<!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 Estimates</TITLE> <META NAME="description" CONTENT="Background Estimates"> <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="node22.html"> <LINK REL="previous" HREF="node20.html"> <LINK REL="up" HREF="node19.html"> <LINK REL="next" HREF="node22.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html1709" HREF="node22.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html1706" HREF="node19.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html1700" HREF="node20.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html1708" 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="tex2html1710" HREF="node22.html">Transformations</A> <B> Up:</B> <A NAME="tex2html1707" HREF="node19.html">Image Manipulations</A> <B> Previous:</B> <A NAME="tex2html1701" HREF="node20.html">Digital Filters</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION00532000000000000000"> Background Estimates</A> </H2> In most astronomical observations the image intensity consists of contributions from both object and sky background. Depending on the complexity of the field and the type of object, different methods are used to estimate and subtract the background intensity. For linear detectors this can be done directly on the intensity calibrated frame while special consideration must be given when a non-linear response transformation is used (i.e. for photographic emulsions) due to the non-gaussian noise distribution. In the latter case a fit is normally done to the original data and the fitted values then transformed to intensities and subtracted. <P> An accurate determination of the background is extremely important for the latter analysis. Therefore, one prefers to use all pixels, which are not contaminated by sources, and fit a low order polynomial surface to the background. Non-linear filters are often used to remove stellar images and other sharp features (see Section <A HREF="node16.html#artifacts">2.2.1</A>) while extended objects are very difficult to eliminate automatically. If only point like objects are analyzed background following methods like the recursive filter described by Martin and Lutz (1979) can be used. <P> Also <!-- MATH: $\kappa \sigma$ --> <IMG WIDTH="31" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img89.gif" ALT="$\kappa \sigma$">-clipping techniques are applied in an iterative scheme where pixels with high residual compared to a low order polynomial fit to the frame are rejected (Capaccioli and Held 1979). In this method areas containing extended objects can be excluded before the iteration starts. In Figure <A HREF="node21.html#fig:bg-fit">2.8</A>, a field with extended objects is shown with the mask defining the areas to be omitted in the computations. <BR> <DIV ALIGN="CENTER"><A NAME="fig:bg-fit"> </A><A NAME="709"> </A> <TABLE WIDTH="50%"> <CAPTION><STRONG>Figure 2.8:</STRONG> Background fitting with an iterative <!-- MATH: $\kappa \sigma$ --> <IMG WIDTH="31" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img91.gif" ALT="$\kappa \sigma$"> technique: (A) original, (B) mask of included areas, and (C) fitted background.</CAPTION> <TR><TD><IMG WIDTH="1106" HEIGHT="365" SRC="img92.gif" ALT="\begin{figure}\psfig{figure=fig8_background.eps,width=15cm,clip=} \end{figure}"></TD></TR> </TABLE> </DIV> <BR> <P> <HR> <!--Navigation Panel--> <A NAME="tex2html1709" HREF="node22.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html1706" HREF="node19.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html1700" HREF="node20.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html1708" 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="tex2html1710" HREF="node22.html">Transformations</A> <B> Up:</B> <A NAME="tex2html1707" HREF="node19.html">Image Manipulations</A> <B> Previous:</B> <A NAME="tex2html1701" HREF="node20.html">Digital Filters</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>