Interferometric null test of a parabolic reflector generating a Hertziandipole field

摘要

Interferometric surface tests of stigmatic aspherics can be carried out in a null test configuration. Null tests requirereference null elements either plane or spherical surfaces or both. A parabolic reflector transforms a plane into a sphericalwave which converges to the focus of the paraboloid. Therefore, a spherical ball lens or a steel ball can be placed into thefocus enabling a double-pass geometry for the null test. Here a Fizeau interferometer geometry has been selected in orderto guarantee invariance against polarization distortions under the assumption that radially polarized laser light is used forthe interferometer. Radial polarized light is necessary to mimic a Hertzian dipole field. Due to the extreme solid angleproduced by the paraboloid the alignment of the setup is very critical and needs auxiliary systems for the control.Aberrations caused by misalignments are removed via fitting of suitable functionals provided through ray-tracesimulations. It turned out that the usual vector approximations fail under these extreme circumstances. Test results aregiven for a paraboloid with 2mm focal length transforming a plane wave into a near dipole wave comprising a solid angleof about 3,4π.