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