<!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>Detector Non-Linearity</TITLE> <META NAME="description" CONTENT="Detector Non-Linearity"> <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="node124.html"> <LINK REL="previous" HREF="node122.html"> <LINK REL="up" HREF="node122.html"> <LINK REL="next" HREF="node124.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html2992" HREF="node124.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html2989" HREF="node122.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html2983" HREF="node122.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html2991" 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="tex2html2993" HREF="node124.html">Removing Cosmic Ray Hits</A> <B> Up:</B> <A NAME="tex2html2990" HREF="node122.html">Photometric Corrections</A> <B> Previous:</B> <A NAME="tex2html2984" HREF="node122.html">Photometric Corrections</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION001021000000000000000"> </A><A NAME="5728"> </A> <BR> Detector Non-Linearity </H2> This section outlines the transformation from raw to radiometrically corrected data. For the purpose of this simple description we make a distinction between linear and non-linear detectors. Data acquired with detectors of the latter category require, at most, bias and dark signal subtraction plus division by a flat field exposure. <P> Most real detectors have nonlinear Intensity Transfer Functions (ITF), <I>e.g.</I> photographic plates, the effects of the dead time at high count rates in photon counting systems and the low light level nonlinearity of Image Dissector Scanners (see M. Rosa, <I>The Messenger</I>, <B>39</B>, 15, 1985). If the ITF is known analytically, the command <TT>COMPUTE/IMAGE</TT> will be sufficient for the correction of the raw data. For example, the paired pulse overlap (dead-time) correction of photoelectric equipment could be corrected for by <TT>COMPUTE/IMA OUT = IN/(1+TAU*IN)</TT>, where <TT>TAU</TT> is the known time constant of the counting electronics an <TT>IN</TT> must be in units of a count rate rather than total counts. If the ITF is defined in tabular form, the command <TT>ITF/IMAGE</TT> can be used to obtain the ITF correction for each image element (pixel value) by interpolation in an ITF table; this command assumes a uniform transfer function over the image field. <P> CCDs are generally more nearly linear than are most other detectors used in astronomy. However, especially in particular pixels or regions, CCDs are clearly non-linear and/or suffer deviating signal zero points. Procedures which may be useful for the treatment of such deficiencies are (partly) described in Appendix B. <P> Problems related to background<A NAME="5737"> </A> estimation and more sophisticated flat-field corrections generally are very instrument dependent. Therefore, it is not possible to give one standard recipe here; check the various instrumental appendices for more specific advice. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html2992" HREF="node124.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html2989" HREF="node122.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html2983" HREF="node122.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html2991" 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="tex2html2993" HREF="node124.html">Removing Cosmic Ray Hits</A> <B> Up:</B> <A NAME="tex2html2990" HREF="node122.html">Photometric Corrections</A> <B> Previous:</B> <A NAME="tex2html2984" HREF="node122.html">Photometric Corrections</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
