<!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 correction</TITLE> <META NAME="description" CONTENT="Bias correction"> <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="node465.html"> <LINK REL="previous" HREF="node463.html"> <LINK REL="up" HREF="node463.html"> <LINK REL="next" HREF="node465.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html7243" HREF="node465.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html7240" HREF="node463.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html7234" HREF="node463.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html7242" 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="tex2html7244" HREF="node465.html">Dark-current and particle hits</A> <B> Up:</B> <A NAME="tex2html7241" HREF="node463.html">Preprocessing</A> <B> Previous:</B> <A NAME="tex2html7235" HREF="node463.html">Preprocessing</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION002781000000000000000"> Bias correction</A> </H2> <P> The Bias is a constant offset (for each pixel) introduced by the readout electronics of the CCD. Its value is usually between 150 and 250 ADU (for CCD's currently in use at ESO). <P> As a first step, the short dark and pre-flashed dark exposures have to be averaged to produce the <TT>BIAS</TT> and pre-flashed <TT>BIAS</TT> frame. A method which performs the averaging and which at the same time discards bad pixels due to e.g. cosmic events is <TT>AVERAGE/WINDOW</TT> Before actually executing this command, prepare a catalogue containing all input files to be averaged via: <P> <BLOCKQUOTE> <TT>CREATE/ICAT catname dirspec</TT></BLOCKQUOTE>Choose appropriate values for the descriptors <TT>FLAT_BKG</TT> and <TT>LHCUTS</TT> and run the command <BLOCKQUOTE> <TT>PREPARE/WINDOW inputspec flatbkg lhcuts</TT></BLOCKQUOTE>where <TT>inputspec</TT> is either a single file name or the catalogue with the list of input files, to be specified as catname.cat. This program writes the descriptors <TT>FLAT_BKG</TT> and <TT>LHCUTS/R/5/2</TT> into each input frame and will also read the descriptor <TT>O_TIME</TT> to get the exposure time, prompting for it if the descriptor is not present. After that the various files can be averaged via:<BLOCKQUOTE> <TT>AVERAGE/WINDOW out = catname.cat bgerr,snoise</TT></BLOCKQUOTE> <P> <HR> <!--Navigation Panel--> <A NAME="tex2html7243" HREF="node465.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html7240" HREF="node463.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html7234" HREF="node463.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html7242" 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="tex2html7244" HREF="node465.html">Dark-current and particle hits</A> <B> Up:</B> <A NAME="tex2html7241" HREF="node463.html">Preprocessing</A> <B> Previous:</B> <A NAME="tex2html7235" HREF="node463.html">Preprocessing</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
