Image restoration is an effective way to improve the quality of images degraded by wave-front aberrations. If the wave-front aberration is too large, the performance of the image restoration will not be good. In this paper, the relationship between the performance of image restoration and the degree of wave-front aberrations is studied. A set of different wave-front aberrations is constructed by Zernike polynomials, and the corresponding PSF under white-light illumination is calculated. A set of blurred images is then obtained through convolution methods. Next we recover the images with the regularized Richardson-Lucy algorithm and use the RMS of the original image and the homologous deblurred image to evaluate the quality of restoration. Consequently, we determine the range of wave-front errors in which the recovered images are acceptable.
An accurate estimation of the point spread function (PSF) is very useful in image restoration. This paper proposes a method of estimating the PSF of space camera based on in-orbit wavefront sensing. Three wavefront sensors are used to measure the wave-front errors of the space camera, with one sensor in the center field and the other two sensors in the two edge fields. The wavefront errors of other fields can be estimated by interpolation. Then, the PSFs of each field can be estimated using the wavefront errors. <p> </p>For monochromatic light, the PSF can be calculated by Fourier Transformation. For a white light imaging system, the PSF is a weighted integration of the monochromatic PSFs. The weighting coefficient is the product of the sun-light spectrum, spectral reflection factor of the object, spectral transmission factor of the optical system, and the spectral response of the image detector. Because different object has a different spectral reflection factor, to be practical and simple, we use an average spectral reflection factor of typical ground scenes instead.<p> </p>It will induce error in the PSF estimations by using the average reflection factor instead of the real one. This error is analyzed in the paper which shows that the error is acceptable.
The image quality of a high resolution space camera may be degraded by wavefront aberrations and optical axis jitters. It is desirable to use an in-orbit instrument to measure these errors and then correct them with on-line adaptive optical technologies or off-line image restoration algorithms. An integrated method of measuring wavefront aberrations and optical axis jitters of the space camera based on the ground scene Shack-Hartmann wavefront sensor is studied. When working in jitter-sensing mode, only parts of the sub-images are read to enhance the frame rate. Factors affect the precision are analyzed. Experiments are made which show that this method has a good performance by choosing proper parameters.
The Modified Hybrid-Input-Output (MHIO) phase retrieval algorithm is proposed for wavefront sensing. The results show that the MHIO algorithm significantly outperforms the Modified Gerchberg-Saxton algorithm (MGS) in large noise. However its dynamic-range is lower than MGS algorithm. It also shows that if combine the MGS algorithm with MHIO algorithm, which is called MGS+MHIO algorithm, then it can retain the property of MGS’s high dynamic-range and MHIO’s accuracy so that outperforms either MGS or MHIO algorithm. Repeating simulation results show that MGS+MHIO algorithm improves RMS of phase error obviously in high dynamic range and large noise.
Large-aperture segmented primary mirror will be widely used in next-generation space-based and ground-based telescopes. The effects of intersegment gaps, obstructions, position and figure errors of segments, which are all involved in the pupil plane, on the image quality metric should be analyzed using diffractive imaging theory. Traditional Fast Fourier Transform (FFT) method is very time-consuming and costs a lot of memory especially in dealing with large pupil-sampling matrix. A Partial Fourier Transform (PFT) method is first proposed to substantially speed up the computation and reduce memory usage for diffractive imaging analysis. Diffraction effects of a 6-meter segmented mirror including 18 hexagonal segments are simulated and analyzed using PFT method. The influence of intersegment gaps and position errors of segments on Strehl ratio is quantitatively analyzed by computing the Point Spread Function (PSF). By comparing simulation results with theoretical results, the correctness and feasibility of PFT method is confirmed.
In the process of high-resolution astronomical observation and space optical mapping, the wavefront aberrations caused by atmosphere turbulence effect lead to reduced resolution of optical imaging sensor. Firstly, on the base of influence of atmosphere turbulence effect for the optical observation system, this paper investigates and analyses the development and technical characteristics of deformable mirror, which is the key device of optical wavefront control technology. In this part, the paper describes the basic principles of wavefront control and measurement using the current production line of deformable mirror, including micro-electromechanical systems (MEMS) deformable mirror which is one of the most promising technology for wavefront modulation and Shack-Hartmann wavefront sensors. Secondly, a new method based on the technology of optical wavefront control and the data of optical path difference (OPD) for simulating the effect of optical transmission induced by turbulence is presented in this paper. The modeling and characteristics of atmosphere turbulence effect applied for optical imagery detector of astronomical observation and space optical mapping has been obtained. Finally, based on the theory model of atmosphere turbulence effects and digital simulation results, a preliminary experiment was done and the results verify the feasibility of the new method. The OPD data corresponding to optical propagation effect through turbulent atmosphere can be achieved by the calculation based on the method of ray-tracing and principle of physical optics. It is a common practice to decompose aberrated wavefronts in series over the Zernike polynomials. These data will be applied to the drive and control of the deformable mirror. This kind of simulation method can be applied to simulate the optical distortions effect, such as the dithering and excursion of light spot, in the space based earth observation with the influence of turbulent atmosphere. With the help of the optical wavefront control technology, the optical sensor and ability of space optical detection system for correcting the target image blurred by turbulence of atmosphere can be tested and evaluated in the laboratory.
Position errors of three-mirror reflective space optical system will affect the whole system badly,
so we have to calibrate these errors in orbit. This paper proposes a new method for the first time, which
is: we can use Stochastic Parallel Gradient Descent algorithm to calibrate three-mirror reflective remote
sensor, this method don't need wavefront sensor to calibrate position errors. It uses root mean square of
radius of image as system merit, through controlling the six position of second mirror to compensate
system error. This method is suitable for the calibration of three mirror reflective space optical system
for its needless of using wavefront sensor. Results of the simulation show that compared with
traditional sensitive matrix inversion algorithm, this method increases the dynamic range of initial
position errors, and it can improve wavefront error from about 1 wave rms to lower than 0.04 wave rms
in the center field of view.
