\section{Production of detector modules}
\subsection{Casting of fibre mats}
Fibre mats are still fragile after taking them  off the winding wheel. It has a tendency to split between adjacent fibres. Fibres near the edges are particularly
prone to becoming separated from the ribbon. For this reason, the ribbon is cast in a bath of glue to ensure a thin protection film around the mat, which also creates a precise flat surface.
To cast fibre mats they are placed in an casting mold made of aluminium (see figure \ref{fig:textCastingMold}. The fibre mat is positioed by means of its pins that are clicked into grooves machined 
at high precision into the casting mold. An endpiece is also added and aligned in the template with respect to the fibre centre by holes. A longhole as used for aligning the fibre mat is shown in 
figure \ref{fig:textPins} (top), 
together with the resulting pin on the fibre mat. Distance holders of 80~$\mu$m height guarantee the proper positioning of the fibre mat in the mold. The mold is closed by means of a 
glass plate and placed in a vertical position. Glue is filled into the mold from bottom to top. To ensure a good wetting of the glue the mold is heated to a temparature of 40$^\circ$. 
After 2 days of curing the casted fibre mat can be taken from the mold. 
The height of the casted fibre mat is 1.45mm to be compared to about 1.2mm as specified for the uncasted fibre mat. A measurement on the uniformity of the fibre mat height and other 
measurements on the mechanical properties of casted fibres mat are summarized in appendix \ref{performance}. Optimization of the process parameters is ongoing. Two lines are followed: First, it is 
envisaged to build a casting mold allowing to cast to fibre mats at once. Secondly, we investigate process parameters (e.g. temparature) to reduce the curing time of the fibre mats in the mold. 
\begin{figure}[h]
       \begin{center}
         \includegraphics[trim = {9cm 1cm 9cm 1cm}, clip,width=0.3\linewidth]{./figs/CastingMold.jpeg}
       \end{center}
		\caption{2.5m casting mold used to cover the raw fibre mats with a thin, protective layer of glue.}
      	\label{fig:textCastingMold}
   \end{figure}



  \begin{figure}[h]
       \begin{center}
         \includegraphics[trim = {6cm 0 6cm 0}, clip,width=0.7\linewidth]{./figs/Pins.pdf}
       \end{center}
		\caption{Top: Grooves in the casting mold used for positioning of pins; Bottom: Pins on a casted fibre mat}
      	\label{fig:textPins}
   \end{figure}

\subsection{Finishing of sub-modules}
To guarantee the optimal light output of the fibre mat special care has to be taken for a very good cut quality transversal to the fibre direction. Before performing that cut the second endpiece is glued
to the fibre mats (see figure \ref{fig:textGluingEndPieces}). The endpieces comprise alignment holes for the positioning of the readout electronics. It needs to be mounted before performing the
optical cut to avoid a gap between optical fibres and readout electronics. The cut is performed with a special three blade diamond milling head shown in figure \ref{fig:textDiamondHead}.
The final step to finish a sub-module is to glue the mirror on the far end of the fibres.

  \begin{figure}[h]
       \begin{center}
         \includegraphics[trim = {0cm 0 0cm 0}, clip,width=0.8\linewidth]{./figs/GluingEndPieces.pdf}
       \end{center}
		\caption{Alignment and gluing of the second end-piece in the gluing jig. }
      	\label{fig:textGluingEndPieces}
   \end{figure}

