<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <!--Converted with LaTeX2HTML 98.1p1 release (March 2nd, 1998) originally by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds * revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan * with significant contributions from: Jens Lippmann, Marek Rouchal, Martin Wilck and others --> <HTML> <HEAD> <TITLE>Detecting and Identifying Arc Lines</TITLE> <META NAME="description" CONTENT="Detecting and Identifying Arc Lines"> <META NAME="keywords" CONTENT="vol2"> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <LINK REL="STYLESHEET" HREF="vol2.css"> <LINK REL="next" HREF="node129.html"> <LINK REL="previous" HREF="node127.html"> <LINK REL="up" HREF="node127.html"> <LINK REL="next" HREF="node129.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html3049" HREF="node129.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html3046" HREF="node127.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html3040" HREF="node127.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html3048" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="icons.gif/contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html3050" HREF="node129.html">Getting the Dispersion Solution</A> <B> Up:</B> <A NAME="tex2html3047" HREF="node127.html">Geometric Correction</A> <B> Previous:</B> <A NAME="tex2html3041" HREF="node127.html">Geometric Correction</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION001031000000000000000"> Detecting and Identifying Arc Lines</A> </H2> <P> For optimum results, it is important to use a comparison spectrum with as high a signal-to-noise ratio of the lines as possible. It is therefore advisible to flat-field<A NAME="5769"> </A> also the arc spectrum in order to correct for small-scale fluctuations. Another good idea is to filter the frame along the slit using the command <TT>FILTER/MEDIAN</TT> with a rectangular filter window of one pixel in the dispersion direction and several pixels in the perpendicular direction. For the row-by-row method one could also consider smoothing the spectra along the slit axis which would provide for a stronger coupling between neighbouring rows and thereby yield a solution which is intermediate between the extremes presented by the two pure methods. <P> <UL> <P> <LI><TT>SEARCH/LONG:</TT> Finds the positions of reference lines in world coordinates. Positions are by default estimated by the center of fitted Gaussians. Other centering methods are available (Gravity, Maximum) but could result in systematic position errors (See Hensberge & Verschueren, Messenger, <B>58</B>, 51). The results are stored in a table called <TT>line.tbl</TT>. The parameter <TT>YWIND</TT> corresponds the half-size of the row averaging window applied to adjacent rows of the spectrum for an improvement of the signal to noise ratio. The parameter <TT>YSTEP</TT> controls the step in rows between successive arc line detections. The value <TT>YSTEP=1</TT> corresponds to the default row-by-row method and larger values can be used to get a quicker calibration. The algorithm detects lines whose strength exceeds a certain threshold (parameter <TT>THRES</TT>) above the local background. The local background results from a median estimate performed on a sliding window which size is controlled by the parameter <TT>WIDTH</TT>. The command <TT>PLOT/SEARCH</TT> allows to check the results at this stage. Note that for a two-dimensional spectrum, both options 1D and 2D can be used (See <TT>HELP PLOT/SEARCH</TT>). <P> <LI>The command <TT>IDENTIFY/LONG</TT> allows an initial interactive identification, by wavelength, of some of the detected lines. Spectral line atlas are provided in the instrument operating manuals. The command <TT>PLOT/IDENT</TT> visualizes the interactive identifications. <P> </UL> <P> <HR> <!--Navigation Panel--> <A NAME="tex2html3049" HREF="node129.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html3046" HREF="node127.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html3040" HREF="node127.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html3048" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="icons.gif/contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html3050" HREF="node129.html">Getting the Dispersion Solution</A> <B> Up:</B> <A NAME="tex2html3047" HREF="node127.html">Geometric Correction</A> <B> Previous:</B> <A NAME="tex2html3041" HREF="node127.html">Geometric Correction</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