Proc. SPIE. 7010, Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter
KEYWORDS: Mathematical modeling, Signal to noise ratio, Astronomy, Detection and tracking algorithms, Imaging systems, Interference (communication), Photonics systems, Space telescopes, Signal processing, Radiometry
Photon imaging system (PIS) is widely applied to astronomy observing, deep space exploration. To investigate the
system performance for space application, the process that target radiance converting to photons is presented. In this
processes, radiometry theory and astronomy units are used. Then a mathematical physical model for the imaging system
is used to calculate the system SNR. In the model, the system dark noise and the signal enhancement process are derived
as mathematical equations. The performance of the detection algorithm is also introduced to predict the PIS range
performance of the probability of detection and correct localization and the probability of false alarm based on SNR. At
last, the actual PIS range performance for space application is discussed and valuable data for space telescopes design and
deep space exploration also are provided.
The wavefront error and polarization of a side mounted infrared window made of ZnS are studied. The Infrared
windows suffer from temperature gradient and stress during their launch process. Generally, the gradient in temperature
changes the refractive index of the material whereas stress produces deformation and birefringence. In this paper, a
thermal finite element analysis (FEA) of an IR window is presented. For this purpose, we employed an FEA program
Ansys to obtain the time-varying temperature field. The deformation and stress of the window are derived from a
structural FEA with the aerodynamic force and the temperature field previously obtained as being the loads. The
deformation, temperature field, stress field, ray tracing and Jones Calculus are used to calculate the wavefront error and
the change of polarization state.
Thermal distortion of the optical elements can greatly reduce the high resolution of the space-borne camera. The
general thermal effect on mirror is analyzed and the optical aberration of the optical surface resulting from 3 kinds of
thermal gradient is discussed. The thermal distortion simulating experiment of a large aperture flat mirror is designed and
the optical aberration is tested on 18" ZYGO with the different axial thermal disturb. The testing results conclude that the
small thermal gradient can greatly affect the wave-front, the aberration of this large aperture flat mirror can be used to
simulate the thermal distortion on space, and MTF is also reduced greatly when this large aperture flat mirror is used in
the real space-borne camera under the same thermal environment. In order to correct the thermal distortion and keep the
high resolution, the 37-units adaptive optics correction close loop experiment is designed and installed in the above
camera. The correction results show that MTF of the testing camera will not reduced greatly under the large thermal
distortion. So employing adaptive optics on a high resolution space camera is the necessary and the valid method to
A novel type of adaptive optical system based on direct optimization of system performance metric is studied. The
optimization method is based on stochastic parallel gradient descent (SPGD) algorithm. Theoretical analysis, computer
simulations, and experimental results under point source and extended source conditions are presented.
When an adaptive optical system is used in an imaging system observing extended targets, a method of correlation can be used to find the relative movements of the sub-images in its Hartman-Shack waterfront sensor. To get a sub-pixel accuracy, a curve fitting method is presented. This paper describes the method in detail, and presents the simulations and experimental results. These results show an accuracy of 0.1 pixels rms.
A mini adaptive optics system is developed in which a micro-machined membrane deformable mirror and the technique of cross-folded optical path are employed. The system has a volume of 30x20x10cm3, a weight of 4kg, while its bandwidth is 17Hz, and the accuracy of its wave-front sensor is ?/15 mis. The system is aimed at improving the image quality of space-based optical systems, but it is versatile. The control host is a PC computer, and various functions are realized, such as, real time display of image or wave-front, testing of the response matrix of the deformable mirror, and closed-loop control. An experimental system is also setup to test the performance ofthe AO system. The results of the experiments show that the AO system is very effective in compensation for thermal deformations and dynamic disturbances.
In order to examine the performance of adaptive optics system in the laboratory environment, a new simple laboratory atmosphere turbulence simulator (ATS) is developed which uses the hot resistance coil to generate the hot air turbulence and the fan to control the intensity of the turbulence. The spatial and temporal characteristics of the turbulence generated by the ATS are tested in different experimental conditions by the test system and then compared with the theoretical Kolmogorov spectrum atmosphere turbulence. The results show that the spatial and temporal characteristics of the turbulence generated by ATS have definite similarity with the Kolmogorov atmosphere turbulence under certain experimental conditions. Finally the reason of the difference with theory Kolmogorov atmosphere turbulence is analyzed.
The disadvantage in expanding dynamic range and restraining noise of the traditional H-S (Hartmann-Shack) wavefront sensor centroid computation method is analyzed. Based on the temporal characteristic of atmosphere turbulence, through determining the changing scope of centroid motion between two adjacent sample periods, a new dynamic tracking centroid method of H-S wavefront sensor is proposed. Using this method, the dynamic range of wavefront sensor can be expanded when atmosphere turbulence is strong; the effect of CCD readout noise and shot noise can be reduced.
An experimental setup of photon counting real-time image acquisition system is introduced, wherein a photon image head (an image intensifier with high radiant emittance gain) is coupled with a high frame rate CCD camera by a super powerful relay lens. The restrictions on luminous emittance of object are analyzed for multiphoton and single photon imaging modes. The methods of determining readout noise are introduced. The application examples of system in adaptive optics wavefront sensor operated with a faint object, and in experimental study on the optical wave-particle duality and the uncertainty relation are presented.