Objective As the main detection spectrum of infrared earth optical payload, infrared spectrum (8-12.5 μm) plays an important role in earth remote sensing. With the development of space imaging optical technology, the requirements for the detection performance of imaging satellites are constantly improving, and the imaging satellites are developing towards high resolution, high spatial resolution and wide radiation. For example, Venezuela’s Remote Sensing Satellite (VRSS) infrared camera, NASA's Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) infrared camera, etc. The infrared imager achieves a spatial resolution of 30 m and a width of 300 km by whiskbroom, which ensures high resolution and improves the imaging width. The influence of stray light on it is particularly prominent when the infrared remote sensing instrument with high resolution and large field of view extracts the remote sensing information of weak targets. If the suppression of stray light is insufficient, the energy distribution on the image plane is uneven, which leads to the decrease of signal-to-noise ratio and modulation transfer function (MTF), and the nonuniformity becomes worse. In severe cases, the detection signal of the detector is annihilated by stray light of background radiation, which results in the failure of the detector. Therefore, the design of stray light suppression for infrared imager is the premise to ensure its on-orbit imaging quality. The long-wave infrared Dewar module is an important part of the imager. Because the optical structure of the Dewar module is close to the detector, the detector is more sensitive to the optical structure, so the design of stray light suppression of the Dewar module is particularly important.
Methods In view of the above requirements, this research analyzed four key surfaces of spurious radiation in opto-mechanical system, including lens, lens barrel, Dewar window and window cap, among which lens barrel was the main source of spurious radiation (Fig.2). Cryogenic optical design was adopted to reduce stray radiation, including 195 K lens, 180 K lens barrel, 200 K Dewar window cap and window design (Tab.2). In order to realize the low-temperature Dewar design, flexible bellows were introduced into the Dewar package structure to increase the thermal resistance between the refrigerator coupling surface and the window cap, and realize the thermal isolation between the 200 K low-temperature window cap and the 240 K expander (Fig.1). The effects of window, window shell, cold screen structure and surface treatment technology of Dewar module on stray light in Dewar were studied (Fig.6, Fig.8, Fig.9).
Results and Discussions Based on the analysis above, the innovative results are as follows. (1) The flexible bellows were introduced into the Dewar package structure to increase the thermal resistance between the coupling surface of the refrigerator and the window cap, and the design of 200 K low temperature window and window cap was realized, and the radiation suppression in the optical machine was at a good level (Tab.2). After the measurement, the temperature gradient of cold end and hot end of bellows reached 37-48 K (Tab.1). (2) The cold screen adopted three-stage baffle design, and the filter was integrated in three bands. Considering the assembly and machining accuracy, the cold screen and the filter bracket were separated. The radiation suppression in the optical-mechanical system was at a good level (Fig.10-12, Tab.3).
Conclusions The main objective is to reduce the radiation stray light of infrared remote sensing instrument with high resolution and large field of view. Reasonable low temperature design is beneficial to restrain the stray radiation of the module, and the flexible bellows shell insulation structure with 0.1 mm wall thickness is an effective means to realize the design of 200 K low temperature window and window cap. As the main source of optical-mechanical stray internal radiation, the influence of lens barrel on it should be considered when considering the processing and design of cold screen and window. This research provides theoretical and technical reference for the design and processing of low-temperature Dewar.
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