Frank Kittmann, Thomas Bertram, Al Conrad, Jan Trowitzsch, Florian Briegel, Juergen Berwein, Lars Mohr




The near infrared interferometer LINC-NIRVANA combines the beams coming from the two primary mirrors of the Large Binocular Telescope to increase the resolution of the science camera image. LINC-NIRVANA is using layer-oriented multi-conjugate adaptive optics (MCAO) to reduce the influence of the atmospheric turbulence. The deformable mirrors of the MCAO systems are conjugated to the ground layer and a second layer in the upper atmosphere, respectively. Ground layer wavefront sensors and high layer wavefront sensors measure the wavefront in these two layers. Due to geometrical constraints unique to this type of instrument, it is not possible to provide a single derotator at the entrance of each incoming beam. Due to that fact, field derotation has to be applied for each wavefront sensor and for the science detector separately. The fields of the high layer wavefront sensors are derotated by the use of K-Mirrors, whereas the ground layer wavefront sensors and the science detector rotate themselves to compensate the field rotation. The derotation axes of the derotators will not perfectly match the field rotation center and projected derotation axis. These resulting effects have to be considered in the tip/tilt control strategy. In this paper we will describe the degree of accuracy required for the field derotation and compare that requirement with our test results. This includes timing and positioning accuracy over the complete derotation trajectory.

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