<!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>Flat Field Correction</TITLE> <META NAME="description" CONTENT="Flat Field 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="node165.html"> <LINK REL="previous" HREF="node158.html"> <LINK REL="up" HREF="node145.html"> <LINK REL="next" HREF="node165.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html3478" HREF="node165.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html3475" HREF="node145.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html3469" HREF="node163.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html3477" 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="tex2html3479" HREF="node165.html">Instrument Response Correction</A> <B> Up:</B> <A NAME="tex2html3476" HREF="node145.html">Echelle Spectra</A> <B> Previous:</B> <A NAME="tex2html3470" HREF="node163.html">Resampling and checking the</A> <BR> <BR> <!--End of Navigation Panel--> <H1><A NAME="SECTION001170000000000000000"> </A> <A NAME="flat-field-correction"> </A> <BR> Flat Field Correction </H1> <P> The main disadvantage of the thin CCD-chips used currently as detectors in some instruments is the generation of interference fringes, that is, intensity fluctuations in the spectra which can be as high as 30% at <!-- MATH: $\lambda>6000$ --> <IMG WIDTH="46" HEIGHT="39" ALIGN="MIDDLE" BORDER="0" SRC="img321.gif" ALT="$\lambda>6000$">Å (York, 1981). These fringes arise from interferences within the silicon for long wavelengths, while for shorter wavelengths they can be due to the interfaces silicon-glass on the back side of the chip. This effect is constant for a given setting and it can be effectively corrected by dividing the object image by a flat-field exposure taken with the same instrument configuration. Before the actual division is carried out, the background levels, both in the object image and flat-field, are subtracted and the flat-field is normalised. <P> The flat-field correction is done with the command <TT>COMPUTE/IMAGE</TT>, this command divides the background subtracted <TT>OBJ</TT> by the normalised flat-field as computed by the command <TT>FLAT/ECHELLE</TT>. <P> <BR><HR> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
