<!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>Order extraction</TITLE> <META NAME="description" CONTENT="Order extraction"> <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="node703.html"> <LINK REL="previous" HREF="node701.html"> <LINK REL="up" HREF="node696.html"> <LINK REL="next" HREF="node703.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html10264" HREF="node703.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html10261" HREF="node696.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html10255" HREF="node701.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html10263" 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="tex2html10265" HREF="node703.html">Flat-fielding</A> <B> Up:</B> <A NAME="tex2html10262" HREF="node696.html">FEROS</A> <B> Previous:</B> <A NAME="tex2html10256" HREF="node701.html">Background subtraction</A> <BR> <BR> <!--End of Navigation Panel--> <H1><A NAME="SECTION003760000000000000000"> Order extraction</A> </H1> <P> After background subtraction, the spectral orders can be extracted. This is done by defining offsets in x-direction (with respect to the center of both fibers) and summing up the flux within a defined slit-width around these offsets. Both fibers are treated completely separately. <P> For reason of efficiency and simplicity, this is done in two steps. <P> <UL> <LI> First, the pixels are re-ordered (rectified) in a new 2-D frame. It is important to note that this step involves <EM>no</EM> resampling. The fractional pixel offsets are kept in a separate file. The corresponding command is RECTIFY/FEROS. <LI> Second, the fluxes are summed up, producing a new 2-D frame (pixel-order space) for each fiber. The corresponding command is EXTRACT/FEROS. </UL> <P> The extraction can be done by straight summation or by using an optimum extraction algorithm which also detects and removes cosmic ray events. Optimum extraction requires a good knowledge of the cross-order profile (COP). For FEROS, this profile is - due to the image slicer - very complicated. It is approximated by fitting the fractional flux per pixel at several distances from the center of the COP as a function of position along the order with a polynomial. Since rebinning has to be avoided, this step is complex and CPU intensive. Thus, the COP can be saved for further use. For FEROS, the COP is typically determined from a flat-field exposure and later used for extraction of object spectra. This assumes that the COP does not change its profile, which can reasonably be assumed only for fiber-linked spectrographs. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html10264" HREF="node703.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html10261" HREF="node696.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html10255" HREF="node701.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html10263" 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="tex2html10265" HREF="node703.html">Flat-fielding</A> <B> Up:</B> <A NAME="tex2html10262" HREF="node696.html">FEROS</A> <B> Previous:</B> <A NAME="tex2html10256" HREF="node701.html">Background subtraction</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
