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<H2><A NAME="SECTION001181000000000000000">
Using a Standard Star</A>
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<P>
The extracted orders, together with the dispersion relation, define
the observed flux as a function of the wavelength for each order:
<BR><P></P>
<DIV ALIGN="CENTER">
<!-- MATH: \begin{equation}
F = F_m(\lambda)
\end{equation} -->
<TABLE WIDTH="100%" ALIGN="CENTER">
<TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><A NAME="echelle-9"> </A><IMG
WIDTH="102" HEIGHT="40"
SRC="img322.gif"
ALT="\begin{displaymath}F = F_m(\lambda)
\end{displaymath}"></TD>
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(7.4)</TD></TR>
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<BR CLEAR="ALL"><P></P>
This flux has to be corrected for two effects in order to get absolute
fluxes: first, for the echelle blaze effect, and second, for the
chromatic response of the instrument. For a given configuration, the
blaze<A NAME="7329"> </A> effect is a function of the position in the order, while the
instrument response is, essentially, a function of the wavelength.
<P>
The solution adopted in the reduction, using the standard star<A NAME="7330"> </A>, is to
correct for both the blaze effect and the instrument response
simultaneously. This is done by comparing a standard star, observed
with the same configuration as the object, to a table of absolute
fluxes. The standard star is reduced exactly as the object and then
correction factors are calculated by comparing the flux values in the
table to the observed counts sampled at the same wavelength intervals
as the fluxes in the table. The resulting response is normalised to an
exposure time of one second. There is no effect due to differences
between the flatfield of the object and the one corresponding to the
standard star given that the flatfields are normalized (see
Section <A HREF="node164.html#flat-field-correction">7.7</A>).
<P>
If, as usually is the case, OBJ and STD were observed through different
airmasses, the spectra have to be corrected for extinction using command
<TT>EXTINCTION/SPECTRUM</TT>. More information about this command is available in
Vol.2, Chapter <A HREF="node676.html#spectral-data">L</A> (Spectral Data).
The command <TT>RESPONSE/ECHELLE</TT> is used to compute the instrument response
as described here. Southern spectrophotometric standards have been published
by Hamuy and al. (1992, 1994).
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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