<!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>Sky background definition</TITLE> <META NAME="description" CONTENT="Sky background definition"> <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="previous" HREF="node155.html"> <LINK REL="up" HREF="node152.html"> <LINK REL="next" HREF="node157.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html3387" HREF="node157.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html3384" HREF="node152.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html3380" HREF="node155.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html3386" 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="tex2html3388" HREF="node157.html">Order Extraction</A> <B> Up:</B> <A NAME="tex2html3385" HREF="node152.html">Background Definition</A> <B> Previous:</B> <A NAME="tex2html3381" HREF="node155.html">Background estimate by filtering</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION001144000000000000000"> </A> <A NAME="sky-bkg"> </A> <BR> Sky background definition </H2> <P> The previous methods allow to correct for the interorder scattered light. The sky background component can be measured in spectra taken with an extended slit using the two commands <TT>DEFINE/SKY</TT> and <TT>EXTRACT/SKY</TT>. The command <TT>DEFINE/SKY</TT> allows to define the offset limits of up to two sky windows, usually on both sides of the spectrum. The command <TT>EXTRACT/ECHELLE</TT> performs an average extraction of the sky background using these offsets, optionally fiters for cosmics, and provides an extracted spectrum in a format similar to the one resulting from <TT>EXTRACT/ECHELLE</TT>. The two extracted images can be subtracted for sky background correction. It can be noted that the background measured by the command <TT>EXTRACT/SKY</TT> includes also the interorder scattered light components and makes in principle unnecessary the correction described in the previous Section. <P> <BR><HR> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
