<!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>Dark-current and particle hits correction</TITLE> <META NAME="description" CONTENT="Dark-current and particle hits 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="node466.html"> <LINK REL="previous" HREF="node464.html"> <LINK REL="up" HREF="node463.html"> <LINK REL="next" HREF="node466.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html7254" HREF="node466.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html7251" HREF="node463.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html7245" HREF="node464.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html7253" 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="tex2html7255" HREF="node466.html">Standard orientation</A> <B> Up:</B> <A NAME="tex2html7252" HREF="node463.html">Preprocessing</A> <B> Previous:</B> <A NAME="tex2html7246" HREF="node464.html">Bias correction</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION002782000000000000000"> Dark-current and particle hits correction</A> </H2> <P> In a similar manner, long dark exposures of the same exposure time can be averaged to produce the <TT>DARK</TT> frames. Please note that the output of <TT>AVERAGE/WINDOW</TT> is in units of the exposure time which is usually in seconds. Use the command <TT>COMPUTE/IMAGE</TT> to multiply the images by the exposure time and to add back the value for <TT>FLAT_BKG</TT>. Long dark exposures, and object and standard star frames contain a huge amount of cosmic events. If multiple exposures exist the above mentioned method can be used to average the frames and to remove the spikes. If not one can obtain good results by applying a median filtering algorithm to the frames (W.K. Pratt, Digital Image Processing, John Wiley & Sons, 1978). The corresponding MIDAS command is:<BLOCKQUOTE> <TT>FILTER/MEDIAN input output 1,1,threshold NA</TT></BLOCKQUOTE>with the threshold depending on the r.m.s. noise in the exposure. The noise in the (pre-flashed) <TT>BIAS</TT> and <TT>DARK</TT> frames can further be decreased by smoothing the images via e.g.<BLOCKQUOTE> <TT>FILTER/SMOOTH input output 10,10</TT></BLOCKQUOTE>After having done all these steps the final (pre-flashed) <TT>BIAS</TT> and <TT>DARK</TT> frames can be subtracted using <TT>COMPUTE/IMAGE</TT> from the <TT>FLAT</TT>, <TT>OBJ</TT> and <TT>STD</TT> frames. Often, <TT>BIAS</TT> and <TT>DARK</TT> contain no large - scale structure. Then, it is preferable to substract a constant, thus avoiding extra noise addition. <P> It is also possible to use several spectra of the same target obtained under the same instrumental configuration. Offsets of the target resulting from the positioning of the target on the entrance slit of the spectrograph and variations of exposure time must be taken into account. The commands <TT>AVERAGE/IMAGE</TT> and <TT>AVERAGE/WEIGHT</TT> offer number of options to compare the images and filter particle hits. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html7254" HREF="node466.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html7251" HREF="node463.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html7245" HREF="node464.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html7253" 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="tex2html7255" HREF="node466.html">Standard orientation</A> <B> Up:</B> <A NAME="tex2html7252" HREF="node463.html">Preprocessing</A> <B> Previous:</B> <A NAME="tex2html7246" HREF="node464.html">Bias correction</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
