[Nexus] UB matrix in NXsample

[Osborn, Raymond]

> On Apr 10, 2017, at 2:19 PM, V. Armando Sole <sole at esrf.fr> wrote:
> 
> Hi Ray,
> 
> On 10.04.2017 19:40, Osborn, Raymond wrote:
>> I suppose we could add a UB
>> matrix to the list of stored parameters in a NeXus file even though it
>> is technically redundant.
> 
> We face the problem that people stores the UB matrix and the diffractometer positions together with the images but we do not have any of the other unit cell parameters.
> 
> I guess the best thing I can do then is to use a not-yet-official-and-perhpas-never-official ub term inside NXsample with dimensions [n_comp, 3, 3].
> 
> Concerning the other question concerning sample orientation specified by 3 floats. Is that intended to be hkl?
> 
> Best,
> 
> Armando

Your example provides a good rationale for having a separate UB matrix field, so I would support it. If I’m able to join the next telco, I will raise it then if others don’t.

I have never used the sample_orientation parameter, but the units are specified as NX_ANGLE, so they must refer to omega, chi, and phi, which Busing and Levy discuss in the section on “Coordinate Transformations.” They are used in the sequence of matrix multiplications that transform (hkl) into goniometer lab coordinates (Eq. 19 in Busing and Levy). The documentation is inadequate because it doesn’t state what order they are specified, but it’s either (omega, chi, phi) or the reverse. The other inadequacy is that there could be multiple (omega, chi, phi) angles if the sample is being scanned. 

Thanks for raising the issue because it is something we should sort out. There are a few questionable decisions taken when we were first formalizing the standard, such as the unit_cell field, which contains a mix of lattice parameters and angles, and therefore have mixed units. They are gradually being fixed. This might just need better documentation. I hope others contribute to this discussion if I have misunderstood the definitions. I am not a crystallographer.

With best regards,
Ray
-- 
Ray Osborn, Senior Scientist
Materials Science Division
Argonne National Laboratory
Argonne, IL 60439, USA
Phone: +1 (630) 252-9011
Email: ROsborn at anl.gov