Why are the mirrors figured as paraboloids?
Mirrors for large reflecting telescopes are polished to precise paraboloidal (figure of revolution like a parabola) or nearly paraboloidal shapes. This shape can perfectly focus starlight from far away into an image just above the mirror. In this manner, a concave mirror acts the same as a camera lens – forming an image for the film or CCD detector to record. The f/number, or focal ratio, is the ratio of the focal length to diameter of the mirror. Our large spun-spun cast mirrors typically have focal ratios of f/1.25 (for 6.5-m mirrors) or f/1.14 (for 8.4-m mirrors). Frequently, a second smaller mirror is used to redirect the image from the prime focus to a more convenient location or image scale. • How is the paraboloidal shape formed? The rough paraboloidal shape is formed when the mirror blank is cast in the spinning furnace. By spinning the furnace at the proper speed while the glass is molten, the surface takes on a paraboloidal shape. By the time the cooling process is complete,
