Null corrector
![Testing a spherical mirror using an interferometer. All surfaces in the tester are either flat or spherical, so the tester itself is fairly easy to fabricate and test. This setup can test a spherical mirror of any size – since the wavefront is spherical, the mirror can be small and close up, or many meters across and further away. The test only requires that the pinhole be located at the center of sphere defined by the mirror's surface.](/Images/godic/202501/30/SphereTest5955.png")
![Adding a null corrector so the interferometer test can measure an aspheric mirror. The null corrector cancels the non-spherical portion of the mirror figure, so when viewed from point A, the combination looks precisely spherical if the mirror under test has the correct figure. Diagram is not to scale – the null corrector is much smaller than shown here.](/Images/godic/202501/30/NullCorrector5955.png")
A null corrector is an optical device used in the testing of large aspheric mirrors. A spherical mirror of any size can be tested relatively easily using standard optical components such as laser, mirrors, beamsplitters, and converging lenses. One method of doing this using a Shack cube is shown at the right, and many other setups are possible. An interferometer test such as this one generates a contour map of the deviation of the surface from a perfect sphere, with the contours in units of half the wavelength used. This is called a null test because when the mirror is perfect, the result is null (no contours at all). If the result is not null, then the mirror is not perfect, and the pattern shows where the optician should polish the mirror to improve it.