<!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>Instrument Description: CASPEC</TITLE> <META NAME="description" CONTENT="Instrument Description: CASPEC"> <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="node478.html"> <LINK REL="previous" HREF="node476.html"> <LINK REL="up" HREF="node455.html"> <LINK REL="next" HREF="node478.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html7392" HREF="node478.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html7389" HREF="node455.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html7383" HREF="node476.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html7391" 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="tex2html7393" HREF="node478.html">Summary of reduction options</A> <B> Up:</B> <A NAME="tex2html7390" HREF="node455.html">Echelle Reduction</A> <B> Previous:</B> <A NAME="tex2html7384" HREF="node476.html">Saving the Data on</A> <BR> <BR> <!--End of Navigation Panel--> <H1><A NAME="SECTION0027120000000000000000"> </A> <A NAME="instrument-description"> </A> <BR> Instrument Description: CASPEC </H1> <P> In the present version we will be concerned with the standard configuration of CASPEC, the Cassegrain Echelle Spectrograph which is in operation at the <I>f</I>/8 Cassegrain focus of the ESO 3.6 m telescope. The instrument has been described in detail by D'Odorico and Tanné (1984) we will include here a summary of the main instrumental characteristics which are relevant for the analysis of the data. <P> In its standard configuration CASPEC uses a a 31.6 lines/mm echelle grating together with a 300 lines/mm grating cross disperser. A short focal length camera (f/1.46) focuses the beam into a thinned, back illuminated CCD consisting of <!-- MATH: $320 \times 512$ --> <IMG WIDTH="98" HEIGHT="39" ALIGN="MIDDLE" BORDER="0" SRC="img958.gif" ALT="$320 \times 512$"> <!-- MATH: $30 \mu m^2$ --> <IMG WIDTH="69" HEIGHT="48" ALIGN="MIDDLE" BORDER="0" SRC="img959.gif" ALT="$30 \mu m^2$">pixels. One pixel on the detector corresponds to an entrance aperture or <!-- MATH: $1.2 \times 0.7$ --> <IMG WIDTH="89" HEIGHT="39" ALIGN="MIDDLE" BORDER="0" SRC="img960.gif" ALT="$1.2 \times 0.7$"> seconds of arc on the sky, the first dimension being in the dispersion direction. The table <A HREF="node477.html#resolution">D.1</A> shows the change in resolution with the order number. <P> <BR> <DIV ALIGN="CENTER"><A NAME="25929"> </A> <TABLE> <CAPTION><STRONG>Table D.1:</STRONG> Resolution</CAPTION> <TR><TD> <DIV ALIGN="CENTER"> <TABLE CELLPADDING=3 BORDER="1" ALIGN="CENTER"> <TR><TH ALIGN="RIGHT"><B>Order </B></TH> <TH ALIGN="LEFT"><B>Resolution </B></TH> <TH ALIGN="CENTER"><B>Central Wavelength </B></TH> </TR> <TR><TD ALIGN="RIGHT"> </TD> <TD ALIGN="LEFT">(Å/pixel)</TD> <TD ALIGN="CENTER">(Å)</TD> </TR> <TR><TD ALIGN="RIGHT">140</TD> <TD ALIGN="LEFT">0.125<IMG WIDTH="22" HEIGHT="39" ALIGN="MIDDLE" BORDER="0" SRC="img961.gif" ALT="$\pm$"> 0.01</TD> <TD ALIGN="CENTER">4062.21</TD> </TR> <TR><TD ALIGN="RIGHT">130</TD> <TD ALIGN="LEFT">0.133</TD> <TD ALIGN="CENTER">4374.95</TD> </TR> <TR><TD ALIGN="RIGHT">120</TD> <TD ALIGN="LEFT">0.143</TD> <TD ALIGN="CENTER">4739.34</TD> </TR> <TR><TD ALIGN="RIGHT">110</TD> <TD ALIGN="LEFT">0.157</TD> <TD ALIGN="CENTER">5170.49</TD> </TR> <TR><TD ALIGN="RIGHT">100</TD> <TD ALIGN="LEFT">0.174</TD> <TD ALIGN="CENTER">5687.50</TD> </TR> <TR><TD ALIGN="RIGHT">90</TD> <TD ALIGN="LEFT">0.194</TD> <TD ALIGN="CENTER">6319.20</TD> </TR> <TR><TD ALIGN="RIGHT">80</TD> <TD ALIGN="LEFT">0.217</TD> <TD ALIGN="CENTER">7109.60</TD> </TR> <TR><TD ALIGN="RIGHT">70</TD> <TD ALIGN="LEFT">0.244</TD> <TD ALIGN="CENTER">8120.00</TD> </TR> </TABLE></DIV> <A NAME="resolution"> </A> </TD></TR> </TABLE> </DIV> <BR> <P> In this configuration, the spectrograph records in a single CCD frame an <IMG WIDTH="62" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img962.gif" ALT="$\sim 900$"> Å wide portion of the spectrum of objects with <IMG WIDTH="74" HEIGHT="41" ALIGN="MIDDLE" BORDER="0" SRC="img963.gif" ALT="$V \leq 15$">with a resolving power of <!