Waineo null test (telescopeѲptics.net).

Probably the simplest way to null paraboloid is to use another reflecting surface. By placing point source of light at the infinity focus of a paraboloid, the collimated light output can be reflected back to it, to produce focus that can be nulled. The disadvantage of this setup is that it requires a flat as large as the paraboloid.

A setup with the flat replaced by a smaller, easy to make reflecting surface, is more practical. And this is what Waineo null test offers: spherical aberration of a concave paraboloid - or a conical surface in general - is compensated for by a smaller concave sphere.

Waineo null test setup: the sphere is facing test surface, both centered around a common axis, with the light source placed inside the focus of the sphere. Diverging light reflected from the sphere is focused by the test surface; if the aberration contribution of a sphere is at the level of mirror tested, and of opposite sign, the final focus will be aberration-free, and the surface tested - if sufficiently accurate - will null when the converging light is intercepted at the focus.

De Waineo null test opstelling.

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