Two-lens afocal compensator for thermal defocus correction of catadioptric system

Abstract

Subject of Research. Traditionally, afocal compensators, located in parallel or in converging beams, are used to correct the aberrations of mirror systems. The additional property of afocality gives the possibility to ignore practically the selection of materials in the design, since, in this case, the achromatic correction is achieved automatically. A change in the ambient temperature leads to a change in the shape of the mirrors and their mutual arrangement, in addition, the optical characteristics of the compensator material change that leads to defocusing. Main Results. Based on the analysis of paraxial formulas valid for the correction of chromatic aberrations and thermal defocusing, we have obtained formulas that enable to evaluate the characteristics of the materials necessary for passive athermalization, that is, to keep image quality when the ambient temperature changes without using a mechanical offset of the sensor. It is shown that in a two-lens compensator used to correct aberrations of two-mirror lens in a converging beam of rays, it is necessary to use a combination of optical glasses and polymer materials for passive athermalization. Practical Relevance. Based on the theoretical correlations obtained, a two-mirror system with an afocal compensator is calculated with a high image quality maintained over a wide temperature range. The use of obtained formulas in practice made it possible to demonstrate the possibility of creation of athermalized catadioptric lens involving combinations of conventional glasses with modern polymeric materials. The method presented is not universal, but it gives the possibility to select materials for calculating the afocal two-lens systems, compensating the thermal defocusing of the image without active correction methods (mechanical shifts).