
\subsection{Control of the tube position}
%  As described in [Assembly.jig] the tubes to be glued are 
% placed on jigs on the granite table.
%---
% overbodig , see jigs.text
% The precise endplug rings glued under the spacer cross-plates
% are held by the precise 'Frascati' jigs (+- 2? etc in z and y)
%  at the ends of the table.
% Inbetween the tubes are held by the semi-precise 'NIKHEF' jigs 
%
% (+- xx in z and +- xx in y). etc
%--
   The position of the tubes placed  in the jigs 
 is checked prior to glueing  by the readings of the
 Planarity Laser systems at HV and RO ends ( see Assembly.QA.Guido ),
 also called Guido Laser system. 
  Deviations from a straight line fit through the data
 of more than $\approx$ 20~\mum require an inspection of the
 associated tube(s).
 At the beginning these deviations were frequent and usually caused
 by omitting to release the 'plug-endcap' or the orientation pin
 from the auxilliary positioning comb attached to the jig.
 Sometimes pieces of hardened glue left on the jig caused the
 deviation. Only in a few cases tubes had to be replaced because
 the plug was inproperly crimped into the tube or the tube itself
 was too crooked to be held straight in the jigs by the vacuum
 suction pads. So far no significant deviations caused by plug
 diameter variations were found.  

   In fig [guido.AB] we plot the average of the residuals from the straight
 line fits for all 6 layers of the first 4 chambers as a function 
 of the position in the jig (footnote) at each side of the
 assembly table. Wether this corresponds to the HV and RO side
 of the chamber depends on which multilayer is being glued.

  \footnote{ As explained in Assembly.QA.Guido the laser system 
 measures the Y position  of the calibration caliper laying
 in the wedge of two adjacent tubes. A deviation can be caused by 
 mispositionning of either of the tubes, and one expects in that
 case two deviations on either side of a displaced tube. }

 The error bars represent the rms of the
 24 deviations found at that position, typically ~5 \mum.
 Systematic  deviations of order +-8 \mum can be recognised.
  These are attributed to the fact that the jigs are 
 assembled out of 4 solid pieces  which were precisely machined
 and then glued together. 
 The fixation of the jigs to the granite table and 
 to the assembly stacking towers on the reference side ( position zero
 in the plots ) also leads to small distortions. However these systematic
 deviations are well within the required precision.

   In fig [guido.CD] we plot the residuals from straight line
 fits throught the data of the X-ray tomograph scans for BOLx??.
 In this case the data from individual layers in each multilayer
 at the HV and RO side  are combined such that they correspond to 
 the same side of the assembly table at which they were glued. 
 In this case the error bars represent the rms of the tomagraph data
 at each tube position.
 The systematic deviations in the X-ray data are clearly correlated with
 and of the same size as the planarity laser data. This shows
 that tube positionning and its QA are  well under control.

























