\appendix
\section{Measurement tools}
\label{tools}
In this section we describe the home-made and other tools to measure the mechanics and to align the assembly station. 

\subsection{The Heigth-meter}
The heigth-meter is a commercial instrument to measure relative heights. It has an intrisic accuracy of 1\um.

\subsection{The malmonitor}
The malmonitor is a homemade instrument to monitor the distance between NIKHEF sub- combs before and during gluing. A NIKHEF comb consists of four sub-combs.

The malmonitor consists of two parts which are placed on the two adjacent sub-combs. Two
RASNIK systems are used to measure the possible shifts and also rotation between these parts with an accuracy of about 5~\um.


\subsection{Laser optics}
We use a Hewlett Packard laser with small (hardly visible) divergence over the 
relevant distance up to 6~m. Several optical instruments are used to measure
angles, distance and straigthness to high accuracy. Intrinsically, the distance and straigthness measurement is accurate to a few micron; the angular measurement is accurate to 10 urad.

However, The obtained accuracy of the measurements is limited by the flatness of the contact surface of the mechanics and instruments and other reproduction errors.

% Most notably is the  use of the laser optics in the measurement of the parallelity of the Frascati combs. The laser is pointed along the X axis op the assebly station. A penta prism is used to reflect the beam by 90 degrees with an accuracy of 10 urad. 
% Then, the Frascati combs are alinged with respect to this beam with an
% accuracy of about 100~\um. As reference contact we used the feet of the combs, but we also performed measurents on the body of the combs. Note that his procedure defines the X axis of the assembly station.

\subsection{Silicon Sensor}
 To measure the straightness we use a silicon sensor with strips in
 both tranverse directions with a pitch of 400\mum with respect to a laser beam. The sensor
 is mounted on a brass support with steel inserts which are positionned
 on small steel rods lying in the wedges of the combs. The laser beam position
is obtained from the centre of gravity average of the strips with an accuracy
of about 20~\um. 


\subsection{Bol-monitor}
\label{sbalmon}
The 'Bol-monitor'  consists of a cross-plate
with two small lenses attached to one of the flanges
 approximately 1.8~m away from each other,
at positions corresponding to two precision wedges in the combs.
This cross-plate is positionned on the stacking towers in the
same way it is done for glueing tube layers. The relative
position of each of the lenses is measured with an Mask/CCD-camera
combination  mounted on a brass fork with steel positionning inserts.
 This fork is placed on two precise steel rods 
lying in the comb such that
the cross plate lens is  hanging aproximately in the middle between the
Mask and the CCD. With the thus created RASNIK system the 
the lens position in Z and Y is read out , while varying
the fork position by +-2mm in the optical axis direction (X)
along the supporting rods. These measurements are done 
at both precision combs, using the same fork/rods combination.
Comparison of the measurements yields the difference in Z
and Y of the jig and reference assembly tower combinations.
The use of two lenses on the cross-plate 
provides two independent measurements. In addition
the whole procedure is repeated with the cross-plate rotated
180 degrees around the vertical axis, thereby interchanging
the lens/fork combinations and yielding additional redundancy.
  The BOL-monitor was used to fix the initial position of
the towers and the measurements will be repeated regularly as part of
the QA/QC procedures.


  An example of such a measurement is shown in Fig~\ref{xyzbol11}.
 Here we plot the difference in $Z$ and $Y$ position of the cross plate
 lenses with respect to the two precision combs, as measured on the North 
 and South side of the assembly table. The position difference is plotted
 as function of the $X$ position of the forks; $X=0$ corresponding to the
 lens being in the middle between the mask and the CCD of
 of the Rasnik system.
 The X values are derived from magnification of the mask image
 on the CCD.
  Straight lines are fit through the two corresponding sets of data 
 at the North and South side respectively. 
 The points show the deviations of the measurements on the North side
 from the line fit throught the measurements on the South side, and
 vice versa.
 The two colors differentiate between the two orientations of 
 the monitor cross-plate, which in principle
 should lead to the same result. The corresponding lens positions are
 indicated by Reference side and NonReference side. 
 For the $dZ$ measurement the the Reference side data are the most reliable,
 as the distance between the reference tower and the lens is short (30~cm)
 and temperature effects can be neglected. For the Non-Ref Z measurements
 the lens is at 210~cm from the reference tower and its position is thus
 subject to temperature differences of the cross-plate and 
 the combs between North
 and South data sets, which are not corrected for.
 For the $Y$ measurements the temperature effects can be neglected .

    From the plot one can see that with both orientations of the cross-plate
 we measure at the Ref side a difference  in $Z$ of  6\mum,
 which is within the 10\mum tolerance.
  This value has remained the same within $\pm$5\mum over 2 years since
 the comb positions were fixed.
 
  The $Y$ differences are much less critical for the chamber assembly,
 since the glueing procedure assures that the distance between the two
 multilayers remains the same on both ends of a chamber and only the
 relative position of the cross-plates glued between them is affected.
 Nevertheless the measurements show that Y differences are smaller than
 20\mum on both Ref and NonRef side of the combs.




\begin{figure}[b]
\setlength{\unitlength}{1cm}
\begin{picture}(14,10)
\put(-1,0){\mbox{\epsfysize=10cm\epsffile{xyzbol11.ps}}}
\end{picture}
\caption{\sl    Results from  BOL monitor measurements for the two orientations
of the monitor Cross-Plate ( 0deg CP and 180deg CP). The upper two plots
show the measured deviations in $Z$ and the lower two plots the deviations
in $Y$. The data are described in the text.}
\label{xyzbol11}
\end{figure}



\subsection{Calibration tower for RASAS system}
\input{caltower.tex}


\subsection{Calibration tower for In-Plane masks}
To calibrate the orientation of the in-plane masks we equipped a tower of the appropriate heigth with a RASNIK lense and CCD. The intrinsic orientation of the CCD was calibrated using the RASAS calibration tower and found to amount 11.7~$\pm$0.11~mrad. 





























