Design of Reflective Optical Systems

Abstract

The primary objective of this research is to develop optical and opto-mechanical design procedures for reflective optical systems. Challenges in fabrication and testing have limited the choice of surfaces used in the design of reflective optical systems to rotationally symmetric surfaces. Freeform surfaces or non-rotationally symmetric surfaces are necessary to meet challenging performance and packaging requirements. To gain familiarity with optical and opto-mechanical design techniques, two systems were considered: a two mirror Ritchey-Chrétien telescope and a Three Mirror Anastigmat.

The two mirror Ritchey-Chrétien optical system using rotationally symmetric hyperbolic surfaces is designed. The opto-mechanical design incorporates the use of radial and axial fiducial surfaces to locate the two mirrors onto a tube relating the optical surfaces to each other and to the detector through a spacer plate.

Optical performance improvement over the two mirror telescope is achieved with the addition of a third mirror. The Three Mirror Anastigmat (TMA) optical design uses off-axis conic sections of a rotationally symmetric system. Further improvement to the optical performance is achieved with a freeform TMA optical system and optical surface fabrication feedback to the designer. Opto-mechanical design of the TMA incorporates the use of a telescope frame to constrain each mirror in six degrees of freedom and relate the optical surfaces to each other and to the detector. The mirrors are held in place through independent mounting clamps to sustain high gravitational acceleration with minimum optical surface distortion.

The two mirror telescope optical performance is limited by optical aberrations especially at high field angles. Locating the mirrors on a tube over-constrains the components and distorts the optical surfaces. Multiple assembly configurations or non-repeatability is due to symmetry of mounting screws. Impressive optical performance, 58 times wavefront error improvement over the two mirror system, is achieved with an unobstructed TMA optical system using freeform surfaces. A snap-together repeatable assembly without adjustments is designed using conventional fiducial techniques and independent mounting clamps minimizing optical surface distortion.