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Use Steca: A typical workflow

Prepare data

Load data files

File/Add files… Select and load data files.
File/Remove selected file(s)… Remove files from the list.

Images in all datasets need to have the same size in pixels (i.e. originate from the same detector). This constraint could be removed in the future, but would that be ever needed?

Currently only the Caress data format can be loaded.

If a file cannot be loaded (unknown or invalid data format, different image size, etc.), an error message is displayed and the process stops.

Load a vanadium correction file

File/Enable correction file… Select and load (enable and disable if it has been loaded) a correction file.
File/Remove correction file Remove the correction file.

Typically, a correction file would contain only one dataset (image). If there are more than one images in the file, their intensities are averaged, pixel-by-pixel.

The image has to have the same pixel size as all other images in used data files.

Set detector geometry

Set the correct detector distance and pixel size. Set the correct beam offset (for X-ray images, in future version).

Image settings

Rotate and/or mirror the image so the beam center is assumed to be to the left of the image. (Select a loaded file and a dataset to show an image.)

Set an image cut (to cut off the dead pixels around the edges of the detector) by specifying the number if pixels to cut off at the:

  • top,
  • bottom,
  • left, and
  • right

of the image.

Select to use the same value for all four sides. Select to show the cut in the image.

If a correction file is loaded, switch to the 'Corr.' tab that shows the correction image. Use to switch correction on. The correction image is then displayed as self-corrected. If it does not contain pixels with zero intensity (“dead” pixels) or if all such pixels are cut off, then the self-corrected inside of the cut has a nice blue colour.

Choose intensity normalization

Choose how the intensity of individual images should be normalized:

  • not at all (disabled, images are assumed to have the same exposure)
  • to Δ time (Δt metadata item)
  • to Δ monitor count (Δmon metadata item)
  • to (fitted) background level.

Normalization to background works only if the background selection has been set and will slow down the program response. It is to be used when Δ time and Δ monitor data are not available or reliable.

Select files, inspect datasets

Select one or multiple files in the 'Files' panel. The datasets from the selected files will be shown in the 'Datasets' panel. The diffractogram of a selected dataset will be calculated and shown in the diffractogram panel.

With the option 'Combine' below the datasets view it is possible to combine a certain number of datasets.

Below the diffractogram there are two check boxes: 'all datasets' and 'fixed scale'. 'All datasets' will calculate a combined diffractogram of all selected files. The combined diffractogram is handy for selecting background and reflection regions. 'Fixed scale' will display diffractograms using a fixed intensity scale, useful to compare diffractograms.

Fitting

Fit the background polynomial

Select the too button on the 'Background' tab and move the mouse over the diffraction diagram. With the primary mouse button select the background regions; clear the regions using the secondary mouse button, or clear everything with the button. To show or hide the fitted polynomial use the button. Choose the degree of the background polynomial. In the diffractogram the gray curve shows the data with out the background reduction and the black curve with the background noise subtracted. For further calculations the black curve is taken. The background polynomial is the curve displayed in green as are the background ranges.

 Fitting background

Specify and fit reflections

Now to the part that is the most fun:

  • Switch to the Reflections tab and enable the selection tool .
  • Add a reflection and select the desired fitting type; Use the primary mouse button to select the reflection region in the diffractogram; repeat for all reflections.
  • To change a reflection region, simply re-select it with the mouse or change the 'min' and 'max' values in the number boxes below the reflections list.
  • To remove a reflection use the button.

An example of reflection-fitting is shown in the image below:

  • The light grey curve represents the measured intensity data.
  • The green horizontal line shows the fitted background and the black curve is the intensity curve adjusted for the background.
  • The blue backround indicates the selected reflection range (2θ).
  • The neon green curve indicates the fitted (Gaussian) peak curve. The small circles indicates the guessed (grey) and fitted (red) peak position and fwhm.

To test the stability of levmar fitting (for example), you may adjust the guessed values in the number boxes and observe how the fit results do (or do not) change.


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