Multi-aperture optical imagers intrigue attention for high-resolution Earth observation optics. One crucial technology in building these optical systems is constructing an on-orbit final alignment method of piston-tip-tilt errors between sub-apertures. We propose using image-based alignment based on the stochastic parallel gradient descent (SPGD) algorithm. We developed a tabletop multi-aperture imaging setup with 37 hexagonal-shaped mirror segments. The piston-tip-tilt state of each segment was controlled by changing the applied voltage to each microelectromechanical systems actuator that supports the mirror segments. The observation targets were a pinhole and extended scenes projected on a display. The experimental results demonstrated the successful aperture synthesis of 37 mirror segments using static and time-varying extended scenes.
In recent years, small satellites have been utilized for remote sensing from Low Earth Orbit (LEO) with a spatial resolution of several meters. However, improving the temporal resolution for LEO remote sensing is challenging because of the short orbital period. Observation techniques using remote sensing from a Geostationary Orbit (GEO), or its nearby orbit are becoming increasingly crucial, particularly in disaster monitoring, due to their ability to provide high-temporal resolution. To improve both temporal and spatial resolutions from GEO, it is necessary to use an optical system with a diameter of several meters due to the diffraction limit. We propose the Formation Flying Synthetic Aperture Telescope (FFSAT). One of the key issues is realizing the optical system with an accuracy of less than 1/10 of the observation wavelength to get synthesized images. We propose a method for estimating and correcting misalignment and optical aberrations using adaptive optics.
Model-free image-based wavefront correction is a useful technique in resource-limited spaceborne imaging systems used for Earth observation. However, such an approach is difficult because correction performance depends on selection of the measured scenes and the evaluation metric. In this study, the relationship is experimentally and numerically investigated using the stochastic parallel gradient descent (SPGD) algorithm. As a result, the combination of deviation-based metrics and scenes with distinct structures was found to facilitate wavefront correction. The results also show that the present approach is useful in not only monolithic but also segmented mirror optics. The proposed method will contribute to realize on-orbit wavefront-sensorless aberration removal required in near-future high-resolution Earth observation systems.
Earth observation satellite with a large mirror telescope has been studied in Japan Aerospace Exploration Agency to offer a fine ground sampling distance from geostationary orbit. Our latest optical design has a 3.6-m primary mirror and thus aims to obtain a ground sampling distance of sub-10 m from an altitude of 36,000 km at visible wavelengths. To achieve diffraction-limited performance in such optics on orbit, the telescope equips with not only actuators to control primary and secondary mirrors but also a deformable mirror (DM) at the exit pupil plane for the fine phasing. For on-orbit wavefront correction with a deformable mirror, a wavefront sensorless image-based aberration correction scheme is advantageous from the viewpoint of severely limited hardware resources in satellites. Phase diversity (PD) and stochastic parallel gradient descent (SPGD) optimization are known for promising image-based approaches. The former is model-based and thus the estimation accuracy of wavefront aberration significantly depends on the model accuracy, while the other requires many measurements to compensate for large aberration. To alleviate these issues, we propose sequential use of PD and SPGD optimization to efficiently reduce wavefront correction. We first developed an optical testbed with an incoherent light source, a MEMS DM, and extended image targets, and then a wavefront correction experiment was carried out. As a result, the proposed method successfully achieved diffraction-limited imaging performance with a small number of measurements. We will also discuss the image dependence of the wavefront correction performance.
Wavefront compensation techniques that do not require wavefront sensors are demanded in the on-orbit telescopes on Earth observation satellites. This is especially true for segmented or sparse aperture telescopes that could realize unprecedented high angular resolution. A promising wavefront sensorless approach is the stochastic parallel gradient descent (SPGD) algorithm. The wavefront correction by the SPGD optimization relies only on the intensity data in the acquired image. However, many previous observation targets are point light sources, not the extended ground scenes generally acquired by Earth observation satellites. This paper derives an efficient wavefront control method for imaging systems for the fast SPGD optimization. Wavefront compensation has been demonstrated by experiment on extended objects in single aperture optics, in which a microelectrome- chanical system deformable mirror controls the wavefront. Subsequent numerical simulations are reported for multi-aperture imaging systems. The paper also discuses a method to reduce the computational cost of SPGD optimization.
