<!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>Airmass Calculation</TITLE> <META NAME="description" CONTENT="Airmass Calculation"> <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="previous" HREF="node134.html"> <LINK REL="up" HREF="node133.html"> <LINK REL="next" HREF="node136.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html3124" HREF="node136.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html3121" HREF="node133.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html3117" HREF="node134.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html3123" 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="tex2html3125" HREF="node136.html">Spectral Analysis</A> <B> Up:</B> <A NAME="tex2html3122" HREF="node133.html">Flux Calibration</A> <B> Previous:</B> <A NAME="tex2html3118" HREF="node134.html">Flux Calibration and Extinction</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION001052000000000000000"> Airmass Calculation</A> </H2> <P> The commands in the previous Section require the airmass<A NAME="5887"> </A> as input parameter. Some instrument/telescope combinations provide raw data files with the proper values of right ascension, declination, siderial time, geographical latitude, duration of measurement and eventually even ``mean'' airmass. In most cases it will however be necessary to compute an appropriate airmass using the <TT>COMPUTE/AIRMASS</TT>. Refer to <TT>HELP COMPUTE/AIRMASS</TT> for the details of the image descriptors which are needed. Otherwise, the required information must be provided by the user on the command line. It is important to keep in mind that ``mean airmass'' and ``mean atmospheric extinction correction'' are different from the values at mid-exposure, especially for larger zenith distances. This is so because the airmass depends non-linearly on zenith distance (<IMG WIDTH="2" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img260.gif" ALT="$\sec z$">) and extinction corrections depend non-linearly on airmass ( <!-- MATH: $10^{-(0.4\times{\rm airm}\times{\rm ELAW}({\rm mag}))}$ --> <IMG WIDTH="187" HEIGHT="25" ALIGN="BOTTOM" BORDER="0" SRC="img261.gif" ALT="$10^{-(0.4\times{\rm airm}\times{\rm ELAW}({\rm mag}))}$">). For reasonable combinations of exposure time and zenith distance, the weighted mean airmass supplied by <TT>COMPUTE/AIRMASS</TT> should be appropriate. <P> <BR><HR> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>