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What is an optic surface figure?

The surface figure, or surface quality orsurface cosmetics all refer to the deviation between an actual optic andits ideal surface. There are basically two set of information that are commonlygiven by manufacturers: the surface flatness and what is referred to as"scratch-dig".

Why is it important?

Well, an optic with a bad rating on surfaceflatness will introduce some wavefront distortions, which are responsible for aberrations and badquality focus. Aberrated wavefront leads to poor ratio (ratio of theobserved intensity at the image plane compared to the theoretical maximumintensity of a perfect optical system), so poor optics makes one loosevaluable optical power at focus. Plus scratches or digs on an opticcreate diffraction and stray light, which no one wants either.

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