-- MATH: $\sim 20,000$ --> <IMG WIDTH="93" HEIGHT="39" ALIGN="MIDDLE" BORDER="0" SRC="img964.gif" ALT="$\sim 20,000$"> and a signal-to-noise ratio <IMG WIDTH="50" HEIGHT="39" ALIGN="MIDDLE" BORDER="0" SRC="img965.gif" ALT="$\geq 10$">. This magnitude limit is set primarily by the readout noise of the chip (50 electrons rms) and by the maximum exposure time of <IMG WIDTH="62" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img966.gif" ALT="$\sim 120$"> min before contamination by cosmic rays becomes a problem. Fainter objects can be observe at lower resolution by binning the CCD data. <P> <I>Scattered light.</I> <P> Assuming a plane grating, used in near-Littrow mode, the blaze function <I>R</I> at wavelength <IMG WIDTH="18" HEIGHT="22" ALIGN="BOTTOM" BORDER="0" SRC="img967.gif" ALT="$\lambda$"> is approximated by <BR><P></P> <DIV ALIGN="CENTER"> <!-- MATH: \begin{equation} R(\lambda) = {\sin^2 \pi \alpha X \over (\pi \alpha X)^2} \end{equation} --> <TABLE WIDTH="100%" ALIGN="CENTER"> <TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><A NAME="echelle-10"> </A><IMG WIDTH="165" HEIGHT="63" SRC="img968.gif" ALT="\begin{displaymath}R(\lambda) = {\sin^2 \pi \alpha X \over (\pi \alpha X)^2} \end{displaymath}"></TD> <TD WIDTH=10 ALIGN="RIGHT"> (20.1)</TD></TR> </TABLE> </DIV> <BR CLEAR="ALL"><P></P> where <IMG WIDTH="19" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img969.gif" ALT="$\alpha$"> is a grating `constant' with value between 0.5 and 1, and <!-- MATH: $X=m(1-\lambda_c(m)/\lambda)$ --> <IMG WIDTH="210" HEIGHT="44" ALIGN="MIDDLE" BORDER="0" SRC="img970.gif" ALT="$X=m(1-\lambda_c(m)/\lambda)$">, in which <I>m</I> is the order number, and <!-- MATH: $\lambda_c(m)$ --> <IMG WIDTH="63" HEIGHT="44" ALIGN="MIDDLE" BORDER="0" SRC="img971.gif" ALT="$\lambda_c(m)$"> is the central wavelength of order <I>m</I>. Both parameters are related through the grating `constant' <I>k</I> by <!-- MATH: $k=m\lambda_c(m)$ --> <IMG WIDTH="124" HEIGHT="44" ALIGN="MIDDLE" BORDER="0" SRC="img972.gif" ALT="$k=m\lambda_c(m)$">. In table <A HREF="node477.html#blaze-parameters">D.2</A> we include approximate values for the parameters <I>k</I> and <IMG WIDTH="19" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img973.gif" ALT="$\alpha$">. These are mean values, given that the actual values for a given observation are a function of the order number and depend also on the instrumental set up. <P> <BR> <DIV ALIGN="CENTER"><A NAME="25944"> </A> <TABLE> <CAPTION><STRONG>Table D.2:</STRONG> Blaze parameters</CAPTION> <TR><TD> <DIV ALIGN="CENTER"> <TABLE CELLPADDING=3 BORDER="1" ALIGN="CENTER"> <TR><TH ALIGN="LEFT"><B>Grating </B></TH> <TD ALIGN="CENTER"><I>k</I></TD> <TD ALIGN="CENTER"><IMG WIDTH="20" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img974.gif" ALT="$\alpha $"></TD> </TR> <TR><TD ALIGN="LEFT">lines/mm</TD> <TD ALIGN="CENTER"> </TD> <TD ALIGN="CENTER"> </TD> </TR> <TR><TD ALIGN="LEFT">31.6</TD> <TD ALIGN="CENTER">568746.</TD> <TD ALIGN="CENTER">0.8</TD> </TR> <TR><TD ALIGN="LEFT">51.</TD> <TD ALIGN="CENTER">344705.</TD> <TD ALIGN="CENTER">0.8</TD> </TR> </TABLE></DIV> <A NAME="blaze-parameters"> </A></TD></TR> </TABLE> </DIV> <BR> <P> <HR> <!--Navigation Panel--> <A NAME="tex2html7392" HREF="node478.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html7389" HREF="node455.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html7383" HREF="node476.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html7391" 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="tex2html7393" HREF="node478.html">Summary of reduction options</A> <B> Up:</B> <A NAME="tex2html7390" HREF="node455.html">Echelle Reduction</A> <B> Previous:</B> <A NAME="tex2html7384" HREF="node476.html">Saving the Data on</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>