ABSTRACT
The LAser raDAR (LADAR) system designed in this study shows a ghost pattern around the object image when operated. The system contains 4 wedge prisms, each with different rotational directions and speeds. Therefore, an efficient and thorough analysis method was established. Ray path analysis was performed, and categorized, for every instantaneous case sampled using a backward ray tracing method. The rays' flux and directions were accumulated according to their path histories. This backward ray tracing was performed repeatedly with different neutral density (ND) filter orientations, until no measurable ghost radiance remained in the field of regard (FOR): a tilt angle of 5°. The ND filter was replaced with a mechanical vignette. Subsequently, the ghost flux was 21% of the total accumulated point cloud, coinciding with the actual measurement of 19%. The final image has significantly improved resolution and shows no ghost reflections where they were previously.
ABSTRACT
The imaging spectrometer generally shows geometrical asymmetric distortions known as the keystone and smile that are different from the regular imaging optical system. The conventional method of measuring such distortions requires a precision movement control stage and specialized optical setup. Moreover, it is even harder to measure other characteristics such as the wave front error (WFE) simultaneously and to repeat the measurements since an accumulated vast number of statistical data is required to calculate the keystone and smile. To overcome these disadvantages, a new and simple method is proposed. The newly proposed method takes images separated in fields and wavelengths utilizing a simple tool called the field identifier (FI). Then, the keystone and the smile are calculated fast and repeatedly from a single measurement image while measuring the WFE with the Shack-Hartmann sensor with the minimum change of the measurement setup. With this method, hyperspectral imager is aligned and its geometrical distortions are measured.
ABSTRACT
A fore optics for the hyperspectral spectrometer is designed, manufactured, assembled, and aligned. The optics has a telecentric off-axis three-mirror configuration with a field of view wider than 14 degrees and an f-number as small as 2.3. The primary mirror (M1) and the secondary mirror (M2) are axially symmetric aspheric surfaces to minimize the sensitivity. The tertiary mirror (M3) is a decentered aspheric surface to minimize the coma and astigmatism aberration. The M2 also has a hole for the slit to maintain the optical performance while maximizing the telecentricity. To ensure the spatial resolution performance of the optical system, an alignment procedure is established to assemble and align the entrance slit of the spectrometer to the rear end of the fore optics. It has a great advantage to confirm and maintain the alignment integrity of the fore optics module throughout the alignment procedure. To perform the alignment procedure successfully, the precision movement control requirements are calculated and applied. As a result, the alignment goal of the RMS wave front error (WFE) to be smaller than 90 nm at all fields is achieved.
ABSTRACT
We developed a new integrated ray tracing (IRT) technique to analyze the stray light effect in remotely sensed images. Images acquired with the Geostationary Ocean Color Imager show a radiance level discrepancy at the slot boundary, which is suspected to be a stray light effect. To determine its cause, we developed and adjusted a novel in-orbit stray light analysis method, which consists of three simulated phases (source, target, and instrument). Each phase simulation was performed in a way that used ray information generated from the Sun and reaching the instrument detector plane efficiently. This simulation scheme enabled the construction of the real environment from the remote sensing data, with a focus on realistic phenomena. In the results, even in a cloud-free environment, a background stray light pattern was identified at the bottom of each slot. Variations in the stray light effect and its pattern according to bright target movement were simulated, with a maximum stray light ratio of 8.5841% in band 2 images. To verify the proposed method and simulation results, we compared the results with the real acquired remotely sensed image. In addition, after correcting for abnormal phenomena in specific cases, we confirmed that the stray light ratio decreased from 2.38% to 1.02% in a band 6 case, and from 1.09% to 0.35% in a band 8 case. IRT-based stray light analysis enabled clear determination of the stray light path and candidates in in-orbit circumstances, and the correction process aided recovery of the radiometric discrepancy.
ABSTRACT
A new and patented polishing tool called Orthogonal Velocity field Tool (OVT) was built and its material removal characteristics from Chemical Vapor Deposition Silicon Carbide (CVD SiC) mirror surfaces were investigated in this study. The velocity field of OVT is produced by rotating the bicycle type tool in the two orthogonal axes, and this concept is capable of producing a material removal foot print of pseudo Gaussian shapes. First for the OVT characterization, we derived a theoretical material removal model using distributions of pressure exerted onto the workpiece surface, relative speed between the tool and workpiece surface, and dwell time inside the tool- workpiece contact area. Second, using two flat CVD SiC mirrors that are 150 mm in diameter, we ran material removal experiments over machine run parameter ranging from 12.901 to 25.867 psi in pressure, from 0.086 m/sec to 0.147 m/sec tool in the relative speed, and 5 to 15 sec in dwell time. Material removal coefficients are obtained by using the in-house developed data analysis program. The resulting material removal coefficient varies from 3.35 to 9.46 um/psi hour m/sec with a mean value of 5.90 ± 1.26(standard deviation). We describe the technical details of the new OVT machine, the data analysis program, the experiments, and the results together with the implications to the future development of the machine.
