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]
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 laser beam director (LBD) is a reflective telescope which has a long optical Coude path. Its optical path can be off
the original aligned if rotational jitter of the telescope turret occurs or structural deformation in the LBD due to external
disturbances occurs. It can make the laser beam deviate from the line of sight of the telescope. Therefore misalignment
must be monitored and corrected. We adapt null optics to the telescope in order to monitor alignment state of the LBD.
Misalignment is corrected in real time through the fast steering mirror located between the primary and secondary mirror
of the telescope. Test results on rotation of the telescope turret show that aligned state of the LBD is kept in spite of
rotational jitter of the turret.
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