Low temperature focal surface presetting technology for light and small infrared camera

This study addresses the challenges of defocusing in lightweight, compact infrared cameras under low-temperature conditions, proposing a novel methodology for accurately presetting the focal plane at room temperature. Traditional solutions, such as mechanical refocusing mechanisms or strict material-based thermalization, often compromise weight constraints or incur high costs, making them unsuitable for miniaturized satellite payloads. The authors introduce a low-cost, high-precision approach combining opto-mechanical co-simulation, low-temperature interferometry, and linear displacement measurement to quantify defocusing effects and enable focal plane presetting.

The research focuses on an 181 mm aperture lightweight infrared camera with a 535 mm focal length operating at 220 K (−50 °C). Key factors contributing to low-temperature defocusing are analyzed, including structural contraction of aluminum alloy components (axial shrinkage of 0.23 mm in the front barrel), curvature radius variations in primary/secondary mirrors (ΔR = −0.12 mm and +0.04 mm, respectively), and refractive index changes in Si/Ge corrective lenses. Through finite element analysis and Zernike polynomial surface fitting, these factors are integrated into optical simulations, predicting a 0.38 mm back-focal-length (BFL) shift.

Experimental validation employs a ZYGO interferometer (3.39 μm wavelength) with a customized cryogenic test setup. The system measures wavefront changes under vacuum conditions, utilizing a reference spherical mirror mounted on a high-precision 5-axis stage. Results confirm a 0.396 mm focal shift from room temperature to −50 °C, demonstrating excellent agreement with simulations (deviation: 0.016 mm < half the 0.07 mm focal depth). The methodology achieves micron-level accuracy (<0.03 mm), validated through MTF testing of a pre-focused camera under simulated space conditions.

Two critical innovations are demonstrated. 1) Focal shift measurement: leveraging power term sensitivity in wavefront interferometry to detect defocusing, combined with submicron displacement tracking, enables direct quantification of thermal-induced focal shifts. 2) Presetting protocol: room-temperature focal positioning compensates for predicted low-temperature shifts, achieving MTF equivalence (0.13 at Nyquist frequency) between ambient and cryogenic environments without mechanical refocusing.

The technology successfully resolves the inherent conflict between miniaturization requirements and thermal stability in spaceborne infrared optics. Experimental verification confirms that the focal position preset error remains within one focal depth (±0.07 mm), meeting stringent imaging performance criteria. This approach eliminates complex cryogenic actuators while accommodating material mismatch in compact designs, offering broad applicability for temperature-sensitive optical systems in lightweight satellite platforms. Future work may extend the methodology to multi-spectral systems and optimize parameter weighting algorithms for improved prediction accuracy.