<!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>Distortion Along the Slit</TITLE> <META NAME="description" CONTENT="Distortion Along the Slit"> <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="node131.html"> <LINK REL="previous" HREF="node129.html"> <LINK REL="up" HREF="node127.html"> <LINK REL="next" HREF="node131.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html3071" HREF="node131.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html3068" HREF="node127.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html3062" HREF="node129.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html3070" 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="tex2html3072" HREF="node131.html">Resampling the Data</A> <B> Up:</B> <A NAME="tex2html3069" HREF="node127.html">Geometric Correction</A> <B> Previous:</B> <A NAME="tex2html3063" HREF="node129.html">Getting the Dispersion Solution</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION001033000000000000000"> Distortion Along the Slit</A> </H2> <P> The distortion<A NAME="5827"> </A> along the slit <!-- MATH: $s = s(X, Y)$ --> <I>s</I> = <I>s</I>(<I>X</I>, <I>Y</I>) can be modelled using an image containing one or several spectra of (ideally: point-) sources with well defined continuum. The command <TT>SEARCH/LONG</TT> can be used to generate a table with the positions of the spectra at several wavelengths. (Contrary to the general conventions used throughout this chapter, for this particular application the dispersion direction must be parallel to the `Y'-axis!) The distortion is then modelled by a two-dimensional polynomial fitted to these positions (using, <I>e.g.</I>, <TT>REGRESSION/TABLE</TT>). If the resulting coefficients are stored in the keyword <TT>COEFY*, (*=I,R,D)</TT> and combined with suitable (if necessary: dummy) coefficients for the dispersion resolution in the keyword <TT>KEYLONG*, (*=I,R,D)</TT>, the command <TT>RECTIFY/LONG</TT> can be applied. <P> Note that these steps are not implemented as a convenient-to-use high-level command procedure. <P> <BR><HR> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
