Optical alignment of the LGS and NGS WFS of ERIS: procedures and first results

ERIS is an adaptive instrument for the UT4 of the Very Large Telescope. It implements both an infrared imager and a spectrograph served by a single-conjugate adaptive optics system able to work with a natural or an artificial guide star. The wavefront correction will be provided by the adaptive secondary mirror of the ESO Adaptive Optics Facility. The natural and laser guide star wavefront sensors are 2 independent Shack-Hartmann sensors with 40 × 40 subapertures arranged on CCD220-based detectors provided by ESO. Moreover, each WFS implements an optical device to register the pupil-actuator geometry and to counter-rotate the actuators pattern on the plane of the lenslet array. The NGS WFS also implements a low-order sensor with 4 × 4 subapertures to work as tip-tilt and truth sensor during the LGS operation. Both the LGS and NGS WFS units of ERIS are now completing their assembly and integration phase in laboratory at the Arcetri Observatory. We present in this paper the procedures we employed to optically align the 2 WFS units, discussing the precision we achieved in the process. Such alignment concept was successfully proven in the FLAO Pyramid WFS and we expect to be able to adopt it also for the Low-Order and Reference (LOR) WFS of MAORY for the ELT.