  Euclid is a space telescope currently developed in the framework of the ESA
Cosmic Vision 2015-2025 Program. It addresses fundamental cosmological
questions related to dark matter and dark energy. The lens system of one of the
two scientific key instruments [a combined near-infrared spectrometer and
photometer (NISP)] was designed, built-up and tested at the Max Planck
Institute for Extraterrestrial Physics (MPE). We present the final imaging
quality of this diffraction-limited optical assembly with two complementary
approaches, namely a point-spread function and a Shack-Hartmann sensor-based
wavefront measurement. The tests are performed under space operating conditions
within a cryostat. The large field of view of Euclid's wide-angle objective is
sampled with a pivot arm, carrying a measurement telescope and the sensors. A
sequence of highly accurate movements to several field positions is carried out
by a large computer controlled hexapod. Both measurement approaches are
compared among one another and with the corresponding simulations. They
demonstrate in good agreement a solely diffraction limited optical performance
over the entire field of view.
