Can a Spherometer measure convex surfaces ?

Question

I know spherometers are used to measure concave hand-ground mirror surfaces, but I want to know what I'd use to measure a convex surface if I tried grinding my own refractor lense.

I wouldn't do it by hand though, I'd try to make like a pottery spinning wheel and automatically do the grinding by keeping the pressure, rpm , and exposure time constant, and just measuring the surface till I got it right. But I'd need something to measure the surface with. What do I do ?

Answer 1

Hi. I do not see why not. The relationship between the three outer points and the center point would have the same magnitude but an opposite sign.

Edit. A refractor lens suffers from chromatic abberation. They usually consist of two pieces of glass with approximately canceling index of refraction values.

Answer 2

You can measure either concave or convex as long as your spherometer is made to do it. Keep in mind that a spherometer only gives you a measure of the focal length, not of the surface shape. The surface of a lens or mirror must be much more precise than is measureable with a spherometer. Once you coarse grind to the right focal length, the spherometer is no longer used. Optical techniques like Ronchi gratings and the Foucault or knife edge test are needed to measure the figure of your surface.

A simple potter's wheel type arrangement is OK for rough grinding but is not precise enough for the rest.