Proceedings PaperActively cophased interferometry with SUN/SIMURIS
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The Solar Ultraviolet Network (SUN) is an instrument based on interferometric concepts, and capable of observations with a spatial resolution of 0.013' (10 km) on the Sun, in the UV and visible wavelength ranges. In this paper we present results on fringe pattern acquisition and stabilization as performed on a Mach-Zehnder set up representative of the interferometer cophasing system. The system algorithm is based on 'white light' fringe tracking controlled in a reference interferometer by a synchronous detection. This servo-system drives a two-stages delay line for real-time compensation of the optical path delays. Acquisition capabilities and stability possibilities are investigated as a function of flux and noise levels. Being stabilized, actively cophased, and in a 'compact' configuration, the SUN interferometer possesses remarkable imaging capabilities allowing high resolution diffraction-limited imaging on an extended field of view of 6 X 6 arcsec2. The dynamics of reconstructed images is superior to 400 for phase stabilities >= (lambda) /6 and photon flux of approximately 10,000 ph s-1 pixel-1 (on average). The SUN instrument is part of the Solar Interferometric Mission for Ultrahigh Resolution Imaging and Spectroscopy (SIMURIS) which was proposed to ESA in the framework of the Next Medium Size Mission (M2) in November 1989, and which completed a First Phase of Study in the context of the Space Station in August 1991.