Small satellites have been used for remote sensing with a spatial resolution of several meters from LEO in recent years. However, it is difficult to increase temporal resolution for LEO remote sensing due to the short orbital period. Therefore, GEO remote sensing which enables observation of high temporal resolution from GEO or its nearby orbit is getting important. In order to obtain enough spatial resolution in GEO remote sensing, an optical system having a diameter of several meters is required because of the diffraction limit. It takes huge cost to realize such a large diameter primary mirror due to manufacturability and required accuracy. To address this problem, we propose a synthetic aperture telescope by small satellites formation flying. The synthetic aperture telescope is composed of several mirror satellites constituting a primary mirror of the telescope and an imaging satellite having a focal plane assembly. By optically synthesizing the light collected by each mirror satellite with the imaging satellite, a virtual large aperture telescope is constructed. In this paper, we assume the observation at near infrared to short wavelength infrared and show the specifications of the system. The apperture telescope and the image processing method used to extract high spatial frequency information from the observed images are also described.
A demand for responsive, high-resolution Earth observations is emerging for mitigating the human suffering and damage that follow large-scale disasters. One of the most promising advances is a sophisticated optical imager with a large, 3.6-m satellite-mounted telescope in geostationary orbit. The imager of the proposed space telescope has a segmented mirror and offers a ground sampling distance of better than 10 m and a latency of shorter than 30 minutes. For the imager to realize diffraction-limited performance, deformable mirrors are planned to be installed at the exit pupil of the telescope system. One candidate for the deformable mirrors in segmented telescope is based on a micro-electromechanical system (MEMS) that offers a small actuator pitch, fine step resolution, and excellent hysteretic motion response. This paper presents the wavefront correction of aberrations with a high and low spatial frequency using MEMS deformable mirror on an optical testbed. The expected image quality is also evaluated through numerical simulation.
For small satellite remote sensing missions, a large aperture telescope which has more than 400 mm of diameter is required to realize less than 1m GSD observations. However, it is difficult or expensive to realize the large aperture telescope using monolithic primary mirror with high surface accuracy. Generally, not only high accuracy of optical surface but also high accuracy of optical alignment is required for large aperture telescope. For a segmented mirror telescope, aligning optical elements in high accuracy is more difficult and more important. For conventional systems, optical alignment is adjusted before launch to achieve desired imaging performance. However, it is difficult to adjust the alignment for large sized optics in high accuracy. Furthermore, thermal environment in orbit and vibration in launch vehicle cause the misalignments of the optics. We are developing an adaptive optics system using a MEMS deformable mirror (DM) for Earth observing remote sensing sensor. Image based adaptive optics system compensate the misalignments and wavefront aberration of optical elements using DM by feedback of observed images. Because, it is difficult to use a reference point source unless the satellite controls its attitude toward a star for Earth observing systems. Furthermore, total amount of incident light can enter an image sensor.
We propose the control algorithm of DM and mirror segments for a segmented mirror telescope by using of observed images. Numerical simulation results represent that misalignment and wavefront aberration of the segmented mirror telescope are corrected and image quality is improved.
For small satellite remote sensing missions, a large aperture telescope more than 400mm is required to realize less than 1m GSD observations. However, it is difficult or expensive to realize the large aperture telescope using monolithic primary mirror with high surface accuracy. A segmented mirror telescope should be studied especially for small satellite missions. We describe the conceptual design of the optical system that involve the segmented primary mirror and adaptive optics system, then we show numerical simulation results of the optical performance.
For small satellite remote sensing missions, a large aperture telescope more than 400mm is required to realize less than
1m GSD observations. However, it is difficult or expensive to realize the large aperture telescope using a monolithic
primary mirror with high surface accuracy. A segmented mirror telescope should be studied especially for small satellite
missions. Generally, not only high accuracy of optical surface but also high accuracy of optical alignment is required for
large aperture telescopes. For segmented mirror telescopes, the alignment is more difficult and more important. For
conventional systems, the optical alignment is adjusted before launch to achieve desired imaging performance. However,
it is difficult to adjust the alignment for large sized optics in high accuracy. Furthermore, thermal environment in orbit
and vibration in a launch vehicle cause the misalignments of the optics. We are developing an adaptive optics system
using a MEMS deformable mirror for an earth observing remote sensing sensor. An image based adaptive optics system
compensates the misalignments and wavefront aberrations of optical elements using the deformable mirror by feedback
of observed images. We propose the control algorithm of the deformable mirror for a segmented mirror telescope by
using of observed image. The numerical simulation results and experimental results show that misalignment and
wavefront aberration of the segmented mirror telescope are corrected and image quality is improved.
We are developing an adaptive optics system for earth observing remote sensing sensor. In this system, high spatial
resolution and high signal to noise ratio has to be realized by a lightweight sensor system due to the launcher’s
requirements. Moreover, simple hardware architecture has to be used to achieve high reliability, low cost, and short
development period. Image based AOS realize these requirements without wavefront sensor. In remote sensing, it is
difficult to use a reference point source unless the satellite controls its attitude toward a star. We propose the multi-mode
phase diversity method using deformable mirror.
