The purpose of the \textit{magnetic spectrometer} is to determine the directions and momenta of secondary particles originating from primary kaon decays. The kinematical constraints needed to reject most of the background require an accurate reconstruction of the track of the daughter charged particle. In particular, in order to achieve proper reconstruction, the needed resolutions are:
\begin{itemize}
\item decay angle: $\Delta \theta_{K\pi} \leq 60$~\si{\milli\radian}
\item momentum: $\Delta p / p \leq 1\%$
\item spatial resolution: $\sigma_{x,y} \leq 130 $~\si{\micro\meter}, in order to correctly trace back the decay vertex.
\end{itemize}

In addition to this, a number of experimental requirements need to be fulfilled. The tracker will be in fact integrated inside the vacuum chamber, and it needs to minimize the amount of material traversed by the particles as much as possible. The detector must also be able to operate in a high rate environment, with a particle flux close to 40~\si{\mega\hertz\per\centi\meter} in the central area.\\

The spectrometer is based on the \textit{straw} technology, its building blocks being ultra-light straw tubes 2.1~\si{\meter} long and 9.8~\si{\milli\meter} in diameter \cite{na62-tdr}. The material employed consists of 36~\si{\micro\meter} thin PET foils coated on the inner side with 50~\si{\nano\meter} of copper and 20~\si{\nano\meter} of aluminium, acting as cathode; the anode is a gold-plated tungsten wire, 30~\si{\micro\meter} in diameter, placed at the centre of the tube.\\

\begin{figure}
\centering
\includegraphics[width=\textwidth]{Images/pdf/na62/detector/straw-view-full}
\caption{Sketch of the four ``views'' composing a straw chamber \cite{na62-tdr}. The last panel shows the superposition of $x$, $y$, $u$ and $v$ views, i.e. a complete station.}\label{img:det:straw-views}
\end{figure}

A simulation performed with Garfield\footnote{\url{http://garfield.web.cern.ch/garfield/}} has allowed to compare the best working points for two different gas mixtures: a slower, but more ``ageing-safe'' isobutane mixture (CO\subscript{2} 90\% iso-C\subscript{4}H\subscript{10} 5\% CF\subscript{4} 5\%), and a faster Ar 70\% CO\subscript{2} 30\% mixture. Considerations about the  possibility of pile-up of hits corresponding to different events have led to the choice of the faster gas mixture.\\

\begin{figure}[t]
\centering
%\subfigure{\includegraphics[width=0.29\textwidth]{Images/pdf/na62/detector/straw_detail_small}}
%\subfigure{\includegraphics[width=0.29\textwidth]{Images/pdf/na62/detector/cedar_detail_small}}
%\subfigure{\includegraphics[width=0.29\textwidth]{Images/pdf/na62/detector/lkr_detail_small}}
\subfigure{\includegraphics[height=9cm]{Images/pdf/na62/detector/straw_detail_small}}\hfill
\subfigure{\includegraphics[height=9cm]{Images/pdf/na62/detector/cedar_detail_small}}\hfill
\subfigure{\includegraphics[height=9cm]{Images/pdf/na62/detector/lkr_detail_small}}
\caption{Details of: the straws for the downstream spectrometer, the CEDAR-KTAG photomultiplier housing frame, the ribbons defining the cells of the LKr calorimeter \cite{newson}.}\label{img:det:details}
\end{figure}

The full spectrometer consists of four chambers. A dipole magnet, placed between the second and the third chamber, generates a vertical field of 0.36~\si{\tesla}, corresponding to a kick of 270~\si{\mega\electronvolt/\textit{c}} directed towards the $x-$axis. Each chamber is composed of four complementary ``views'' ($x$, $y$, $u$ and $v$), allowing for a redundance of 100\%. Each view, finally, is made of 256 \textit{straw} tubes. \figurename~\ref{img:det:straw-views} shows the layout of the views composing a STRAW chamber, while a detailed view of the tubes composing a plane is visible in the first panel of \figurename~\ref{img:det:details}.
