<!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>Purpose</TITLE> <META NAME="description" CONTENT="Purpose"> <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="node498.html"> <LINK REL="up" HREF="node498.html"> <LINK REL="next" HREF="node500.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html7686" HREF="node500.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html7683" HREF="node498.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html7679" HREF="node498.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html7685" 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="tex2html7687" HREF="node500.html">Retrieving demonstration and calibration</A> <B> Up:</B> <A NAME="tex2html7684" HREF="node498.html">Introduction</A> <B> Previous:</B> <A NAME="tex2html7680" HREF="node498.html">Introduction</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION003011000000000000000"> Purpose</A> </H2> <P> As described in Chapter <A HREF="node676.html#spectral-data">L</A>, the reduction of long slit spectra comprises three main steps: <P> <DL COMPACT> <DT>1. <DD>Pre-reduction: <BR> - removal of cosmic rays <BR> - flat-fielding <BR> - dark subtraction. <DT>2. <DD>Geometric corrections: <BR> - correction for distortions perpendicular to the dispersion direction (<I>e.g.</I>, due to differential refraction in the earth's atmosphere) <BR> - determination of the dispersion curve <BR> - resampling of the data to constant step in wavelength. <DT>3. <DD>Final photometric operations : <BR> - sky subtraction <BR> - flux calibration. </DL> <P> Section <A HREF="node501.html#c-book">G.3</A> is a summary of a typical session but without much explanation on the methods or general advice. First-time users may find it useful to follow it as it stands, step by step, in order to become acquainted with the plain technical operation of the available commands. Section <A HREF="node521.html#ap:XLong">G.4</A> presents the Graphical User Interface XLong. The interface XLong is a convenient way to gain visibility on the parameter values, avoid command syntax problems and understand the organisation of the context. <P> <BLOCKQUOTE> <DIV ALIGN="CENTER"> <B>Note</DIV> <I>Although this Appendix is intended to be self-contained, it cannot serve as a substitute for the <TT>HELP</TT> information available for all the individual commands mentioned. Full advantage of the flexibility of the tools offered for the reduction of long slit spectra can be taken only on the basis of the command's detailed explanations in Volume A and more specifically in Chapter 6 (long-slit spectra) of Volume B.</I></B></BLOCKQUOTE> <P> <HR> <!--Navigation Panel--> <A NAME="tex2html7686" HREF="node500.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html7683" HREF="node498.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html7679" HREF="node498.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html7685" 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="tex2html7687" HREF="node500.html">Retrieving demonstration and calibration</A> <B> Up:</B> <A NAME="tex2html7684" HREF="node498.html">Introduction</A> <B> Previous:</B> <A NAME="tex2html7680" HREF="node498.html">Introduction</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
