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<H3><A NAME="SECTION003034100000000000000">
Line Detection in the Comparison Spectrum</A>
</H3>
<P>
The first step consists of detecting the emission lines of the comparison
spectrum to produce the line table <B>line.tbl</B>. It can be noted that
the command <TT>SAVE/LONG</TT> will store all keyword values as descriptors
of this table, so that a session can be saved only if the table <B>line.tbl</B>
is available. The commands:
<BR> <BR>
Midas...> <TT>SET/LONG WLC=<B>ccd0003</B> THRES=20. WIDTH=8 YWIDTH=3 YSTEP=1</TT>
<BR>
Midas...> <TT>SEARCH/LONG</TT>
<BR>
<BR>
initialize the line searching parameters and <TT>SEARCH/LONG</TT> performs
the following operations:
<P>
<UL>
<LI>Average 2*
<!-- MATH: $\fbox{YWIDTH}$ -->
<IMG
WIDTH="110" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1022.gif"
ALT="\fbox{YWIDTH}">+1 rows of image
<!-- MATH: $\fbox{WLC}$ -->
<IMG
WIDTH="66" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1023.gif"
ALT="\fbox{WLC}">
in the Y
direction at a step
<!-- MATH: $\fbox{YSTEP}$ -->
<IMG
WIDTH="88" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1024.gif"
ALT="\fbox{YSTEP}">
<LI>for each resulting row, detect emission lines above
a local background computed by median estimate in a sliding window
of size
<!-- MATH: $\fbox{WIDTH}$ -->
<IMG
WIDTH="93" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1025.gif"
ALT="\fbox{WIDTH}">
and with a peak intensity
<!-- MATH: $\fbox{THRES}$ -->
<IMG
WIDTH="89" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1026.gif"
ALT="\fbox{THRES}">
counts
above the local background;
<LI>determine line centers by fitting Gaussians to the line profiles.
</UL>
<P>
The parameter
<!-- MATH: $\fbox{YWIDTH}$ -->
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SRC="img1028.gif"
ALT="\fbox{YWIDTH}">
can be used to improve the signal-to-noise
ratio of each row by averaging adjacent rows. The parameter
<!-- MATH: $\fbox{YSTEP}$ -->
<IMG
WIDTH="88" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1029.gif"
ALT="\fbox{YSTEP}">is by default equal to 1 which corresponds to the standard row-by-row
method but can be set to a larger value to accelerate the wavelength
calibration. The detections are stored in the following columns of the
table <B>line.tbl</B>: <BR>
<P>
<TABLE CELLPADDING=3>
<TR><TD ALIGN="LEFT">Column label</TD>
<TD ALIGN="LEFT">:X</TD>
<TD ALIGN="LEFT">- X-positions of spectral lines</TD>
</TR>
<TR><TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT">:Y</TD>
<TD ALIGN="LEFT">- Y-position (row)</TD>
</TR>
<TR><TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT">:PEAK</TD>
<TD ALIGN="LEFT">- peak intensity of lines</TD>
</TR>
</TABLE> <BR>
<P>
Results can be checked with the command <TT>PLOT/SEARCH</TT>. For
one-dimensional spectra,
<BR> <BR>
Midas...> <TT>PLOT/SEARCH</TT> <BR>
<P>
plots the central row
<!-- MATH: $\fbox{YSTART}$ -->
<IMG
WIDTH="103" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1030.gif"
ALT="\fbox{YSTART}">
of the spectrum as well as
the position of the detected lines. This behavior
is obtained for 2D spectra by the additional option 1D, as in:
<BR> <BR>
Midas...> <TT>PLOT/SEARCH 1D</TT> <BR>
<BR>
whereas typing the command without option provides a bi-dimensional
graph of the X- and Y-positions of the detected lines.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
</ADDRESS>
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