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 designed wedge scanner has conventional raster scan pattern canceling the movement along one axis by rotating opposite direction, in which case the alignment of these two wedge scanners affects the imaging performance of the system. To align the wedge scanners more efficiently and effectively, 2nd split rays are used. The ray tracing was performed to predict the precision achievable. The sensitivity according to the movement of the scanner is dependent on the refractive index and slant angle of the wedge and was about 350 times more sensitive when the angle between two vertexes of wedge scanners reaches 1°. The goal was successfully achieved below 0.6 mm in the screen 5 m away, which is 1/4 of the alignment goal.
An active range detection system with high power laser source requires a high precision of pulse timing. Although the trigger time stamp can be obtained electrically on the circuit board level, it is crucial to detect the actual transmitting laser pulse signal to monitor the malfunction of the laser. To detect the signal, inserting a reflector in the optical path is suggested so that the photodiode can detect the signal directly. To insert the reflector and pulse detection photodiode in the optical path of the transmitting laser, it is required to confirm the obscuration ratio of the laser module by the shadow of the reflector. The shape of the reflector was set to be rectangular due to the manufacturability although the elliptical reflector shows a better performance in the aspect of the vignette. As a result, it is confirmed that the obscuration is tolerable enough to cover the requirement of 0.8 of the nominal irradiances of the signal of each pixel when the reflector angle is set to be 47 ° and insertion depth is between 0.9 mm and 2 mm.
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 designed laser radar (LADAR) system utilize a Geiger focal plane array (FPA) of 16x16 resolution which limits the field of view (FOV) of the system. To overcome this, an additional scanner module consists of 4 rotational wedges was placed in front of the optical system. Naturally, the cases to analyze as a part of the stray light analysis became extremely complex and abundant. To sort out the major cases out of all possible scenarios, the stray light analysis was performed separately according to the modules. The ghost analysis was performed backward from the FPA to the front of the optics.
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]
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.
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.
Image mosaic technique is widely used in a field of remote sensing research. However, in case of Geostationary Ocean
Color Imager’s (GOCI’s) mosaic image which is consist of 16 slot images, the radiance level discrepancy was noticed in
the cloudy circumstance next to each other slot when acquiring the imagery data in the low Sun elevation angle. We
provided, in this study, the in-depth stray light analysis results in order to find out this discrepancy phenomenon, and
performed to compare the stray light pattern via a bright target movement.
Stray light analysis as the first step was completed with ray tracing technique based on ASAP program, and we
suggested that unwanted radiations from the Earth bright target or the atmosphere such as cloud are major candidates of
stray light in the problematic images. For embodying GOCI operational concept, we constructed the Integrated Ray
Tracing model consisting of the Sun model as a light source, a target Earth model, and the GOCI optical system model.
In the second step, we investigated the stray light pattern at each slot image including unwanted random source from out
of field, and then constructed the simulated mosaic bias image reached at the detector plane. In the simulated bias, the
ray path followed the procedures that light travels from the Sun and it is then reflected from the Earth section of roughly
2500km * 2500km in size around the Korea peninsula with 16 slots.
Lastly, we analyzed stray light pattern in the third step for the real image environment acquired at UTC-03 16th, October,
2011. In addition, verification was performed to compare the difference among slot boundaries for moving bright target.
The designed Active LDR(Laser Detection and Ranging) System contains high-power Laser and its diameter is
approximately 24mm. Although the laser transmitting channel and receiving optic channel are completely separated
from each other and doesn’t share any of the optical components in design, each channel shares 4 wedge scanners, which
are to overcome the narrow FOV(Field of View) of the optical system. Any backward reflection back to the fiber laser
end must be carefully studied since it can damage the LD(Laser Diodes), the inner components of the laser unit because
of the high amplification factor of the laser unit. In this study, the stray light caused by the transmitting channel’s laser
and inner reflection by optical components were analyzed by ASAP(Advanced System Analysis Program) software. We
also can confirm the operability and stability of the system by more than 6 months of operation of the system.
In this report, we present laboratory test simulation for directional responsivity of a global Earth albedo monitoring instrument. The sensor is to observe the Sun and the Earth, alternately, and measure their shortwave (<4μm) radiations around the L1 halo orbit to obtain global Earth albedo. The instrument consists of a broadband scanning radiometer (energy channel instrument) and an imager (visible channel instrument) with ±2° field-of-view. In the case of the energy channel instrument, radiations arriving at the viewing ports from the Sun and the Earth are directed toward the pyroelectric detector via two spherical folding mirrors and a 3D compound parabolic concentrator (CPC). The instrument responsivity is defined by the ratio of the incident radiation input to the instrument output signal. The radiometer’s relative directional responsivity needs to be characterized across the field-of-view to assist output signal calibration. For the laboratory test, the distant small source configuration consists of an off-axis collimator and the instrument with adjustable mounts. Using reconstructed 3D CPC surface, the laboratory test simulation for predicting the instrument directional responsivity was conducted by a radiative transfer computation with ray tracing technique. The technical
details of the laboratory test simulation are presented together with future plan.
