<!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="node158.html">
<LINK REL="previous" HREF="node152.html">
<LINK REL="up" HREF="node145.html">
<LINK REL="next" HREF="node158.html">
</HEAD>
<BODY >
<!--Navigation Panel-->
<A NAME="tex2html3398"
HREF="node158.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next"
SRC="icons.gif/next_motif.gif"></A>
<A NAME="tex2html3395"
HREF="node145.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up"
SRC="icons.gif/up_motif.gif"></A>
<A NAME="tex2html3389"
HREF="node156.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous"
SRC="icons.gif/previous_motif.gif"></A>
<A NAME="tex2html3397"
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="tex2html3399"
HREF="node158.html">Wavelength Calibration</A>
<B> Up:</B> <A NAME="tex2html3396"
HREF="node145.html">Echelle Spectra</A>
<B> Previous:</B> <A NAME="tex2html3390"
HREF="node156.html">Sky background definition</A>
<BR>
<BR>
<!--End of Navigation Panel-->
<H1><A NAME="SECTION001150000000000000000"> </A>
<A NAME="order-extraction"> </A>
<BR>
Order Extraction
</H1>
Individual echelle orders are extracted by adding the pixel values
over a numerical slit running along the orders, with the position of
the slit center defined by equation <A HREF="node150.html#echelle-2">7.2</A>. The width of the
slit is one pixel and its length, as well as an optional offset to
shift the slit prependicular to the dispersion direction, are defined
by the user.
<P>
The pixel values in the numerical slit are found by linear
interpolation of the values in the image. The extracted flux number
is the weighted average of these interpolated pixel values. There are
three types of weights (<I>w</I>) depending on the selected option:
<P>
<UL>
<LI>option <TT>LINEAR</TT>: <I>w</I> = 1,
<LI>option <TT>AVERAGE</TT>: <I>w</I>= 1/<I>L</I> where <I>L</I> is the length of the slit,
<LI>option <TT>OPTIMAL</TT>: weights optimising the signal to noise ratio
of the extracted spectrum. This algorithm is based on (Mukai, 1990)
</UL>
<P>
There are several effects to consider when defining the length of the
extraction slit. If the length is too small, the orders are only partially
extracted and they present a periodic variation due to the
inclination of the orders with respect to the lines in the image.
On the other side, if the slit is too large, the extracted flux will
include noisy pixels from the flatfielded background, when
the flat field correction is applied.
<P>
The command <TT>EXTRACT/ECHELLE</TT> is used for the order extraction. The method
is selected using echelle keyword <TT>EXTMTD</TT>.
<P>
<HR>
<!--Navigation Panel-->
<A NAME="tex2html3398"
HREF="node158.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next"
SRC="icons.gif/next_motif.gif"></A>
<A NAME="tex2html3395"
HREF="node145.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up"
SRC="icons.gif/up_motif.gif"></A>
<A NAME="tex2html3389"
HREF="node156.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous"
SRC="icons.gif/previous_motif.gif"></A>
<A NAME="tex2html3397"
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="tex2html3399"
HREF="node158.html">Wavelength Calibration</A>
<B> Up:</B> <A NAME="tex2html3396"
HREF="node145.html">Echelle Spectra</A>
<B> Previous:</B> <A NAME="tex2html3390"
HREF="node156.html">Sky background definition</A>
<!--End of Navigation Panel-->
<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
</ADDRESS>
</BODY>
</HTML>
