<!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>Background Correction</TITLE> <META NAME="description" CONTENT="Background Correction"> <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="node474.html"> <LINK REL="previous" HREF="node472.html"> <LINK REL="up" HREF="node469.html"> <LINK REL="next" HREF="node474.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html7352" HREF="node474.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html7349" HREF="node469.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html7343" HREF="node472.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html7351" 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="tex2html7353" HREF="node474.html">Flat Field correction</A> <B> Up:</B> <A NAME="tex2html7350" HREF="node469.html">Reduction using Standard Stars</A> <B> Previous:</B> <A NAME="tex2html7344" HREF="node472.html">Wavelength calibration</A> <BR> <BR> <!--End of Navigation Panel--> <H3><A NAME="SECTION0027101400000000000000"> Background Correction</A> </H3> <P> Two background corrections can be performed with the package: <P> <B>Interorder background:</B> The scattered light in the interorder region is measured at a certain number of positions and the background map is provided by polynomial or spline interpolation. Interorder positions can be visualized with the commands: <P> <BLOCKQUOTE> <TT>SET/ECH BKGVISU=YES <BR> LOAD/ECHELLE</TT></BLOCKQUOTE> <P> and the interpolation and background subtraction is performed using <TT>SUBTRACT/BACKGROUND</TT>. This command also update the image descriptors to avoid background subtraction to be performed again by the <TT>FLAT, RESPONSE</TT> or <TT>REDUCE</TT> commands. <P> <B>Sky background:</B> Sky windows are defined on both side of the orders with the comamnd <TT>DEFINE/SKY</TT> and sky background is extracted using <TT>EXTRACT/SKY</TT>. The extracted sky can be subtracted from the extracted spectrum like in: <P> <BLOCKQUOTE> <TT>DEFINE/SKY object 2 CURSOR <BR> EXTRACT/SKY object sky FILTER <BR> SET/ECHELLE EXTMTD=AVERAGE SLIT=8 <BR> EXTRACT/ECH object ext <BR> COMPUTE/IMAGE sub = ext - sky <BR></TT></BLOCKQUOTE> <P> <BR><HR> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