In the remote sensing researches, the reflected bright source from out of FOV has effects on the image quality of wanted signal. Even though those signal from bright source are adjusted in corresponding pixel level with atmospheric correction algorithm or radiometric correction, those can be problem to the nearby signal as one of the stray light source. Especially, in the step and stare observational method which makes one mosaic image with several snap shots, one of target area can affect next to the other snap shot each other. Presented in this paper focused on the stray light analysis from unwanted reflected source for geostationary ocean color sensor. The stray light effect for total 16 slot images to each other were analyzed from the unwanted surrounding slot sources. For the realistic simulation, we constructed system modeling with integrated ray tracing (IRT) technique which realizes the same space time in the remote sensing observation among the Sun, the Earth, and the satellite. Computed stray light effect in the results of paper demonstrates the distinguishable radiance value at the specific time and space.
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.
Accurate identification and understanding of spectral bio-signatures from possible extra terrestrial planets have received
an ever increasing attention from both astronomy and space science communities in recent years. In pursuance of this
subject, one of the most important scientific breakthroughs would be to obtain the detailed understanding on spectral biosignatures
of the Earth, as it serves as a reference datum for accurate interpretation of collapsed (in temporal and spatial
domains) information from the spectral measurement using TPF instruments. We report a new Integrated Ray Tracing
(IRT) model capable of computing various spectral bio-signatures as they are observed from the Earth surface. The
model includes the Sun, the full 3-D Earth, and an optical instrument, all combined into single ray tracing environment in
real scale. In particular, the full 3-D Earth surface is constructed from high resolution coastal line data and defined with
realistic reflectance and BSDF characteristics depending on wavelength, vegetation types and their distributions. We first
examined the model validity by confirming the imaging and radiometric performance of the AmonRa visible channel
camera, simulating the Earth observation from the L1 halo orbit. We then computed disk averaged spectra, light curves
and NDVI indexes, leading to the construction of the observed disk averaged spectra at the AmonRa instrument detector
plane. The model, computational procedure and the simulation results are presented. The future plan for the detailed
spectral signature simulation runs for various input conditions including seasonal vegetation changes and variable cloud
covers is discussed.
KEYWORDS: Ray tracing, Coastal modeling, Performance modeling, Atmospheric modeling, Sun, Sensors, 3D modeling, Monte Carlo methods, Instrument modeling, Light sources
The Geostationary Ocean Colour Imager (GOCI) is a visible band ocean colour instrument onboard the
Communication, Ocean, and Meteorological Satellite (COMS) scheduled to be in operation from early 2010. The
instrument is designed to monitor ocean water environments around the Korean peninsula in high spatial and temporal
resolutions. We report a new imaging and radiometric performance prediction model specifically designed for GOCI.
The model incorporates the Sun as light source, about 4000km x 4000km section of the Earth surrounding the Korean
peninsula and the GOCI optical system into a single ray tracing environment in real scale. Specially, the target Earth
section is constructed using high resolution coastal line data, and consists of land and ocean surfaces with reflectivity
data representing their constituents including vegetation and chlorophyll concentration. The GOCI instrument in the IRT
model is constructed as an optical system with realistic surface characteristics including wave front error, reflectivity,
absorption, transmission and scattering properties. We then used Monte Carlo based ray tracing computation along the
whole optical path starting from the Sun to the final detector plane, for simultaneous imaging and radiometric
performance verification for a fixed solar zenith angle. This was then followed by simulation of red-tide evolution
detection and their radiance estimation, in accordance with the in-orbit operation sequence. The simulation results prove
that the GOCI flight model is capable of detecting both image and radiance originated from the key ocean phenomena
including red tide. The model details and computational process are discussed with implications to other earth
observation instruments.
In recent years, high performance visible and IR cameras have been used widely for tactical airborne reconnaissance.
The process improvement for efficient discrimination and analysis of complex target information from active battlefields
requires for simultaneous multi-band measurement from airborne platforms at various altitudes. We report a new dual
band airborne camera designed for simultaneous registration of both visible and IR imagery from mid-altitude ranges.
The camera design uses a common front end optical telescope of around 0.3m in entrance aperture and several relay
optical sub-systems capable of delivering both high spatial resolution visible and IR images to the detectors. The camera
design is benefited from the use of several optical channels packaged in a compact space and the associated freedom to
choose between wide (~3 degrees) and narrow (~1 degree) field of view. In order to investigate both imaging and
radiometric performances of the camera, we generated an array of target scenes with optical properties such as reflection,
refraction, scattering, transmission and emission. We then combined the target scenes and the camera optical system into
the integrated ray tracing simulation environment utilizing Monte Carlo computation technique. Taking realistic
atmospheric radiative transfer characteristics into account, both imaging and radiometric performances were then
investigated. The simulation results demonstrate successfully that the camera design satisfies NIIRS 7 detection criterion.
The camera concept, details of performance simulation computation, the resulting performances are discussed together
with future development plan.
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