User:Boesecke

from Peter Boesecke (ESRF, Grenoble)

The discussion in what format small angle scattering data should be saved is already going on for many years: XML, HDF5, NEXUS etc. I am convinced that this is not the most important question that must be answered. If somebody would decide: "It is BXHN" (just a name) I would not feel better because the most important part would still be missing: What data must be saved, what data are needed for analysis? As beamline scientist and local contact at X-ray scattering beamlines I have always tried to save as much useful metadata as possible together with the (2D) scattering data. The parameter set was evolving with time and I have tried to make sure that it does not become contradictory and remains well-defined. Many metadata parameters are not needed very often, e.g. the proposal number, but some are practically always needed, e.g. the wavelength.

The standard geometry is a small angle scattering experiment where the (1D, 2D) detector is perpendicular to the primary beam (required metadata: pixel size in each direction, wavelength, distance, beam center). I need to be able to analyze in a similar way standard geometry data and data from more complicated geometries, e.g. inclined detectors. Therefore, parameters are added that default to the standard case when not specified. An inclined detector is described by additional rotation angles that are zero when not defined. It is now also necessary to distinguish the beam center and the point of normal incidence. Sometimes it can also become more comfortable to look to the data in a different way, e.g. to construct a physical detector mask. In this case distances in mm are much more adequate than in 1/nm.

A description of this parametrization it is shown here.

It must be possible to extract the described parameters from a standard SAS file (task: what are the corresponding NEXUS parameters?). How could/must they be saved after refinement in a standard SAS file? I surely do not want to overwrite any raw data, e.g. the monochromator rotation. For me it would be sufficient to write all my parameters. But, whenever possible, I would prefer a common way.

For statistical error propagation calculations I propose to store, optionally, the variance of each pixel (see: Peter Boesecke, "Reduction of scattering data" J. Appl. Cryst. (2007). 40, s423–s427)

Additionally to describe a given experimental geometry it should be possible to save experimental conditions (temperature, pressure etc.) in a defined way and to look-up experiments in a data base. But this is out of the scope of this contribution.