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 displacements of parts of the huge, 5m x 3m,
frames are unavoidable. The required accuracy of modules and wires has
to be obtained by an alignment procedure using data. Such an alignment
procedure 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 microns). This initernal wire precision then leaves only a few alignment 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 and will survey their position.
Frrst thoughts about alignment and module positioning: Attempt
to suammarize the mechanical constraints for module and
stations
of the OT.