Alignment


It is difficult, if not impossible, to guarantee the position of all 
wires within one half station to the necessary accuracy by
construction. Torsions, tippings and other dsiplacements of parts of
the huge, 5m x 3m frames are unavoidable.
The needed accuracy of modules and wires has to be obtained by an alignment 
procedure using real data. Such an alignment program has the following
requirements:

- in order to keep the number of alignment constants small enough the
  wire positions within one module has to be guaranteed by
  construction to the  final accuracy ( <100 mu). This then leaves 
  only a few constants to be determined for each module.

- the position of a module in the x-y plane within one half station 
  will have to be determined with data.
  The initial construction accuracy of the module positions should 
  be in the order  of 0.5 mm over the full width of a station.  

- on time scales of weeks the positions of the individual modules
  should stay stable within the required accuracy.
  Short term variations would require an monitoring system
  for all modules which is a major complication and hard to realise.

- within one half station we will prevent independent movements of
  individual modules along z. Also, we must prevent module bendings
  larger than about 1mm.
 
- half stations can be treated as independent. There is no need for
  a  relative positioning of the stations of better than about 1 mm. 
  The positions of the half stations, on the other hand, must be 
  reproducible. A laser based monitoring system will guarantee the 
  correct repositioning of all chambers after maintenance.