Studies Of Static Correction Technology For Aberrations And Boresight Error Of Aircraft Conformal Optical Windows

Different from conventional spherical or flat windows, aircraft conformal optical windows can satisfy the contour and shape need of the fuselage, and thus improve the aerodynamic performance and enhance the stealthy effectiveness of an aircraft. Conformal optical windows will play important roles on future aircraft. However, the conformal optical window introduces dynamic aberrations and dynamic boresight error in the optical sytem which vary with the look angle, because of the unusual shape of the window and the tilt of the window relative to the gimbaled imaging system. This creats challenge for conformal optical design, and the difficulty is the correction of both the dynamic aberrations and dynamic boresight error. According to the dynamic characteristics of the aberrations and boresight error for the conformal optical window, it is naturally considered to use dynamic solutions to the aberrations and boresight error for the window, such as the use of a dynamic corrector to correct dynamic aberrations and the addition of an angle compensator in the control system to compensate dynamic boresight error. But the use of dynamic solutions reduces the system stability and increases the complexity of the system. A static solution implies the dynamic aberrations and dynamic boresight error for the conformal optical window are compensated by utilizing static optical correction elements. The static solution offers the advantage of simplicity, stability and low cost. In view of without feasible static solutions to aberrations and boresight error for conformal optical windows at the available technology, the static technology for correcting aberrations and boresight error of aircraft conformal optical windows is studied in this dissertation. The static solutions, which can achieve the correction of both dynamic aberrations and dynamic boresight error for conformal optical windows, are explored. The main contents and conclusions are as follows:(1) The optical characteristics of the conformal window and the structure characteristics of the conformal window optical system are studied, and a conformal window optical system is configured. The relationship between Zernike polynomials and aberrations is studied, and the aberration characteristics of the conformal optical window are analyzed by utilizing the Zernike polynomials. Based on the vector aberration theory, the aberration characteristics of off-axis optical systems and plane symmetric optical systems are analyzed respectively, and then the vector aberration characteristics of the conformal optical window is analyzed. The analysis results show that the conformal optical window introduces large amounts of astigmatism. The nodes of the astigmatism field are far away from the image plane, which leads to the constant astigmatism contribution over the field of view on the image plane. Butthe magnitude and direction of the astigmatism vary with the look angle.(2) The reason for boresight error occurrence for the conformal optical window is analyzed, and a model for the boresight error of conformal window optical systems is established. Based on this model, the dynamic boresight error introduced by the conformal optical window is calculated and analyzed. The analysis results indicate that the conformal optical window introduces substantial boresight error, which increases with the increase of look angle. An approach using static tilted or decentered optical elements for the dynamic boresight error correction for conformal optical windows is proposed. The principle of the static solution is discussed and design examples are presented. The design results show that static tilted or decentered optical elements are effective in correcting dynamic boresight error introduced by conformal optical windows. Furthermore, this static solution can also help the asymmetric aberration correction for the conformal optical window.(3) A solution using tilted non-rotationally symmetric optical elements for the dynamic aberrations and dynamic boresight error correction for conformal optical windows is proposed. The tilted non-rotationally symmetric correction elements are fixed behind the conformal optical window, having no change in position and direction with the variation of look angle. The optical characteristics of non-rotationally symmetric surface shape are studied. Aiming at the astigmatism introduced by the conformal optical window, a solution using tilted doubly-curved optical elements for the dynamic aberrations and dynamic boresight error correction for conformal optical windows is proposed. The principle of the static solution is discussed and doubly-curved correction elements are designed. These elements correct both the dynamic aberrations and dynamic boresight error for the conformal optical window. In order to make the corrector meet the need of more complex aberration correction, a solution using tilted freeform optical elements for the dynamic aberrations and dynamic boresight error correction for conformal optical windows is proposed. The principle of the static solution is discussed. Based on the principle, freeform correction elements are designed, which make the system achieve diffraction limited performance, and correct dynamic boresight error for the conformal optical window at the same time.(4) A solution using tilted and decentered rotationally symmetric optical elements for the dynamic aberrations and dynamic boresight error correction for conformal optical windows is proposed. The tilted and decentered rotationally symmetric correction elements are fixed behind the conformal optical window, having no change in position and direction with the variation of look angle. The principle of the static solution is discussed, and tilted and decentered rotationally symmetric correction elements are designed. The design result shows that tilted and decentered rotationally symmetric optical elements can correct both thedynamic aberrations and dynamic boresight error for the conformal optical window.(5) It is theoretically proved that the inner surface of the conformal optical window can be used to correct the asymmetric aberration introduced by the conformal optical window at zero degree look angle. Based on the proof, a solution using the inner window surface and tilted fixed correctors for the dynamic aberrations and dynamic boresight error correction for conformal optical windows is proposed. Fixed correctors are rotationally symmetric optical elements and fixed behind the conformal optical window, having no change in position and direction with the variation of look angle. The principle of the static solution is discussed,and a design example is presented. The design results show that this static solution can achieve the correction of both the dynamic aberrations and dynamic boresight error for the conformal optical window. Moreover, fixed correctors have rotationally symmetric surface shape, which facilitates the fabrication and testing.