Stray light analysis of off-axis four mirrors system based on two-dimensional function PST

Off-axis four mirrors systems are opto-mechanical core components of multiband common aperture airborne observation and aiming systems to adapt to their future development. This type of systems is easily affected by strong stray light outside the field of view. The point source transmittance (PST) of stray light in those systems is generally required to be no more than 10⁻⁴ order of magnitude. Accordingly, omnidirectional scanning calculation of PST is the key to analyze and suppress stray light of them. For the asymmetry of the optical system, PST as a two-dimensional function varying with the horizontal and vertical angle of stray light was set up to evaluate the influence of external stray light. Besides, the conversion relationship between the horizontal and vertical angle and the process quantity of rotation about coordinate axis required for opto-mechanical modeling was established. The simulation control program was compiled, which realized the automatic tracking of stray light and the two-dimensional scanning calculation of PST by calling LightTools software. For a multiband common aperture off-axis four mirrors system, the PST distribution of stray light in all directions within the whole incident hemispherical space was calculated, so as to screen out 3 series of stray light that had a great impact on the application of airborne observation and then find their transmission paths and key surfaces. Based on this, the inner baffles and vanes were designed to optimize the stray light trap structure in the system, so that the peak value of PST of the system was reduced from the original 10⁻¹ level to 10⁻⁴ level, especially less than 10⁻⁷ outside the avoidance angle range, which could meet the application requirements of airborne photoelectric observation and aiming systems. It provides a basis for the analysis and suppression of stray light of off-axis four mirrors systems.