EXPGUI has an ingenious method of manually describing a fixed background. The user clicks on the points that define the background; after which EXPGUI will fit Chebyshev coefficients to these background points and save them into the GSAS EXP file. To then change from a fixed background to a refined background at a later stage can thus done seamlessly with very reasonable starting values generated by the EXPGUI manual background fitting. A November 2002 note from Brian Toby:
EXPGUI Homepage with download and installation information on EXPGUI for Windows and UNIX
Download FTP Site and Mirrors
Click here to download the example files used in this tutorial run-through |
In EXPGUI, load the EXP file that you intend to manually fit the background. (the example file provided above is a combined X-ray / Neutron refinement)
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Before manually fitting the background, run Powpref. To manually fit the background, via the EXPGUI menu, select Powder, bkgedit to bring up the BKGEDIT screen.
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Select the Histogram pattern of interest then use "Click and Drag" via the mouse to zoom up as required
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Click on the Add Icon and click to define the background points. (you can always delete points using the Delete Icon)
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Now define the number of Chebyshev terms to fit the background to. In the following, we will start with 6 terms. (If you get a divide by zero error, check that you have the latest version of EXPGUI - post 15th November 2001).
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Now fit using the Start Fit Icon.
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Slowly increase the number of terms until you get the best fit possible without over-fitting.
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When you have the best fit possible, click on the Save in EXP file & Exit, which will then save the Chebyshev terms into the GSAS EXP file.
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