<!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>Standard construction</TITLE> <META NAME="description" CONTENT="Standard construction"> <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="node659.html"> <LINK REL="previous" HREF="node657.html"> <LINK REL="up" HREF="node657.html"> <LINK REL="next" HREF="node659.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html9729" HREF="node659.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html9726" HREF="node657.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html9720" HREF="node657.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html9728" 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="tex2html9730" HREF="node659.html">Alternative construction</A> <B> Up:</B> <A NAME="tex2html9727" HREF="node657.html">Construction of Flat Fields</A> <B> Previous:</B> <A NAME="tex2html9721" HREF="node657.html">Construction of Flat Fields</A> <BR> <BR> <!--End of Navigation Panel--> <H3><A NAME="SECTION003432100000000000000"> </A> <A NAME="irac2:standard-construction"> </A> <BR> Standard construction </H3> There are two ways to produce flat fields for use with IR data. The standard method is to take 'LAMP ON' and 'LAMP OFF' exposures of the calibration spot on the telescope dome. The 'LAMP ON' exposure contains light from both the calibration lamp, which should provide the even illumination needed for the flat field, together with potentially uneven illumination from the telescope and instrument environments. To retain only the even calibration illumination, the ``LAMP OFF'' image must be subtracted from the ``LAMP ON'' image. This subtraction also removes the bias pattern from the flat field image. <P> The subtracted image should then be normalised for later use as a flat field. <P> In MIDAS, you may directly use <TT>COMPUTE/IMAGE</TT> to perform the subtraction. The image mean may then be calculated using <TT>STATISTICS/IMAGE</TT> or similar. The ``LAMP on - LAMP OFF'' image can then be divided by this value using <TT>COMPUTE/IMAGE</TT> for normalisation. Alternatively, the procedure MKFLAT/IRAC2 combines these actions. To improve the signal-to-noise ratio of the flat-field, a number of flat field images may be used together. These should be combined using <TT>COMBINE/CCD</TT> (see later). <P> <HR> <!--Navigation Panel--> <A NAME="tex2html9729" HREF="node659.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html9726" HREF="node657.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html9720" HREF="node657.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html9728" 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="tex2html9730" HREF="node659.html">Alternative construction</A> <B> Up:</B> <A NAME="tex2html9727" HREF="node657.html">Construction of Flat Fields</A> <B> Previous:</B> <A NAME="tex2html9721" HREF="node657.html">Construction of Flat Fields</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
