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What's under a mirror?
07/04/2005
The segments of the Gran Telescopio CANARIAS' (GTC's) primary mirror have to move with great precision in order to form one single surface. One of the actors in this 'drama' is the structure made up of the segment and subcell supports, which will be mounted on the giant cell to provide an interface with the telescope structure.
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Now that the mechanical structure of the GTC is in place, work on adjusting the above elements has started. It's like a jigsaw puzzle that, once finished, will see the structure in its entirety on site.
We have already mentioned several times that the position of the primary mirror segments will have to be precisely controlled if they are going to work together as one optical surface.
The segments will have to be mounted very carefully in the perpendicular. We will do this by using sensors to measure the relative position of the segments. Once in place, the positioners and actuators will continuously correct each segment's position.
The position of the segments parallel to one another is less exacting, given that the curvature of each of them is minimal. Even so, the margin of error is only 0.25 mm.
It is not possible to manufacture the primary mirror support structure with this degree of precision in each dimension. Substructures, or 'subcells', will therefore be used: there will be one for each of the segments, and they will be installed under strict and exhaustive metrological controls in order to achieve the level of accuracy required.
The subcells will support the whole structure of the segments: the mirror itself, its support system and the positioners - around 540 kg in all.
The subcells will also make it possible to install an elevation mechanism which, like the jack used to change the wheels of a vehicle, will lift a mirror up to the level of those adjacent to it so that it can be mounted and unmounted using a crane.
The segment support system, including its subcells, has been produced by Spanish companies: the main contractor is CESA, which has subcontracted work on the subcell support systems to RAMEM using designs produced by LIDAX.
Natalia R. Zelman