Calibration of an area detector in GSAS-II
In this tutorial, data collected with a Perkin-Elmer area detector at APS 11-ID-C with a wavelength 0.10798 A, where the detector was intentionally tilted at 45 degrees, are used. Note that menu entries are listed in bold face below as Help/About GSAS-II, which lists first the name of the menu (here Help) and second the name of the entry in the menu (here About GSAS-II).
If you have not done so already, start GSAS-II.
Use the Data/Read image data menu item to read the data file into the current GSAS-II project.
A file selection dialog will be shown; its appearance will depend on your OS. Change the search directory to /Exercises/images and then select the file
La_hex_+45deg-00015.tif and press Open.
At this point the data tree window will have several entries; the highlighted one specifies the contents of the data window as Image Controls
The plot window shows the image; notice the elliptical shape of the powder diffraction rings. The blue X marks the default beam location (image center); your calibration will correctly place it. Also notice that the values shown in the plot window status bar (at bottom of plot window) change as you move the cursor over the plot; these values are at present in error as they are computed from the default calibration values. They will be correct after you finish the calibration.
Note that, alas, very few image formats contain all of the important metadata about the image, so it needs to be added manually. In this case set the wavelength to 0.10798. It is also helpful to change the display of the image so that it is easier to see the diffraction rings. Lowering the maximum intensity to 10,000 to 20,000 counts will help (note that this can be done by moving the slider, or by typing a value in the box and then clicking on another control or pressing the Enter or Tab key). You may also wish to select another color scheme using the Color bar selector.
Set the material used as a sample in the box labeled Calibrant; select LaB6 SRM660a
Use the Operations/Calibrate menu item (note this menu is on the Image Controls window or for the Mac, the Image Controls window must be active to place this into the system menu bar). At this point the status line on the bottom of the Image Controls window changes with a prompt to select points for calibration.
Use the left mouse button to click on at least five locations on the innermost ring. As each point is defined, a red "+" is added to the plot. You need not hit the ring exactly as the code will search for the locally highest point.
To remove a point added in error click on that point with the right mouse button [on the Mac, if you have a single-button mouse, hold the Command (⌘) key down and click].
When done, press the right mouse button well away from any points that have been added [on the Mac, if you have a single-button mouse, hold the Command (⌘) key down and click].
The calibration is then performed. First the rings are located and an ellipse is optimized for each ring. The indexed rings are first shown in red as the calibration proceeds and center of each ellipse is noted with a blue "+". When all rings have been fitted, the entire ensemble of rings is refitted to the detector parameters and the rings are then shown in blue. The beam position (blue X) is now correctly positioned.
The derived calibration results are shown in the Image Controls window.
and the console window shows details of the fit; a trial refinement for the wavelength is also given for you to consider.
To see the actual points selected by the program, click on the show ring picks? check box
When this is done, it sometimes occurs that the points selected for the outermost rings are scattered between that and the next ring. This can be repaired by changing the pixel search range to 10 and using the Operations/Recalibrate menu item.
Assuming that these calibration results will be applied to other images in the same GSAS-II project, click on Use as default for all images. You may also save a copy of the calibration values to a file (name.imctrl) with the Operations/Save Controls menu item. To save the project, including the now-derived calibration information, use the File/Save Project menu item associated with the data tree window; as this project has not been saved before the file save dialog will need a file name. Dont enter the extension; it will be set to .gpx.