Takao Endo, Mitsubishi Electric Corporation; Yoshichika Miwa, Mitsubishi Electric Corporation; Toshiyuki Ando, Mitsubishi Electric Corporation; Yasutaka Fujii, Mitsubishi Electric Corporation; Takashi Takanezawa, Mitsubishi Electric Corporation; Yutaka Ezaki, Mitsubishi Electric Corporation
Keywords: Wavefront sensing, Shack-Hartmann wavefront sensor, Adaptive optics, Active optics
Abstract:
Adaptive and active optics were originally developed in the field of astronomy to remove the image aberration induced by wavefronts propagating through Earth’s atmosphere, and by gravitational deformation or misalignment of the telescope optics. Adaptive optics systems allow compensating the aberrations of an incoming wavefront by using a deformable mirror in order to correct the degrading effects of atmospheric turbulence. A conventional wavefront sensor (WFS) generally requires a single or multiple point source beacons, either a natural guide star (NGS) or an artificial laser guide star (LGS), as the reference wavefront. However, for passive, remote imaging applications, such a reference source is not generally available. Therefore, we have already developed extended-scene Shack Hartmann (SH) WFS for space-based and ground-based active/adaptive optics systems. In this paper, we discuss the correlation-based wavefront detection algorithm and the preliminary results of the prototype SH-WFS.
SH-WFS is fundamentally based on the geometrical optics, and consists of an array of lenses for wavefront division and typically CCD or CMOS image sensor. It uses an array of the focal plane image of each lenslet, which is called Hartmanngram, to measure the deformation of the incoming wavefront. The principle of the conventional point-source image based SH-WFS is to determine the local wavefront tilt from the measurement of the displacement of the focal point of a lenslet array. In the case of a point source as a reference of the wavefront, the sub-image created by the sub-aperture also produces a point source image. Therefore, the spot locations can be estimated by calculating the centroids of these spot images. However, in the case of the extended source as reference beacon, these centroiding algorithms can severely degrade the performance of conventional SH-WFS. In this paper, we proposed correlation-based wavefront sensing as a means of improving performance in these applications. The extended-scene SH-WFS algorithm is introduced to determine the relative positions by cross-correlation between a particular extended-scene sub-image and a reference sub-image, each of which is produced by one element of Hartmanngram. In order to compensate up to the fifth order of Zernike polynomial with an accuracy of 5 / 100?, we set the number of pupil divisions to 12 × 12 and the number of pixels per sub-aperture to 100 × 100 pixels for prototype SH-WFS. In this design, the accuracy of 1/50 pixel is required to measure the sub-pixel displacement of the sub-image, and then we evaluated the accuracy of the sub-pixel shift for prototype SH-WFS in the laboratory. As a result, we confirmed the accuracy of 1/69 pixels can be satisfied. This corresponds to an accuracy of less than 4 / 100? of the Zernike polynomial.
Date of Conference: September 17-20, 2019
Track: Adaptive Optics & Imaging