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.
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.
In order to ensure the on-orbit performance of space laser communication terminal(SLCT), the optical performance test under thermal vacuum conditions must be completed on the groud. In this paper, according to the requirements of SLCT, thermal vacuum optical performance test system was designed and developed. Its main testing capabilities include the divergence angle, polarization state, wave aberration, transmission power. Several SLCTs were tested by the system, the results show that the overall performance of the test system is stable and the thermo-optical test of SLCT can be completed well.
Absolute measurement with Phase Measuring Deflectometry (PMD) is gaining importance in industry, but the accuracy of deflectometry metrology is strongly influenced by the level of calibration. In order to improve the accuracy of the PMD to a level where it competes with interferometry, a reference calibration process is commonly carried out to carefully calibrate the systematic errors. The systematic errors obtained by measuring a high quality reference surface can be subtracted from the measurement of a test surface to get its actual surface, however, it could introduce the surface error of reference into the measurement. To alleviate this problem, this paper introduces a technique named “rotational shear phase measuring deflectometry”, this technique have the ability of removing the rotationally asymmetric systematic errors from the test surface without using a reference surface. The validity of this technique has been demonstrated by simulation and our experimental results.
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