Null corrector

A null corrector is an optical device used in the testing of large aspheric mirrors.

However, the mirrors used in modern telescopes are not spherical – they are rotations of parabolas or hyperbolas, since these more complex shapes reduce optical aberrations and give a larger field of view.

An error in building the null corrector led to the mirror in the Hubble Space Telescope being ground to the wrong shape.

[3] Originally, there was no easy way to test a null corrector, so mirror fabricators needed to take extra care that the lenses were correct and spaced correctly (this second part, spacing, was the source of the Hubble null corrector failure).

[4] This procedure was used to test (and find an error in) the null corrector used for the MMT Observatory single-mirror retrofit.

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.
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.