<!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>What was the measurement?</TITLE> <META NAME="description" CONTENT="What was the measurement?"> <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="node636.html"> <LINK REL="previous" HREF="node634.html"> <LINK REL="up" HREF="node634.html"> <LINK REL="next" HREF="node636.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html9448" HREF="node636.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html9445" HREF="node634.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html9439" HREF="node634.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html9447" 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="tex2html9449" HREF="node636.html">What was measured?</A> <B> Up:</B> <A NAME="tex2html9446" HREF="node634.html">Required observational data</A> <B> Previous:</B> <A NAME="tex2html9440" HREF="node634.html">Required observational data</A> <BR> <BR> <!--End of Navigation Panel--> <H3><A NAME="SECTION003261100000000000000"> What was the measurement?</A> </H3> The measurements themselves should be stored in a column with the label SIGNAL. These are Real*4 data; the format specified in the table may depend on the instrument. Often the readings are recorded as integers; then assume the decimal point at the end of the field. <P> Programs will expect that SIGNAL represents the integration of the photon flux for the exposure time given in the EXPTIME column (see section <A HREF="node638.html#exptime">I.6.1</A>). That is, it is the <I>ratio</I> of SIGNAL to EXPTIME that represents an actual photon count <I>rate</I>, or intensity, if the exposure times vary. <P> This assumes the data are in arbitrary intensity units (times time). Sometimes one must deal with data in the form of magnitudes, as in re-reducing old data, or data measured from strip-charts with a magnitude ruler. In that case, the column label should be RAWMAG; see Table <A HREF="node642.html#tbl:other-obs-cols">I.18</A>. Programs will expect RAWMAG to be the usual negative logarithms of intensities; that is, RAWMAG values only use EXPTIME in determining weights. <P> Except for pulse counting with photomultipliers, where reasonably accurate models exist for the nonlinearity, the SIGNAL values should have been corrected for nonlinearity. In any case, the EXPTIME values should have been corrected for differences between nominal and actual exposure times. Such corrections are especially important for CCD data, where they can vary across the frame (see Stetson [<A HREF="node649.html#S89">7</A>]). <P> <HR> <!--Navigation Panel--> <A NAME="tex2html9448" HREF="node636.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html9445" HREF="node634.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html9439" HREF="node634.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html9447" 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="tex2html9449" HREF="node636.html">What was measured?</A> <B> Up:</B> <A NAME="tex2html9446" HREF="node634.html">Required observational data</A> <B> Previous:</B> <A NAME="tex2html9440" HREF="node634.html">Required observational data</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