We are developing an adaptive optics system for earth observing remote sensing sensor. In this system, high spatial
resolution has to be achieved by a lightweight sensor system due to the launcher’s requirements. Moreover, simple
hardware architecture have to be selected to achieve high reliability. Image based AOS realize these requirements
without wavefront sensor. In remote sensing, it is difficult to use a reference point source unless the satellite controls its
attitude toward a star. We propose the control algorithm of the deformable mirror on the basis of the extended scene
instead of the point source.
We are developing an adaptive optics system for earth observing remote sensing sensor. In this system, high spatial resolution has to be achieved by a lightweight sensor system to be used for small satellites. Moreover, simple hardware architecture has to be selected to achieve high reliability and low development cost. Image based AOS realize these requirements without wavefront sensor. In remote sensing, it is difficult to use a reference point source unless the satellite controls its attitude toward a star. We propose the control algorithm of the deformable mirror on the basis of two methods; model-based wavefront estimation method and direct optimization of acquired images. We described simulation results of the proposed methods.
We are developing an adaptive optics system for earth observing remote sensing sensor. In this system, high spatial
resolution has to be achieved by a lightweight sensor system due to the launcher’s requirements. Moreover, simple
hardware architecture has to be selected to achieve high reliability. Image based AOS realize these requirements without
wavefront sensor. In remote sensing, it is difficult to use a reference point source unless the satellite controls its attitude
toward a star or it has a reference point source in itself. We propose the control algorithm of the deformable mirror on
the basis of the extended scene instead of the point source. In our AOS, a cost function is defined using acquired images
on the basis of the contrast in spatial or Fourier domain. The cost function is optimized varying the input signal of each
actuator of the deformable mirror. In our system, the deformable mirror has 140 actuators. We use basis functions to
reduce the number of the input parameters to realize real-time control. We constructed the AOS for laboratory test, and
proved that the modulated wavefront by DM almost consists with the ideal one by directly measured using a Shack-
Hartmann wavefront sensor as a reference.
Satellite hyperspectral imaging sensors suffer from ''smile'' and ''keystone'' properties, which appear as distortions of
spectrum images. The smile property is a center wavelength shift and the keystone property is a band-to-band
misregistration. These distortions degrade the spectrum information and reduce classification accuracies. Furthermore,
these properties may change after the launch. Therefore, in the preprocessing of satellite hyperspectral images, the
onboard correction of the smile and keystone properties is an important issue as well as the radiometric and geometric
correction. The main objective of this work is to propose the prototype of the preprocessing of hyperspectral image with
consideration of smile and keystone properties. Image registration based on phase correlation is used for detecting the
optical properties. Cubic spline interpolation is adopted to modify the spectrum because of its good trade-off between the
smoothness and shape preservation. Smile and keystone detection simulation using the EO-1 Hyperion imagery taken at
various times in the past nine years proved that the optical properties have been changing due to the onboard secular
distortion. Therefore, onboard optical properties should be updated periodically and built into the radiometric and
geometric corrections for future satellite hyperspectral sensors. The proposed method may be the prototype of the
preprocessing of future satellite hyperspectral sensors.
Remote sensing missions have been conventionally performed by using satellite-onboard optical sensors with
extraordinarily high reliability, on huge satellites. On the other hand, small satellites for remote-sensing missions have
recently been developed intensely and operated all over the world. This paper gives a Japanese concept of the
development of nano-satellites(10kg to 50kg) based on "Hodoyoshi" (Japanese word for "reasonable") reliability
engineering aiming at cost-effective design of optical sensors, buses and satellites. The concept is named as "Hodoyoshi"
concept. We focus on the philosophy and the key features of the concept. These are conveniently applicable to the
development of optical sensors on nano-satellites. As major advantages, the optical sensors based on the "Hodoyoshi"
concept are "flexible" in terms of selectability of wavelength bands, adaptability to the required ground sample distance,
and optimal performance under a wide range of environmental temperatures. The first and second features mentioned
above can be realized by dividing the functions of the optical sensor into modularized functional groups reasonably. The
third feature becomes possible by adopting the athermal and apochromatic optics design. By utilizing these features, the
development of the optical sensors become possible without exact information on the launcher or the orbit. Furthermore,
this philosophy leads to truly quick delivery of nano-satellites for remote-sensing missions. On the basis of the concept,
we are now developing nano-satellite technologies and five nano-satellites to realize the concept in a four-year-long
governmentally funded project. In this paper, the specification of the optical sensor on the first satellite is also reported.
