That completes our tour of the structure! The question now is, how will this giant move with such absolute precision? After all, this mobile mass - 350 tonnes of steel, motors, mirrors, cables and scientific instruments - will glide along, virtually unhindered by friction and with microscopic precision, tracking objects billions of light years away across the Universe.
To locate an object in the sky, the telescope will move on two axes - azimuth (horizontal) and elevation (vertical). This works like cannon onboard a warship: first, the cannon rotates on its base, then it seeks the target in the vertical.
Once located, the object must be tracked, because the earth is rotating. The telescope has to be able to move, to counter the earth’s rotation, so that the image of the object being observed stays in the field of view. If the telescope’s axis of rotation were parallel to that of the Earth’s, only one type of movement would be needed to track an object. Since its axis is not the same as the Earth’s, though, the telescope needs to rotate on three axes: azimuth, elevation and an axis that compensates for the rotation of the Earth.
This third movement is called field rotation and, on the GTC, it will be achieved by mounting the instruments on mechanical rotators.