Investigation of TMA systems with different freeform surfaces
Three mirror anastigmats (TMA) are telescopic optical systems with only plane symmetry, that allow for good image quality without any central obscuration. The complexities of manufacturing and alignment can be reduced by fabricating the first mirror and the third mirror in one piece and defining a common axis of all the mirrors. It is attractive to use off-axis used aspheres and to come to an acceptable performance with the smallest number of freeform surfaces. In this paper, different types of freeform surfaces are considered to evaluate their potential. In the performed case study, the correction of spherical aberration and coma is best corrected in the pupil with the second mirror and to select the Zernike representation with remaining x-symmetry is one of the best ways to do this. The use of the Chebyshev polynomials also gives good results. Furthermore it is found, that the first mirror and the third mirror are quite beneficial to be modelled as off-axis aspheres of the Q-type. The result shows that a combination of two Q-aspheres with a Zernike surface at the second mirror is one of the most favorable combinations.