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<H1><A NAME="SECTION0027120000000000000000">&#160;</A>
<A NAME="instrument-description">&#160;</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 Tann&#233; 
(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&nbsp;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&nbsp;<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">&#160;</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">&nbsp;</TD>
<TD ALIGN="LEFT">(&#197;/pixel)</TD>
<TD ALIGN="CENTER">(&#197;)</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">&#160;</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="$\sim 900$">
&#197; 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">&#160;</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&nbsp;<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">&#160;</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">&nbsp;</TD>
<TD ALIGN="CENTER">&nbsp;</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">&#160;</A></TD></TR>
</TABLE>
</DIV>
<BR>
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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