Ground layer correction: the heart of LINC-NIRVANA

The delivered image quality of ground-based telescopes depends greatly on atmospheric turbulence. At every observatory, the majority of the turbulence (up to 60-80% of the total) occurs in the ground layer of the atmosphere, that is, the first few hundred meters above the telescope pupil. Correction of these perturbations can, therefore, greatly increase the quality of the image. We use Ground-layer Wavefront Sensors (GWSs) to sense the ground layer turbulence for the LINC-NIRVANA (LN) instrument, which is in its final integration phase before shipment to the Large Binocular Telescope (LBT) on Mt. Graham in Arizona. 19 LN is an infrared Fizeau interferometer, equipped with an advanced Multi-Conjugate Adaptive Optics (MCAO) module, capable of delivering images with a spatial resolution equivalent to that of a ~23m diameter telescope. It exploits the Layer-Oriented, Multiple Field of View, MCAO approach 3 and uses only natural guide stars for the correction. The GWS has more than 100 degrees of freedom. There are opto-mechanical complexities at the level of sub- systems, the GWS as a whole, and at the interface with the telescope. Also, there is a very stringent requirement on the superposition of the pupils on the detector. All these conditions make the alignment of the GWS very demanding and crucial. In this paper, we discuss the alignment and integration of the left-eye GWS of LN and detail the various tests done in the lab at INAF-Padova to verify proper system operation and performance.