In accordance with the requirements of a satellite-borne remote sensing camera, the lens and focal detector work at different temperatures, which results in a large temperature difference between them. The cold radiation of the focal detector window makes the surface parameters and temperature gradient of the lens change greatly, which cannot meet the imaging requirements. In order to eliminate the influence of radiant heat difference, this paper proposes a design of adding a light blocking window between the detector window glass and the lens, which is used to elininate the cold radiation of the focal window glass so that the imaging quality of the lens system can meet the requirements. Through comparative simulation, it is proved that the design form of this paper can eliminate the influence of the radiant heat of the focal plane, which verifies the rationality and correctness of this design and provides a certain design guidance for the design of high-precision lens to eliminate the radiant heat difference.
Objective Temperature is an important parameter that affects the performance of the lens system of high-precision satellite-borne remote sensing camera. Temperature change produces temperature gradient, which results in thermal stress and thermal deformation. And the mirror element curvature radius, center thickness, air interval, refractive index change greatly, especially on the edge area of optical element. Even edge collapse and warping phenomenon appeared, resulting in mirror element tilt, imaging plane drift and other problems, which affects the imaging quality of camera. So athermalization design and temperature control must be adopted for remote sensing camera. Many athermalization designs are based on the temperature gradient introduced by heat conduction to affect the mirror displacement of optical elements, but the effect of temperature gradient introduced by radiant heat difference is less studied. Therefore, based on the surface parameter change of a satellite-borne remote sensing camera lens assembly caused by the cold radiation of the focal plane detector window glass, which is not satisfied for optical image quality, a design for eliminating the radiant heat difference of the focal plane window glass is proposed, which provides some guidance for the design of the radiant heat difference between the eliminating components of the satellite-borne high-precision camera.
Methods In order to eliminate the radiant heat difference between the focal plane and the optical lens, a blocking light window is designed between the focal plane detector window glass and the mirror 8(Fig.4). The blocking light window is fixed on the focal surface box shell through adhesive. Through active temperature control, the focal surface box temperature is stabilized at 14 ℃, and the temperature of the blocking light window fixed on the focal surface box shell is also stabilized at 14 ℃, so that a radiation barrier is formed between the focal plane detector and the mirror 8 components. Through the 14 ℃ blocking light window, the cold radiation of the focal plane window glass is blocked, so that the surface parameters of lens 8 and the overall temperature difference of the lens can be repaired, and the imaging quality requirements of the lens system can be met.
Results and Discussions With the addition of the light blocking window, the body temperature of mirror 8 is 18.32-19.15 ℃, the gradient temperature difference of mirror 8 is reduced from 2.7 ℃ without the blocking light window to less than 1 ℃, the temperature difference is increased by 1.7 ℃ (Fig.6). The surface profile parameters of mirror 8 can be repaired by the temperature difference gradient within 1 ℃, which can meet the requirements for camera imaging quality. As a result, the modulation transfer function (MTF) of the entire lens system is repaired from 0.57 to 0.75 (Fig.8), improving the image quality of the camera.
Conclusions In order to eliminate the influence of the cold radiation of the window glass of the focal plane detector on the surface parameter of the mirror near it, this paper adopts a design form of adding a blocking light window to block the cold radiation effect so as to eliminate the radiant heat difference between the mirror and the focal plane detector. The surface parameters of the mirror near the focal plane detector and the MTF of the whole lens system are greatly improved, which verifies the correctness and rationality of the design, and can provide a certain guidance for the design of eliminating the radiant heat difference of the satellite-borne high-precision remote sensing camera.
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