99 lines
3.6 KiB
HTML
99 lines
3.6 KiB
HTML
<HTML>
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<HEAD>
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<TITLE>TRICS PSD Peak Search</TITLE>
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</HEAD>
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<BODY>
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<H1>TRICS PSD Peak Search</H1>
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<P>
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For almost any measurement at TRICS a UB matrix has to be determined
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beforehand. In order to do this a couple of peak must be located by
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some means. This section describes how the computer can help in
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finding an initial set of peaks.
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</P>
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<p>
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The algorithm is quite simple: It consists of a big loop over ranges
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of the four circle angles two theta, omega, chi and phi. At each
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position a counting operation is performed. Then peaks are located on
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all three detectors through a local maximum search. For this, the
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<a href="lowmax.htm">local maximum search</a> module is used.
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If a candidate
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peak is found, it is refined in omega and written to a file. The
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tricky bit is the adjustement of the local maximum search parameters
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in order to minimize false maxima caused by a spicky background or
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powder lines.
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</p>
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<p>
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The peak search facility need a lot of parameters in order to
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operate. This includes angle ranges, count parameters and the maximum
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search parameters. Commands are provided for adjusting these
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parameters. The general operation of these commands follow a pattern:
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typing the command alone prints the current values of the
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parameters. In order to set new values the command name must be typed
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plus new values for all the parameters listed by this command. An
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Example:
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<pre>
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ps.sttrange
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</pre>
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prints the range in two theta for the peaksearch.
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<pre>
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ps.sttrange startval endval step
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</pre>
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sets new values for the two theta range and prints them afterwards.
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The following commands are provided:
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<dl>
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<dt>ps.sttrange
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<dd>adjustment of the two theta range for the peak search.
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<dt>ps.omrange
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<dd>adjustment of the omega range for the peak search.
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<dt>ps.chirange
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<dd>adjustment of the chi range for the peak search.
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<dt>ps.phirange
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<dd>adjustment of the phi range for the peak search.
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<dt>ps.countpar
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<dd>adjustment of the counting parameters for the peak search.
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<dt>ps.scanpar
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<dd>adjustment of the parameters used by ps.scanlist for scanning
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located peaks. See below.
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<dt>ps.maxpar
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<dd>Adjusts the maximum finding parameters for the peak search. These
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parameters need some explanation:
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<dl>
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<dt>window
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<dd>window is the size of the quadratic area which will be searched
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around each point in order to determine if it is a local maximum.
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<DT>threshold
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<dd>This is a minimum intensity a candidate local maximum must have
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before it is accepted as a peak. The value given is multiplied
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with the average counst on the data frame before use. This threshold
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is the strongest selection parameter.
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<dt>steepness
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<dd> A candidate peak should drop of towards the sides. This is
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tested for by checking if the pixels on the borders of the local
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maximum detection window are below maximum value - steepness.
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<dt>cogwindow
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<dd>In order to refine the peaks position a center of gravity
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calculation is perfomed. For this calculation pixels within the
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cogwindow around the candidate peak position are considered.
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<dt>cogcontour
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<dd>In order not to base the COG calculation on background pixels,
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only pixels above cogcontour * maxvalue are used for the
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calculation. With the spicky background at TRICS .5 seems a good value.
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</dl>
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<dt>ps.list
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<dd>lists all parameters for the peak search.
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<dt>ps.listpeaks
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<dd> lists all the peaks already found.
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<dt>ps.run filename
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<dd>starts the peak search and stores peaks identified in file
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filename.
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<dt>ps.continue
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<dd>continues a peak search which was interrupted for one reason or
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another.
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<dt>ps.scanlist
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<dd>performs an omega scan for each reflection found in the current
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peak list.
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</dl>
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</P>
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</BODY>
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</HTML>
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