  \begin{figure}[h]
       \begin{center}
         \includegraphics[width=0.7\linewidth]{./figs/DiamondHead.pdf}
       \vspace*{0.5cm}
       \end{center}
		\caption{Three blade diamond milling head used for the optical cut of the fibre mat}
      	\label{fig:textDiamondHead}
   \end{figure}
The resulting sub-module is robust and handleable without fear of damage. At that stage it is foreseen that the sub-modules undergo a 
thorough test procedure to guarantee its correct mechanical properties and functionality. Now sub-modules are safely protected and ready to be shipped to the module assembly centres. 
\clearpage
\subsection{Cutting sub-modules}
To reach a minimal loss of acceptance at the boundary of two neighbouring sub-modules they have to be cutted to the appropriate width with a precision of better than 150$\mu$m. To guarantee 
that precision two parallel cuts are performed at the same time using a double blade circular mill. The set-up for cutting the sub-modules and the circular mill are shown in figure \ref{fig:textLongitudinalCut}. The choice of the blade has been taken after a series of test. The advantage of that particular mill is that it efficiently removes the chips during milling and 
provides a reasonable cooling due to the good thermal conductivity of the blade. A uniformity of better than 100~$\mu$mm can be achived as shown in appendix \ref{performance}. 
Optimization of the processing parameter to perform the longitudinal cut is ongoing.  
  \begin{figure}[h]
       \begin{center}
         \includegraphics[width=0.48\linewidth]{./figs/LongitudinalCut.pdf}
         \includegraphics[width=0.48\linewidth]{./figs/KreisFraeser.pdf} 
       \end{center}
		\caption{Cutting a fibre mat along the fibres. The right picture shows the circular mill used for that cut. }
      	\label{fig:textLongitudinalCut}
\end{figure}

\subsection{Module assembly}
A detector module is assembled from 8 sub-modules prepared as described before. This is done by means of a full size (5m x 0.53m) template, machined from two pieces at very high precision. 
This procedure has been successfully applied in the construction of the OT detector modules of similar size (5m x 0.34m). The advantage is the template gives the precise alignment of the 
detector modules and the reproducability is intrinsically guaranteed. The first step of module assembly is to align the 8 sub-modules in the template. 
To align the fibres the template contains a groove. The alignment pins in the sub-module (see figure \ref{fig:textPins}) are 
clicked into the groove. Like this the alignment is transferred from the pins produced during winding of the fibre mat, via the sub-module to the final detector module. After alignment of 
sub-modules the end-plug is positioned across the end-pieces. This is done by alignment pins and the 
corresponding alignment holes in the endpieces and the template. 
Glue is applied and the half panel (one piece of 5m x 0.53m) is positioned on top. A good bond is ensured by pressing the panels 
by means of weights to the sub-modules. 


  \begin{figure}[h]
       \begin{center}
         \includegraphics[width=0.9\linewidth]{./figs/ModuleAssembly_Step1.pdf}
       \end{center}
		\caption{Exploded view of set-up in first step of module assembly.}
      	\label{fig:textModuleAssembly_Step1}
\end{figure}

  \begin{figure}[h]
       \begin{center}
         \includegraphics[width=0.9\linewidth]{./figs/GluingHCandEndplug1.pdf}
       \end{center}
		\caption{Gluing a half panel to a sub-module while constructing a testbeam module made of one sub-module. In the final production this step is performed for eight sub-modules at a time.}
      	\label{fig:textGluingHCandEndplug1}
   \end{figure}

Figure \ref{fig:textGluingHCandEndplug1} shows the corresponding production step while constructing a testbeam module made from one sub-module.  
\\
The good alignment of the detector modules using that method is demonstrated in figure \ref{fig:alignment} in appendix \ref{performance}. For that measurement a detector half module (one CFRP glued, 
the other not) is laid on coordinate measurement table. No mechanical fixation is used. The edges of the pins are measured with a precision of $~\sim 10 - 15 \mu$m. 
The deviation from a straight line is shown. The RMS of that measurement is 21$\mu$m. 
\\
After curing ($\sim$24hours) the half module is taken out of the template, turned over. End-plugs and are aligned on the half module in a different template, glue is applied and the second half 
panel is bonded to the module. The final step in the module assembly is to close the module by wrapping it in a light and gas tight envelope. Gas-tightness is required to avoid humidity penetrating to 
the cold part of the detector. 
The concept has been proven by the construction of a full width, 1m long detector module using this assembly strategy. It has been equipped with 4 sub-modules (3 dummy modules and 1 fibre mat).
% is shown in figure \ref{1mModule}. 
% \begin{figure}[h]
%       \begin{center}
%         \includegraphics[width=0.9\linewidth]{./figs/1mModule.jpg}
%       \end{center}
%		\caption{Gluing a half panel to a sub-module while constructing a testbeam module made of one sub-module. In the final production this step is performed for eight sub-modules at a time.}
%      	\label{fig:1mModule}
%   \end{figure}

\clearpage