On an optical-infrared telescope, the secondary mirror grasps light from the primary mirror and reflects it to the Cassegrain focus or the tertiary mirror (so that it reaches the Nasmyth and folded Cassegrain foci). The GTC's secondary mirror is made from a substrate of beryllium, a material that is much stronger and lighter than glass, coated with a layer of nickel. It has a semi-hexagonal shape (which is adapted to the shape of the primary mirror), a convex hyperbolic surface, weighs just 38 kg and is 1.2m in diameter. The complex control and motion mechanisms of the secondary mirror allow vibrations to be compensated for and, above all, for differential measurements to be made in the thermal infrared range (radiation with the longest wavelength in the infrared range). This technique, which is also known as chopping, allows the secondary mirror to oscillate rapidly. .
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