To assemble and align the optical system, it is required to minimize the sensitivity of the optical system while maximizing the performance. However, sensitivity increases as the performance is optimized. To balance the sensitivity and performance, an efficient sensitivity analysis method is established to confirm the sensitivity right after the optical design is complete. The simplified analysis method reduces the individual analysis time to 1/5 of the full analysis. As a result, the final performance met the requirement and the sensitivity was minimized reducing the complexity and time of the final design process to 1/5 compared to the previous program.
The BlackBody Unit (BBU) mounted in front of the IR optical system is located off the optical path to avoid any thermal noise induced by the unit. To perform a thermal calibration of the optical system, a mirror is inserted in the optical path to refract the optical path toward the BBU during the calibration mode. To maximize the performance of the BBU, it is required to fill the Focal Plane Array (FPA) with the thermal irradiance as even as possible. To verify the performance of the BBU during the calibration mode, a ray tracing is performed. As a result, it is confirmed the uniformity of the thermal signal to be smaller than 0.01K when the BBU has the thermal distribution of 1K.
The designed infra-red optical system requires the thermal compensation of 3.5 um according to the environmental thermal variation. To compensate the depth of focus, the mechanical barrel should move not only the amount of movement required but also the appropriate direction. However, the current single shell structure cannot compensate the amount and direction required. To achieve the goal, a multi-layer shell structure is required. However, it is highly limited to select the material appropriate for the space craft. As a result, the goal of 3.5 um and required direction is achieved with multi-material shell structure.
The optical system of the entire mechanical and optical components consist of all silicon carbide (SiC) is designed, manufactured and aligned. The Korsch type Cassegrain optical system has 3-mirrors, the primary mirror (M1), the secondary mirror (M2), the folding mirror (FM) and the tertiary mirror (M3). To assemble the M3 and the FM to the rear side of the M1 bench, the optical axis of the M3 is 65.56 mm off from the physical center. Due to the limitation of the mass budget, the M3 is truncated excluding its optical axis. The M2 was assigned to the coma compensator and the M3 the astigmatism respectively as per the result of the sensitivity analysis. Despite of the difficulty of placing these optical components in their initial position within the mechanical tolerance, the initial wave front error (WFE) performance is as large as 171.4 nm RMS. After the initial alignment, the sensitivity table method is used to reach the goal of WFE 63.3 nm RMS in all fields. We finished the alignment with the final WFE performance in all fields are as large as 55.18 nm RMS.