In this study, we investigated the impact of ghost images on the modulation transfer function (MTF) of a Korsch-type telescope using nonsequential ray-tracing simulations and the experimental measurements of the knife-edge method with a collimator and light source targets. Our findings showed that ghost images introduce a directional bias into the edge spread function depending on the field position, which affects the line spread function and MTF. Furthermore, our measurement results demonstrated that ghost images can significantly affect the MTF on the edge field of the green channel. The ghost-to-signal ratio in the multispectral (MS) green channel was approximately 2.5%, which is approximately 0.25% higher than that in the panchromatic channel. To estimate the impact of ghost images in the MS green channel, we performed a parametric analysis using a nonsequential ray-tracing simulation, exploring potential strategies, such as adjusting the window thickness, the distance between the detector and the window, the transmittance of the window surface, and the reflectance of the detector surface. By comparing the positions and intensities of the ghost images obtained from the simulations with those measured experimentally, we identified the simulation input parameters that best reproduced the measured results. Our study provides valuable insight into the importance of managing ghost images when designing and operating Korsch-type telescopes to achieve the optimal image quality.
KEYWORDS: Control systems design, Satellites, Telecommunications, Free space optical communications, Acquisition tracking and pointing, Optical tracking, Design and modelling, Wavefronts, Beam steering, Optical simulations
This paper discusses a testbed that implements a beam acquisition and tracking system essential for inter-satellite optical communication. It summarizes the results of optical and control system design. Recent research on free-space optical communication has been thriving due to the increasing demand for data transmission in clustered operations of small satellites. However, since the distances between satellites are often hundreds of kilometers, a beam acquisition and tracking system is necessary to establish communication by accurately detecting and tracking the position of the target satellite. The optical system of the testbed, developed by Telepix, shows the attenuated output and flatness of wavefront propagated over long distances. The control system applies an adaptive control method to manage disturbances, resulting in the successful reduction of beam pointing errors to the desired level, as demonstrated by simulation results. In the future, this technology holds potential for various applications, including clustered operations of small satellites using free-space optical communication, as well as communication between ground stations and deep-space optical communication.
We developed Korsch type telescope for the compact advanced satellite payload. The optical system has a diameter of 600 mm with F/11. All mirror component was developed to have better performance than the tolerance allocations. The alignment of three mirrors are guided using the algorithm, merit function regression method. During this process we found a new approach for the far field focus optimization. To minimize aberration, we have been focused on the main Zernike term (astigmatism and coma), but the focal plane position is also important to proceed to the next step. We will present our optimization process, and the result of performance.
We first studied the characteristics of alignment performances of two computer-aided alignment algorithms i.e. merit
function regression (MFR) and differential wavefront sampling (DWS). The initial study shows i) that, utilizing damped
least square algorithm, MFR offers accurate alignment estimation to the optical systems with non-linear wavefront
sensitivity to changes in alignment parameters, but at the expense of neglecting the coupling effects among multiple
optical components, and ii) that DWS can estimate the alignment state while taking the inter-element coupling effects
into consideration, but at the expense of increased sensitivity to measurement error associated with experiment apparatus.
Following the aforementioned study, we report a new improved alignment computation technique benefitted from
modified MFR computation incorporating the concept of standard DWS method. The optical system used in this study is
a three-mirror anastignmat (TMA) based optical design for the next generation geostationary ocean color instrument
(GOCI-II). Using an aspheric primary mirror of 210 mm in diameter, the F/7.3 TMA design offers good imaging
performance such as 80% in 4 um in GEE, MTF of 0.65 at 65.02 in Nyquist frequency. The optical system is designed to
be packaged into a compact dimension of 0.25m × 0.55m × 1.050m. The trial simulation runs demonstrate that this
integrated alignment method show much better alignment estimation accuracies than those of standard MFR and DWS
methods, especially when in presence of measurement errors. The underlying concept, computational details and trial
simulation results are presented together with implications to potential applications.
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.
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