<!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>Introduction</TITLE> <META NAME="description" CONTENT="Introduction"> <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="node652.html"> <LINK REL="previous" HREF="node650.html"> <LINK REL="up" HREF="node650.html"> <LINK REL="next" HREF="node652.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html9641" HREF="node652.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html9638" HREF="node650.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html9632" HREF="node650.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html9640" 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="tex2html9642" HREF="node652.html">Online Reduction</A> <B> Up:</B> <A NAME="tex2html9639" HREF="node650.html">IRAC2 Online and Off-line</A> <B> Previous:</B> <A NAME="tex2html9633" HREF="node650.html">IRAC2 Online and Off-line</A> <BR> <BR> <!--End of Navigation Panel--> <H1><A NAME="SECTION003410000000000000000"> </A> <A NAME="irac2:introduction"> </A> <BR> Introduction </H1> In many ways, IR array data is similar to that taken with optical CCDs; however, there are a number of important differences that are mainly due to the large, variable sky and instrumental backgrounds that arise at these longer wavelengths. In extreme cases, the objects of interest can be several thousand times fainter than the sky background. This, together with the variable sky background and the unusual bias patterns of some readout methods, implies that objects of interest are generally not visible in a single image. <P> For the above mentioned reasons, fully automated reduction is difficult to achieve, and is unlikely to produce optimally reduced data even if achievable. It is thus inevitable that a more ``hands on'' approach be adopted for the data reduction, with the astronomer monitoring the quality of such things as flat fielding and sky subtraction much more carefully than is usually the case for optical CCD data. <P> In addition, if observers are to get the most out of their observations, some online processing of the data at the telescope is required. The IRAC2 context in MIDAS includes a number of commands that are most useful at the telescope. <P> The first part of this appendix concentrates on the data reduction that can be done at the telescope. The second part describes what is required for off-line reduction of IRAC2 data (and IR data in general), with a specific view towards use of standard MIDAS routines. <P> The IRAC2 context can be activated by the command <TT>SET/CONTEXT IRAC2</TT>. Together with the IRAC2 context, the CCDRED context is also activated. The CCDRED context contains many useful commands for image combining, mosaicing, etc. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html9641" HREF="node652.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html9638" HREF="node650.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html9632" HREF="node650.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html9640" 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="tex2html9642" HREF="node652.html">Online Reduction</A> <B> Up:</B> <A NAME="tex2html9639" HREF="node650.html">IRAC2 Online and Off-line</A> <B> Previous:</B> <A NAME="tex2html9633" HREF="node650.html">IRAC2 Online and Off-line</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
