source: trunk/tutorial3/alumina4.html @ 623

Last change on this file since 623 was 623, checked in by toby, 11 years ago

# on 2002/07/18 20:45:10, toby did:
NIST alumina tutorial

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  • Property rcs:date set to 2002/07/18 20:45:10
  • Property rcs:rev set to 1.1
  • Property rcs:state set to Exp
  • Property svn:keywords set to Author Date Revision Id
File size: 4.6 KB
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1<html>
2<head>
3   <title>EXPGUI Alumina Tutorial, part #4</title>
4</head>
5
6<style>
7A:link {text-decoration:none}
8A:vlink {text-decoration:none}
9</style>
10
11<BODY BGCOLOR="#FFFFFF"
12      topmargin="0" leftmargin="0" marginwidth="0" marginheight="0" 
13      text="#000000" link="#0033ff" vlink="#0033ff" alink="#0033ff">
14
15<?
16   include("/var/www/include/navigation.inc");
17   include("/var/www/include/utility.inc");
18?>
19<blockquote>
20
21<TABLE BORDER BGCOLOR="#FFFF40" ALIGN=RIGHT>
22<TR><TH><A  Href="index.html">Intro</A> 
23</TH><TH><A Href="alumina5.html">Next page</A>
24</TH><TH><A Href="alumina3.html">Previous page</A>
25</TH></TR></TABLE><BR CLEAR=ALL>
26
27
28<CENTER>
29<H1><HR noshade width="75%" size="2" align="center">
30GSAS/EXPGUI Alumina tutorial (part 4)<BR>
31Changing the Background Function
32<HR noshade width="75%" size="2" align="center">
33</H1>
34</CENTER>
35<I>GSAS offers approximately 10 different background functions (not all are
36implemented in EXPGUI). For each of these functions, the number of terms
37to be used is adjustable. The more terms the more complex the shape that
38can be fit.
39Each of these background functions
40has different shapes, and in theory, each function will have advantages
41under different circumstances.
42However, this author
43finds that the Shifted Chebyschev (type #1) is preferable to the
44others for the vast majority of Rietveld refinements and almost never uses
45any other function.</I>
46<P>
47When a histogram is first added in GSAS, the background function is set to the
48Cosine Fourier series option (type #2) with two adjustable terms.
49In this section of the tutorial, we change the background function and the
50number of terms.
51<P><HR noshade width="50%" size="4" align="center"><P>
52To change the background function, press the "Edit Background" button on the
53histogram panel.
54<P>
55<img alt="tutorial screen dump" src="4a.gif" align=RIGHT>
56<BR clear=all>
57<P><blockquote><I>
58Note the "Refine background" check box has been selected -- this means that
59the background parameters will be refined (optimized) when GENLES is run.
60The damping parameter to the immediate left is set to 0 -- this means that
61the full computed shift will be applied. In cases where a refinement has
62trouble reaching a minimum, it may be advantageous to increase damping
63(a setting of 1 implies 90% of the computed shift will be applied and
64a damping setting of 9 yields a shift of 10%.)
65<P>
66Every refinable parameter in
67GSAS has a refinement flag (either for the group, as in this case,
68or for each individual parameter) and a damping parameter. The appearance
69of the check box is slightly different in windows: a "x" appears in the
70box when the parameter is selected.
71</I></blockquote>
72
73<P><HR noshade width="50%" size="4" align="center"><P>
74<img alt="tutorial screen dump" src="4b.gif" align=RIGHT>
75After the "Edit Background" button is pressed,
76The "Edit Background" window opens, as is shown to the right.
77In this window both the function type and the
78number of terms can be changed.
79<BR clear=all>
80<img alt="tutorial screen dump" src="4c.gif" align=RIGHT>
81Clicking on the "Function type" menu offers a choice of function types,
82as is shown to the right. Choose function type #1, the Shifted
83Chebyschev function.
84<BR clear=all>
85<P><blockquote><I>
86Note that the "Fit Background Graphically" invokes the EXPGUI
87<a href="http://www.ncnr.nist.gov/xtal/software/liveplot.html#bkgedit">
88BKGEDIT</A> program, which is used to fit a background function
89directly to user-supplied points. This is a very useful initial way to
90fit the background in difficult cases.
91</I></blockquote>
92<P><HR noshade width="50%" size="4" align="center"><P>
93<img alt="tutorial screen dump" src="4d.gif" align=RIGHT>
94Also in the "Edit Background" window, press on the "Number of terms" button
95and change this number to 6, as is shown to the right. Note that with
96the Chebyschev polynomial arbitrary (including zero) starting values for the
97background terms is acceptable.
98<P>
99After the "Set" button is pressed, the changes are then seen on the histogram
100panel, as shown below.
101Note that by default, the refine background flag is set, allowing these
102parameters to be optimized.
103<BR clear=all><P>
104<img alt="tutorial screen dump" src="4e.gif" align=RIGHT>
105<BR clear=all>
106<HR>
107Previous: <A HREF="alumina3.html">
108Specifying Powder Diffraction Data
109</A>
110<BR>
111Next step: <A HREF="alumina5.html">
112Initial Fitting: Refine Scale Factor and Background
113</A>
114
115<HR>
116</blockquote>
117<P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR>
118<font size=-1><? lastmod(); ?> 
119<BR>$Revision: 623 $ $Date: 2009-12-04 23:09:16 +0000 (Fri, 04 Dec 2009) $</font>
120</body>
121</html>
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