The purpose of this paper is to develop sub-pixel registration method for adaptive optics system using phase diversity
wavefront sensing with a spatial light modulator (SLM). The SLM which is used for wavefront compensation applies
multiple time-series known wavefronts as a priori information to the optical system. By using the SLM for the phase
diversity generator, it is possible to select the optimal number and shape of phase diversities for various kinds of natural
modes of wavefront aberrations which are represented by the Zernike polynomials. In this case, a misregistration of
several diversity images has to be compensated before using phase diversity algorithm. We extracted phase diversity
method to estimate not only wavefront aberration but also parallel shift between images simultaneously. The suggested
method was validated by numerical simulations, and the high estimation accuracy of the distorted wavefront was
demonstrated, and nearly diffraction limited images were acquired by wavefront compensation by preventing noise due
to misregistrations.
The purpose of this paper is to develop adaptive optics system which estimates degrading factors using observed images
and automatically compensates the distorted wavefronts using a spatial light modulator (SLM). The system estimates
optical wavefront aberrations by phase diversity method. The SLM which is used for wavefront compensation applies
multiple time-series known wavefronts as a priori information to the optical system. By using the SLM for the phase
diversity generator, it is possible to select the optimal number and shape of phase diversities for various kinds of natural
modes of wavefront aberrations which are represented by the Zernike polynomials. In laboratory test, wavefront
aberrations were generated by optical misalignments, and they were estimated as the coefficients of Zernike polynomials
for an extended scene. The suggested method was validated by numerical simulations and laboratory tests. The high
estimation accuracy of the distorted wavefront was demonstrated, and nearly diffraction limited images were acquired by
wavefront compensation.
We propose an adaptive optics system using a Liquid crystal on Silicon Spatial Light Modulator (LCOS-SLM) for
wavefront control. The phase diversity technique is used as a wavefront sensor, which estimates a wavefront aberration
using processing images acquired by mission sensor instead of using additional wavefront sensor hardware. Because of
simplicity in a hardware architecture, the phase diversity technique is suitable especially for a light weight remote
sensing satellite. In the conventional phase diversity method, prior information, which is defined as a phase diversity, is
applied to the optical system by defocusing. Then wavefront aberrations are estimated using the phase diversity and
acquired images. For generating prior information, we uses the LCOS-SLM which generate arbitrary wavefront shape. In
this cases, the selection of the phase diversity affect the estimation accuracy of the wavefront aberration. This paper
describes the selection strategy of phase diversities, and then validates it by laboratory test.
We propose an adaptive optics system for a lightweight remote sensing sensor. The phase diversity (PD) technique, in which known wavefronts (phase diversity) are applied to the optics and the inherent aberrations are estimated using the acquired images without a priori information, is a key to realizing the system. For the reduction of computing cost and the enhancement of the estimation accuracy of aberration, a spatial light modulator (SLM) is adopted not only for the wavefront compensator but also for the PD generator. The SLM produces arbitrary "aberration modes" that are each represented by a Zernike polynomial. Therefore, optimal phase diversities are applied to the optical system and particular modes are effectively obtained, which makes it possible to overcome the conventional PD generated by defocusing that describes only the quadratic form and lacks information of a particular mode. To solve the complex inverse problem of phase diversity with low computing cost, a general regression neural network (GRNN) is used. Moreover, principal component analysis compresses the input data for GRNN by extracting information from collected images in Fourier space, and reduces computation cost considerably without degrading estimation accuracy. The mathematical model is implemented and its performance is validated by numerical simulation.
We propose an adaptive optics system for a lightweight remote sensing sensor. The phase diversity (PD) technique, in
which known wavefronts (Phase Diversity) are applied to the optics and the inherent aberrations are estimated using the
acquired images without a priori information, is a key to realizing the system. For the reduction of computing cost and
the enhancement of the estimation accuracy of aberration, a spatial light modulator (SLM) is adopted not only for
wavefront compensator but also for PD generator. The SLM produces arbitrary "aberration modes" that are each
represented by a Zernike polynomial. Therefore, optimal phase diversities are applied to the optical system and particular
modes are effectively obtained, which makes it possible to overcome the conventional PD generated by defocusing that
describes only quadratic form and lacks information of a particular mode. In order to solve the complex inverse problem
of phase diversity with low computing cost, a general regression neural network (GRNN) is used. Moreover, principal
component analysis compresses the input data for GRNN by extracting information from collected images in Fourier
space, and reduces computation cost considerably. The performance is validated by numerical simulation, and the result
of experiment using SLM is described.
Fourier transform spectrometer (FTS) has fast optics, and it can realize high resolution within the range from visible light to thermal infrared radiation. FTS intrinsically has the problem that it takes long time to obtain spectrum, because it needs mechanical scanning. But we developed spaceborne FTS system which has the ability of high speed scanning and data handling. By high speed scanning, FTS makes it possible to have high altitude resolution in occultation, and imaging in nadir observation.
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