > or _diffrn_orient_matrix.subsample_id tag to separate the > matrices for the different grains. Would you care to suggest > the tags for the GRAIN or SUBSAMPLE category itself? > > Regards, > Herbert > > At 12:47 AM +0200 5/4/07, Jon Wright wrote: >> 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 > > _______________________________________________ imgcif-l mailing list imgcif-l@iucr.org http://scripts.iucr.org/mailman/listinfo/imgcif-l
Reply to: [list | sender only]
- Follow-Ups:
- Re: [Imgcif-l] imgCIF Standard Axis definition (Harry Powell)
- References:
- [Imgcif-l] imgCIF Standard Axis definition (David Brown)
- Re: [Imgcif-l] imgCIF Standard Axis definition (Jon Wright)
- Re: [Imgcif-l] imgCIF Standard Axis definition (Herbert J. Bernstein)
- Prev by Date: Re: [Imgcif-l] imgCIF Standard Axis definition
- Next by Date: Re: [Imgcif-l] imgCIF Standard Axis definition
- Prev by thread: Re: [Imgcif-l] imgCIF Standard Axis definition
- Next by thread: Re: [Imgcif-l] imgCIF Standard Axis definition
- Index(es):