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Fig. 4.


Autoguider images. Its objective lens is a ring-shaped Fresnel array (top left): a thin copper plate with several thousand subapertures. An image of the Sun is formed by diffraction and interference through the holes. Top center: two enlarged views of the Fresnel array holes. Top right: image of a monochromatic point source (point spread function) given by the Fresnel array. Bottom: control screen (photo by Géraldine Pedezert) of the autoguider during the solar eclipse(2015-03-20). The solar disk is correctly located, even though only a crescent remains visible.

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