Abstract
Focus adjustment in satellite optical payloads is critical, especially for CubeSats, which generally lack dedicated focus mechanisms due to stringent constraints in size, weight, and power. The harsh space environment, characterized by cyclic thermal variations, can induce significant thermal deformation in optomechanical structures, leading to misalignment and degraded image quality that may compromise mission performance. While larger satellites routinely integrate precise focus adjustment systems into their designs, CubeSats face considerable challenges in implementing such mechanisms, although a few recent missions have successfully demonstrated effective focus adjustment capabilities.
To address this challenge, we propose a dual-method strategy that combines passive pre-compensation with fine-tuning via controlled thermal expansion. The first method employs a thermo-elastic analysis of the expected orbital environment to predict the anticipated focal shift resulting from thermal deformation of the optical assembly. Based on these predictions, an intentional misalignment is introduced during ground-based detector alignment to pre-compensate for the thermal effects encountered in orbit. For the BlueBon payload, the thermo-elastic analysis estimated a focal shift of approximately 98.8 µm under orbital conditions, and the detector was pre-adjusted accordingly after aligning it to the optimal focus in a 20°C laboratory setting. This approach primes the optical system to perform optimally once deployed in space.
Extensive preliminary experiments were conducted using the Bluebon optical payload in an optical thermal vacuum chamber (OTVC) to simulate the harsh conditions of space. Focus variation was evaluated over a temperature range from –10°C to 20°C using a through-focus modulation transfer function (MTF) measurement technique. A notable discrepancy between the predicted focal shift and the measured shift led to a refinement of the thermo-elastic model. It was determined that an additional 10 µm adjustment was required for optimal alignment.
To resolve this issue, a second method was devised to fine-tune the detector’s position through a pre-run process. This method leverages the mechanical properties of the structure surrounding the detector and its electronic modules. The detector module, which comprises a sensor and its associated electronics, is housed within a metal structure engineered for efficient heat dissipation. During operation, rapid heat dissipation causes slight thermal expansion of the metal, resulting in a minor positional shift. OTVC experiments revealed that a 1°C change in the detector electronics produces an approximate focal shift of about 1.1 µm. Consequently, achieving the required 10 µm shift necessitates a 9°C temperature change, implying that a 10-minute pre-run is sufficient prior to in-orbit imaging.
This dual-method strategy offers a robust and practical passive focus control approach for CubeSat optical payloads, significantly mitigating the need for complex active focusing mechanisms. Furthermore, this approach allows for focal position adjustments at the imaging location by considering orbital environmental conditions, ensuring consistently the highest optical imaging performance (MTF) through planned pre-run procedures. Future work will further refine the thermo-elastic model and assess the long-term stability of these methods under actual spaceflight conditions.
Pre-Launch Evaluation of Detector Misalignment to Compensate for Structural Focus Shifts of BlueBON Spectral Imager in Space
Focus adjustment in satellite optical payloads is critical, especially for CubeSats, which generally lack dedicated focus mechanisms due to stringent constraints in size, weight, and power. The harsh space environment, characterized by cyclic thermal variations, can induce significant thermal deformation in optomechanical structures, leading to misalignment and degraded image quality that may compromise mission performance. While larger satellites routinely integrate precise focus adjustment systems into their designs, CubeSats face considerable challenges in implementing such mechanisms, although a few recent missions have successfully demonstrated effective focus adjustment capabilities.
To address this challenge, we propose a dual-method strategy that combines passive pre-compensation with fine-tuning via controlled thermal expansion. The first method employs a thermo-elastic analysis of the expected orbital environment to predict the anticipated focal shift resulting from thermal deformation of the optical assembly. Based on these predictions, an intentional misalignment is introduced during ground-based detector alignment to pre-compensate for the thermal effects encountered in orbit. For the BlueBon payload, the thermo-elastic analysis estimated a focal shift of approximately 98.8 µm under orbital conditions, and the detector was pre-adjusted accordingly after aligning it to the optimal focus in a 20°C laboratory setting. This approach primes the optical system to perform optimally once deployed in space.
Extensive preliminary experiments were conducted using the Bluebon optical payload in an optical thermal vacuum chamber (OTVC) to simulate the harsh conditions of space. Focus variation was evaluated over a temperature range from –10°C to 20°C using a through-focus modulation transfer function (MTF) measurement technique. A notable discrepancy between the predicted focal shift and the measured shift led to a refinement of the thermo-elastic model. It was determined that an additional 10 µm adjustment was required for optimal alignment.
To resolve this issue, a second method was devised to fine-tune the detector’s position through a pre-run process. This method leverages the mechanical properties of the structure surrounding the detector and its electronic modules. The detector module, which comprises a sensor and its associated electronics, is housed within a metal structure engineered for efficient heat dissipation. During operation, rapid heat dissipation causes slight thermal expansion of the metal, resulting in a minor positional shift. OTVC experiments revealed that a 1°C change in the detector electronics produces an approximate focal shift of about 1.1 µm. Consequently, achieving the required 10 µm shift necessitates a 9°C temperature change, implying that a 10-minute pre-run is sufficient prior to in-orbit imaging.
This dual-method strategy offers a robust and practical passive focus control approach for CubeSat optical payloads, significantly mitigating the need for complex active focusing mechanisms. Furthermore, this approach allows for focal position adjustments at the imaging location by considering orbital environmental conditions, ensuring consistently the highest optical imaging performance (MTF) through planned pre-run procedures. Future work will further refine the thermo-elastic model and assess the long-term stability of these methods under actual spaceflight conditions.