Observatorio Astrofisico de Javalambre T250 Telescope Control

Observatory Sciences is contracted to provide the control system for a 2.5m wide-field survey telescope based in Spain. The T250 telescope is being built by AMOS in Belgium for the OAJ (Observatorio Astrofisico de Javalambre). Together with its smaller sister T80 telescope, the T250 will provide Spain with state of the art facilities for wide area astrophysical surveys.

The facility will be used by the Javalambre Physics of the Accelerating Universe Astrophysical Survey (J-PAS) to map the observable universe in 56 colours. This multipurpose astronomical survey will observe more than 8000 square degrees, about a fifth of the whole sky, mapping stars, galaxies, supernovas, quasars and solar system objects with extreme accuracy. The construction of the T250 telescope called for an innovative design that would address the technical challenge of a large aperture of 2.5m combined with a wide field of view of 3 degrees. To meet these requirements, the telescope is built around a RitcheyChretien-like design, equipped with a field corrector and de-rotator. The focal plan corresponds to a Cassegrain layout, and the M1 and M2 mirrors have hyperbolic aspheric surfaces.

The telescope is equipped with a field corrector which is located beyond the central hole of M1. It consists of three lenses of fused silica with four aspheric surfaces and diameters in the range 500 ?? 600mm. The two mirrors and the three lenses are designed to optimise the polychromatic image quality and maintain a low distortion over the entire field of view. Because of its fast optics, the T250 is a very compact telescope, with a height of about 6.5m and an outer diameter of 5m. It will weigh around 40 tonnes. The telescope will use a large 1 gigapixel camera, being built by E2V, Chelmsford. E2V is a leader in telescope imaging devices, with the likes of the Hubble Space Telescope and Gaia, an ESA mission to record over one billion stars, among its projects. The gigapixel camera for the T250 will use 14 newly developed E2V CCD290-99 high performance imaging sensors. The 85 megapixel devices will be back-thinned and given a multi-layer, anti-reflection coating for maximum sensitivity. They are a 9k x 9k pixel format, with multiple outputs for rapid readout times, and are mounted in a precision package to allow them to be assembled into a mosaic, providing an image area that is nearly 0.5m in diameter. The focal plane assembly will also include the telescope guide and wavefront sensors. The whole focal plane will then be contained in a custom cryogenic camera, with vacuum and cooling components and integrated electronics which will provide state-of-the-art low noise for maximum sensitivity.

Observatory Sciences will provide the telescope control system which will co-ordinate several subsystems including the secondary mirror hexapod motion control, the telescope dome, the guiding system and a wavefront sensor system used to monitor and improve the image quality. The guiding system will incorporate four CCDs at the corners of the telescope field of view which will be used to monitor the position of guide stars. This technique will allow automatic correction of the elevation, azimuth and rotation axes positions. Designed, manufactured, assembled and tested in AMOS premises in Belgium, the T250 is scheduled for delivery to CEFCA (Center for the Study of Physics of the Cosmos of Aragon) in Teruel Aragon later this year. The 2.55m mirror will be completely manufactured in-house, and will represent the largest mirror ever polished at AMOS.

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