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Design of a four-mirror astrometric telescope for light bending measurements

We present a new design of a four mirrors telescope for astrometric measurement to be used in the GAME mission, currently under study at the Astronomical Observatory of Turin, Italy. The main aim of GAME - Gamma Astrometric Measurement Experiment - is to measure the γ parameter of the Parameterized Post- Newtonian formulation by looking at the deflection of light produced by the Sun's gravitational curvature, as in the Dyson, Eddington et al. 1919 experiment, using a dedicated, space based dual-field telescope. A first design has been presented in recent years, based on a Cassegrain scheme with a mask in front of the primary mirror to realize multiple aperture Fizeau interferometry. The new design still implements a Fizeau interferometer, but the telescope layout is based on a Korsch-like scheme with four conical mirrors, long focal length, and without the use of exotic surfaces (aspheric or polynomial) as adopted in other long focal astrometric instruments. A different combination scheme of the two lines of sight makes the dimensioning of the primary mirror more relaxed allowing us to work with smaller surfaces and therefore to achieve a more compact payload configuration. The design of the instrument and the masked interferometry approach allow us to maximize the astrometric performances and at the same time to improve the baffling, minimizing the amount of stray light from the Sun. In this paper we describe the mission profile, the observation principle, the new instrument layout and the expected performances.