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.
A series of measurements of keystone and smile is required to assemble and aligning hyperspectral imagers. An efficient measurement method was proposed to calculate the keystone and the smile in a single measurement image by implementing an additional tool called the Field Identifier (FI) [1]. The measurement method is simple enough to make it possible to measure the wave front error (WFE) making minimum change from each measurement setup. To evaluate the accuracy of the measurement method, the positional data of 380 points were collected and 3σ was calculated along spectral and spatial axis. The measurement error calculated as 3σ is to be less than 1/10 of the performance goal to prove the effectiveness of the measurement method.
A three-mirror off-axis optical system was designed as a fore optics of a hyperspectral imager. The secondary mirror (M2) has an obscuration hole in the middle for the slit assembly. Despite of the disadvantages of having a slit hole in the mirror surface such as stray light defect and mirror surface fabrication difficulty, the configuration has great advantages of maximizing telecentricity while maintaining the wide field of view (FOV). [1] To evaluate the integrity of the optical system design, the stray light effect was analyzed including the spectrometer channel and confirmed that it has little effect on the image performance. Also, the RMS wave front error (WFE) of M2 is measured to be 20.12 nm exceeding our expectation including the edge of the hole. As a result, the optical system is aligned to have WFE less than 90 nm RMS in all fields. [1]
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.
We report an innovative simple alignment method for a VNIR spectrometer in the wavelength region of 400–900 nm; this device is later combined with fore-optics (a telescope) to form a f/2.5 hyperspectral imaging spectrometer with a field of view of ±7.68°. The detector at the final image plane is a 640×480 charge-coupled device with a 24 μm pixel size. We first assembled the fore-optics and the spectrometer separately and then combined them via a slit co-located on the image plane of the fore-optics and the object plane of the spectrometer. The spectrometer was assembled in three steps. In the initial step, the optics was simply assembled with an optical axis guiding He-Ne laser. In the second step, we located a pin-hole on the slit plane and a Shack-Hartmann sensor on the detector plane. The wavefront errors over the full field were scanned simply by moving the point source along the slit direction while the Shack-Hartmann sensor was constantly conjugated to the pin-hole position by a motorized stage. Optimal alignment was then performed based on the reverse sensitivity method. In the final stage, the pin-hole and the Shack-Hartmann sensor were exchanged with an equispaced 10 pin-hole slit called a field identifier and a detector. The light source was also changed from the laser (single wavelength source) to a krypton lamp (discrete multi-wavelength source). We were then easily able to calculate the distortion and keystone on the detector plane without any scanning or moving optical components; rather, we merely calculated the spectral centroids of the 10 pin-holes on the detector. We then tuned the clocking angles of the convex grating and the detector to minimize the distortion and keystone. The final assembly was tested and found to have an RMS WFE < 90 nm over the entire field of view, a keystone of 0.08 pixels, a smile of 1.13 pixels and a spectral resolution of 4.32 nm.
The designed Infra-red optical system with multi-magnification shows non-uniform thermal distribution only in Wide
FOV and suspected to be narcissus effect. To analyze the system’s artifacts more effectively, the optical system design
was imported to analysis codes. Initial ray tracing was performed with a point source from the detector to identify main candidates of Narcissus effect by analyzing irradiance distribution and flux distribution. As a second step, a planer source was created at the detector and traced again. As a result, four major candidates were selected and the major contributor was identified among them. To confirm the result with experiment, replacement optical component was manufactured. We can confirm that the Narcissus effect was improved significantly by replacing the identified
component.
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