<!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 Definition</TITLE> <META NAME="description" CONTENT="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="next" HREF="node157.html"> <LINK REL="previous" HREF="node151.html"> <LINK REL="up" HREF="node145.html"> <LINK REL="next" HREF="node153.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html3341" HREF="node153.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html3338" HREF="node145.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html3332" HREF="node151.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html3340" 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="tex2html3342" HREF="node153.html">Bivariate polynomial interpolation</A> <B> Up:</B> <A NAME="tex2html3339" HREF="node145.html">Echelle Spectra</A> <B> Previous:</B> <A NAME="tex2html3333" HREF="node151.html">Removal of particle hits</A> <BR> <BR> <!--End of Navigation Panel--> <H1><A NAME="SECTION001140000000000000000"> </A> <A NAME="background-definition"> </A> <BR> Background Definition </H1> <P> The estimation of the background is one of the critical points in the reduction of echelle spectra for two reasons. On one side, a correct estimate of the background level is necessary to compute the true flux of the object spectrum; on the other side, a wrong estimate of the background in either the flat field image (<TT>FLAT</TT>), the object (<TT>OBJ</TT>) or (optionally) the standard star (<TT>STD</TT>), will severely affect the accuracy with which instrumental effects - such as the blaze - can be corrected for. The background in an echelle image consists of: <P> <UL> <LI>a constant offset introduced by the electronics (bias), <LI>an optional constant pedestal due to the pre-flashing of the CCD, <LI>the dark current, <LI>general scattered light, <LI>diffuse light in the interorder space coming from adjacent orders. <LI>sky background spectrum </UL> <P> The first three components are removed during the preprocessing as described in Appendix <A HREF="node455.html#app:echelle">D</A>. Correction for the general scattered light and diffuse light background is presented in the three following sections. Correction for the sky background is presented in Section <A HREF="node156.html#sky-bkg">7.4.4</A>. <P> The remaining background due to scattered light is estimated from points located in the interorder space. These locations are defined when performing the order definition (command <TT>DEFINE/ECHELLE</TT> or <TT>DEFINE/HOUGH</TT>) and can be displayed by command <TT>LOAD/ECHELLE</TT>. The order definition is used to create a table back.tbl containing the background positions. Unscanned areas of the <TT>CCD</TT> can be avoided in the background estimate by using the command <TT>SCAN/ECHELLE</TT>. If bright features (e.g. sky lines) are located at the same location as background points, these locations must be unselected from table back.tbl using <TT>SELECT/BACKGROUND</TT>. Selection of the method is possible by assignment of a value to the echelle keyword <TT>BKGMTD</TT> and the background estimate, subtraction and update of the descriptor <TT>BACKGROUND</TT> is performed by the command <TT>SUBTRACT/BACKGROUND</TT> <P> <BR><HR> <!--Table of Child-Links--> <A NAME="CHILD_LINKS"> </A> <UL> <LI><A NAME="tex2html3343" HREF="node153.html">Bivariate polynomial interpolation</A> <LI><A NAME="tex2html3344" HREF="node154.html">Smoothing spline interpolation</A> <LI><A NAME="tex2html3345" HREF="node155.html">Background estimate by filtering</A> <LI><A NAME="tex2html3346" HREF="node156.html">Sky background definition</A> </UL> <!--End of Table of Child-Links--> <HR> <!--Navigation Panel--> <A NAME="tex2html3341" HREF="node153.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html3338" HREF="node145.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html3332" HREF="node151.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html3340" 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="tex2html3342" HREF="node153.html">Bivariate polynomial interpolation</A> <B> Up:</B> <A NAME="tex2html3339" HREF="node145.html">Echelle Spectra</A> <B> Previous:</B> <A NAME="tex2html3333" HREF="node151.html">Removal of particle hits</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
