<!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>Bias and Dark Subtraction</TITLE> <META NAME="description" CONTENT="Bias and Dark Subtraction"> <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="node126.html"> <LINK REL="previous" HREF="node124.html"> <LINK REL="up" HREF="node122.html"> <LINK REL="next" HREF="node126.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html3014" HREF="node126.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html3011" HREF="node122.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html3005" HREF="node124.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html3013" 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="tex2html3015" HREF="node126.html">Flat-Fielding</A> <B> Up:</B> <A NAME="tex2html3012" HREF="node122.html">Photometric Corrections</A> <B> Previous:</B> <A NAME="tex2html3006" HREF="node124.html">Removing Cosmic Ray Hits</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION001023000000000000000"> Bias and Dark Subtraction</A> </H2> <P> The dark signal is exposure time dependent. The one appropriate for a given observation has to be derived by interpolation between dark frames of different exposure times. Note that long dark exposures first have to be cleaned of particle hits (see above). Bias and dark signal will usually be determined and subtracted in the same step. <P> In a CCD with good cosmetic properties, bias and dark signal<A NAME="5750"> </A><A NAME="5751"> </A> are the same for all pixels. Then, only a number (obtained from, <I>e.g.</I>, <TT>STATISTICS/IMAGE</TT>) should be subtracted in order to prevent the noise in the dark frames to propagate into the reduced spectra. If, because of local variations, this is not possible, some suitable smoothing should still be attempted. <P> <BR><HR> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
