
\subsection{Landing Gear}

  When the spacer is brought down on the sphere towers directly
from the crane, its initial position at contact may be off by a few mm
in X and Z. Without glue between tubes on the spacer and  tubes in the 
combs, the spacer will glide smoothly to its proper position
when the weight is taken off the crane. However with glue ropes
between the tubes this is not so easy, especially since the 
sag compension system reduces the weight pressing on the
sphere towers.
 To avoid large position deviations at initial contact
we use an auxialiary positionning system dubbed 'Landing Gear'.
 A threaded aluminum block holding a steel bolt  is attached to each end 
of the long beams.
 The bolts have a cone-shaped end which fits into a cone-shaped pit
in a small  aluminum plate screwed on the table. The X-Z position
of these 4 plates is adjusted  in a trial run with the
spacer resting on the sphere towers and no glue on the tubes.
By the turning the bolts the spacer is pushed up from the table
by a few mm without changing its X-Z position by more than 0.1 mm,
which is checked by RASAS. The spacer is lifted from the table
and glue is put on the tubes. The spacer is brought down again and now rests on
the bolts and plates. The bolts are released and the positionning is
taken over by the sphere towers. A picture of a Landing Gear unit is shown 
in fig [landing].


