Astronomers demand diffraction-limited optics, and when the optic is a liquid the requirement calls for pure rotation. Mercury-mirror telescopes need spindles with high load capacity, high stiffness, high accuracy, and speed control to a part-per-million. Balance and leveling are also crucial. Tilt stiffness is necessary to avoid a runaway condition where an off-axis load gains force if the mercury runs sideways. [1]
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