Commissioning and characterising MUSE AO degraded mode

The Multi Unit Spectroscopic Explorer (MUSE) is an integral field spectrograph on the Very Large Telescope Unit Telescope 4. The MUSE adaptive optics observing capabilities include a wide field (1 square arcmin), ground layer seeing-enhanced AO mode (WFM-AO), and a narrow field (7.5”×7.5”), laser tomography AO mode (NFM-AO). The MUSE AO observations use four laser guide stars (4LGS) to correct the atmospheric turbulence. The MUSE AO observations routinely improve image quality by a factor of 2 in the seeing-enhanced wide field (WFM-AO) observations and deliver image quality down to 50-60 milliarcsecond (mas) in the laser tomography AO (NFM-AO) mode. The 4LGS and AO systems at UT4 are stable, but there is still some chance that one of the lasers or Wavefront Sensors (WFS) is out of service, leaving us with 3LGS. In the last few years, we have successfully commissioned and characterized the MUSE AO degraded mode (3LGS mode) for both MUSE WFM-AO and NFM-AO modes. In this paper, we present the commissioning activities, the procedure developed to switch between 4LGS and 3LGS mode, and the performance characterization of the MUSE-NFM AO degraded mode.