To illustrate his point, Booth points at removable, millimetre-wide “shim packs” between the giant beams. These are necessary for tiny structural adjustments as the telescope’s massive concrete base shrinks over time. That concrete, when it was laid, needed to be extraordinarily flat, he says, to an accuracy of 1/3,600th of a degree, and to account for the Earth’s curvature too. (When they were finished, the engineers used the excess wet concrete to make a basketball court on the mountainside, because it was too far to the nearest city to return it before it hardened.)
