Almost all high-contrast imaging coronagraphs proposed until now are based on passive coronagraph optical
components. Recently, Ren and Zhu proposed for the first time a coronagraph that integrates a liquid crystal array (LCA)
for the active pupil apodizing and a deformable mirror (DM) for the phase corrections. Here, for demonstration purpose,
we present the initial test result of a coronagraphic system that is based on two liquid crystal spatial light modulators
(SLM). In the system, one SLM is served as active pupil apodizing and amplitude correction to suppress the diffraction
light; another SLM is used to correct the speckle noise that is caused by the wave-front distortions. In this way, both
amplitude and phase error can be actively and efficiently compensated. In the test, we use the stochastic parallel gradient
descent (SPGD) algorithm to control two SLMs, which is based on the point spread function (PSF) sensing and
evaluation and optimized for a maximum contrast in the discovery area. Finally, it has demonstrated a contrast of 10-6 at an inner working angular distance of ~6.2 λ/D, which is a promising technique to be used for the direct imaging of young exoplanets on ground-based telescopes.
During finely grinding the best fit sphere and initial stage of polishing, surface error of large aperture aspheric mirrors is
too big to test using common interferometer. Foucault test is widely used in fabricating large aperture mirrors. However,
the optical path is disturbed seriously by air turbulence, and changes of light and dark zones can not be identified, which
often lowers people's judging ability and results in making mistake to diagnose surface error of the whole mirror. To
solve the problem, the research presents wavefront retrieval based on Foucault test through digital image processing and
quantitative calculation. Firstly, real Foucault image can be gained through collecting a variety of images by CCD, and
then average these image to eliminate air turbulence. Secondly, gray values are converted into surface error values
through principle derivation, mathematical modeling, and software programming. Thirdly, linear deviation brought by
defocus should be removed by least-square method to get real surface error. At last, according to real surface error, plot
wavefront map, gray contour map and corresponding pseudo color contour map. The experimental results indicates that
the three-dimensional wavefront map and two-dimensional contour map are able to accurately and intuitively show
surface error on the whole mirrors under test, and they are beneficial to grasp surface error as a whole. The technique can
be used to guide the fabrication of large aperture and long focal mirrors during grinding and initial stage of polishing the
aspheric surface, which improves fabricating efficiency and precision greatly.
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