The multispectral high-resolution imaging system is composed of four wide-spectrum cameras, which can detect targets in the visible, short-wave infrared, medium-wave infrared and long-wave infrared spectrum simultaneously. In order to reduce the size of the system, the four wide-spectrum cameras use the same telescope. To achieve operating wavelengths covering four wavebands, the telescope uses a fully reflective design with a coaxial aspherical surface with a hole in the center of the primary mirror. If aberration-free testing is used, the central aperture will occur, resulting in incomplete aperture of surface shape detection. Thus, in this paper, a compensator is designed which can achieve the whole aperture of surface testing without centralized obscuration. At the same time, the impact of the misalignment of compensator in the optical path during the test is analyzed. The optical testing path adopts infinite conjugate working distance to reduce one adjustment amount of compensator. And the first surface of compensator is coordinated with the interferometer to adjust the angle of compensator quickly and accurately, which further reduces the measurement error introduced by the optical testing path. The design of the compensator can realize the control of sensitive misalignment, reduce the surface measuring error caused by the compensator misalignments, and eventually reduce the precise processing error.
When measuring image qualities of large aperture cameras, many factors like people moving around, blowing of air conditioners outlets, thermal convection, etc., will give rise to air turbulence (AT). AT mainly induces non-uniform distribution of air components in the image chain of large aperture camera image quality measurement systems, which will lead to variations of system wavefront errors. Thus, AT will introduce errors to measurement results of traditional image quality evaluation methods. Those errors increase with the camera aperture and are usually time-varying. This paper proposes a method to calibrate AT in real time when measuring image qualities of large aperture cameras. A defocused star point target (DSPT) is added to traditional test targets (TTT). The camera under test can capture images of TTT and DSPT simultaneously. The distance between the effective area of TTT and the DSPT is carefully designed so that the corresponding images do not overlap with each other. We calibrate AT induced wavefront errors by processing the DSPT images with phase retrieval method. Experimental results of AT induced wavefront errors calibrated by the proposed method are presented.
The national major scientific research instrument project: “The accurate infrared solar magnetic field measurements system” (AIMS) is under construction. The figure of the primary of the AIMS can be measured using a computer generated hologram (CGH) test during the polishing process, however, a distortion correction procedure is needed to mapping the coordinates of the mirror and the pixels of fringes due to the large distortion exists in the CGH test configuration, and the mapping relationship need to be re-calibrated after the primary mirror was reassembled, which makes the test process cumbersome. In this paper, a sub-aperture stitching equipment was established, which uses a two\dimensional guide that can move a 450mm reference flat mirror to any position that can cover the aperture of the primary mirror. The surface shape requirement of the 450 mm flat mirror was given by Monte-Carlo analysis and further the figure was tested by using Ritchey-Common technique. Furthermore, a sub-aperture stitching test system was established and a modified simultaneous fitting algorithm was proposed to stitch the sub-aperture wavefront together, the correctness of the technique was verified by a full aperture figure test experiment. Finally, we applied the developed approach to the site test of figure of the AIMS primary mirror.
Large aperture space cameras are widely used in high-resolution remote sensing, ultra-weak space targets detection, high-precision astronomical observation, etc. Modulation transfer function (MTF) can directly reflect the information transmission ability of a space camera. Usually, the image quality of a camera is evaluated by measuring its MTF. The MTF of a space camera needs to be measured during alignment of the optical system and electronic system, preset of the vacuum focus position, mechanical experiment, thermal experiment, etc. This paper suggests a method to model the imaging chain of large aperture space camera MTF measurement systems. The model comprehensively considers the factors of the aberration of the collimator, air turbulence, temperature variation, gravity, and environmental vibration. We calibrate the aberration of the collimator, air turbulence, temperature variation, and gravity induced error by measuring the corresponding wavefront error with a 4D laser interferometer. A star point target is placed in the focus position of the collimator to calibrate the environmental vibration through extracting the centroids of the point target images captured by the space camera. The impacts of the previous factors on the MTF measurement results of the space camera are obtained with the proposed model and the corresponding calibration data. The proposed method can evaluate the impacts of different factors on the MTF measurement results and can guide the measurement of large aperture space camera MTF.
AIMS is an infrareds optical system for the accurate measurement of solar magnetic field, which is a national major scientific research project currently developed. The guiding optical system of AIMS is an off-axis Gregorian system, due to the designed minimum angle between the optical axis of the optical system and horizontal plane is 14.036°, a sub-aperture stitching test approach is developed to test the wavefront of the system. That makes the process of precision alignment of the system very difficult and laborious. Therefore, we developed a two-step alignment approach that based on merit function regression method, the developed method can make alignment of AIMS guiding optical system efficiency and accuracy. In this paper, we explain the detailed two-step alignment method and apply it to the real alignment of AIMS guiding system. Aided with sub-aperture stitching measurements, the AIMS guiding system is aligned and the results show that in 0.076λrms wavefront error in effective aperture was achieved.
Sub-aperture stitching interferometry can be used for measurement of wavefront of large aperture optical system. A variety of sub-aperture stitching algorithms have been studied to reconstruct the sub-aperture data to obtain full aperture wavefront. The simultaneous fitting method plays an important role among those stitching algorithms which uses a series of global polynomials to accomplish the fitting of the test wavefronts, however, it can only be applied in the case of there have no overlap between each sub-apertures. Therefore, a modified simultaneous fitting method is proposed and is applied to measure the wavefront of large aperture optical system. The proposed algorithm is applicable whether there exists overlap between each sub-aperture or not. The numerical simulation is carried out to evaluate the accuracy of the algorithm. Further, a practical stitching experiment that test an optical system with a diameter of 850mm was implemented to demonstrate the modified algorithm.
SVOM-VT has entered the initial phase development stage, and encircled energy is its key performance index. In the development process, it is necessary to determine the encircled energy of the optical lens stage and the system stage. The image recording of a CCD detector includes two imaging processes: one is the pixel integration imaging process, in which the output signal of each pixel is proportional to the area integration of the incident light intensity on the surface of the pixel; the other is the discrete sampling process, in which the continuous graphical object is sampled discretely at the sampling interval of the center distance of the pixel. Based on the data of SVOM-VT, the effect of CCD under-sampling on the encircled energy of detection camera is characterized by simulation and test. Imaging process of CCD pairs of scattered speckles from the lens is a two-dimensional discrete sampling process, as well as the sampling process of discrete signals. This process will lead to low-frequency noise (under-sampling noise) in the sampling of high-frequency signals by CCD detectors, resulting in spectrum aliasing (low-frequency signal distortion) of image signals. Intuitively, the original image is broadened. When the sampling density is increased, this will not be the case.
For a wavefront tested by Shack-hartmann wavefront sensor, the zonal integration method is often chosen by researchers to solve the reconstruction problem. But it has shown an unacceptable result when the phase derivative data is distributed on an unconnected domain, the obtained wavefront will contain different piston error on each subdomain. Therefore, a new zonal wavefront estimation algorithm is proposed to deal with this drawback, which uses a simultaneous fit method to correct piston error of each subdomain. The validity of the algorithm is verified by a numerical simulation and experimental results.
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