The use of sparse aperture can reduce the size and weight of the large aperture telescope. The sphere or aspheric surface commonly used is difficult to increase the field of view of the system and improve the image quality. Compared with spherical or aspherical surfaces, optical freeform surface has more design freedoms. This paper designs a two-mirror sparse aperture telescope. The primary mirror is made of three sub-mirrors arranged in the Golay3 configuration while the primary is a freeform surface defined by Zernike polynomials. The results show that the full field of view increases up to 0.32° in the optical system when the primary mirror uses a freeform surface. The image quality meets the requirements form its modulation transfer function.
Sparse aperture structure can solve the problems that single-aperture structure with large aperture is difficult to process. It can solve the problem that large aperture is easy to be deformed and can reduce the weight and size of the optical system. The most ideal state of the sparse aperture is to obtain more light information with the smallest light-passing area. The current goal is to obtain the best image quality by researching the arrangement of sparse aperture. However, most of the current sparse aperture structures have the same sub-aperture’s diameter, which leads to a rapid decline of system's modulation transfer function in the mid-frequency band. In this paper, the structure of three sub-apertures surrounding the large sub-aperture called quasi four-mirror structure is proposed through theoretical analysis and MATLAB simulation. The proportion of the diameter of the central mirror of the structure and the diameter of the surrounding sub-aperture is established. This proportional relation can ensure that the actual cutoff frequency is maximized while the filling factor of the entire system is minimized, thereby obtaining higher frequency information. The structure also has a feature that allows uniform acquisition of the mid-frequency information to obtain more detail information of image. The results of imaging simulation show that the imaging quality of the structure is better than that of the four-mirror structure when the filling factor and the light-passing area are equal. The sparse aperture structure of the quasi four-mirror structure proposed in this paper can be applied not only to large-scale astronomical telescopes, but also to medical endoscopes.
Sparse aperture optical system is arranged by a number of small apertures or reflective optical systems according to certain rules. It reduces the processing difficulty, the weight and the cost of the telescope system while its resolution is equivalent to that of a single-aperture telescope system. In this paper, the Cassegrain telescope system with three sub-mirror sparse aperture primary mirror as the spherical surface is used as the initial structure and optimized. The freeform surface is introduced into the sparse aperture optical system to increase the freedoms of optical design, balance aberrations and improve the imaging quality. On this basis, the three sub-mirror sparse aperture with freeform surface is designed and its image quality is analyzed.
KEYWORDS: Imaging systems, Sensing systems, Objectives, Genetic algorithms, Error analysis, Contrast transfer function, System on a chip, Optical transfer functions, Americium, Physics
The phase diversity wave-front sensing (PDWFS) technique is a posteriori image-based wave-front sensing method which utilizes two images collected simultaneously whose pupil phase differs from each other in a known manner, typically the defocus phase diversity. Here, we present a new method of implementing phase diversity on the sparse aperture imaging system that adds an intentional piston phase to one subaperture. The objective function is firstly derived for the sparse aperture imaging system, then the genetic algorithm is used to minimize the objective function to estimate the piston errors of the subapertures. Digital simulations are conducted for varying amounts of piston phase diversity and levels of noise, the performance of sub-aperture phase diversity is evaluated by comparing with the conventional defocus phase diversity. The results show that the conventional defocus phase diversity performs better than the sub-aperture phase diversity when there is no noise, while the sub-aperture phase diversity outperforms the conventional defocus phase diversity when the noise strength increases. Sub-aperture phase diversity may be an useful alternative if the conventional defocus phase diversity method fails.
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