After the NACO instrument was decommissioned from Nasmyth B of UT4 in August 2013 to provide space for MUSE, the instrument was re-mounted on UT1 at Nasmyth A in September 2014, in place of CRIRES, which is being upgraded. Since problems with the CONICA detector's wire connections could not be recovered during recommissioning, it was decided to replace the CONICA detector with the old ISAAC long wavelength detector, which is also sensitive from 1 to 5 μm and has very similar features and noise properties. During 4–12 January 2015, engineers and astronomers were able to re-commission NACO and return it to operations. The adaptive optics performance is nominal and all offered modes for Periods 94 and 95 have been tested and validated on-sky.
Recommissioning of NACO
However, anomalously-high thermal background has been measured on the ISAAC Aladdin array (~70 electrons/pixel/second), whose effect is to slightly reduce the NACO dynamic range and detection limits for broad-band filters combined with long exposure times (e.g., ~ 3 mag. sensitivity loss in J-band, but full characherization is underway). This effect can be considered as negligible for the long-wavelength L'-band set-ups and the execution of the least-affected science programmes has already been started. Meanwhile, the operations team is working to resolve this issue to enable the instrument to be recovered to full performance.