LINC-NIRVANA is the Fizeau beam combiner for the LBT, with the aim to retrieve the sensitivity of a 12m telescope
and the spatial resolution of a 22.8m one. Despite being only one of the four wavefront sensors of a layer-oriented
MCAO system, the GWS, which is retrieving the deformation introduced by the lower atmosphere, known to be the main
aberration source, reveals a noticeable internal opto-mechanical complexity.
The presence of 12 small devices used to select up to the same number of NGSs, with 3 optical components each,
moving in a wide annular 2'-6' arcmin Field of View and sending the light to a common pupil re-imager, and the need to
obtain and keep a very good super-imposition of the pupil images on the CCD camera, led to an overall alignment
procedure in which more than a hundred of degrees of freedom have to be contemporary adjusted.
The rotation of the entire WFS to compensate for the sky movement, moreover, introduces a further difficulty both in the
alignment and in ensuring the required pupil superposition stability.
A detailed description of the alignment procedure is presented here, together with the lessons learned managing the
complexity of such a WFS, which led to considerations regarding future instruments, like a possible review of numerical
versus optical co-add approach, above all if close to zero read-out noise detectors will be soon available.
Nevertheless, the GWS AIV has been carried out and the system will be soon mounted at LBT to perform what is called
the Pathfinder experiment, which consists in ground-layer correction, taking advantage of the Adaptive Secondary