The GRAVITY+ project includes the upgrade of the Very Large Telescope Interferometer infrastructure and of the instrument GRAVITY to improve sky coverage, high contrast capabilities, and faint science. The improved sky coverage is obtained via the implementation of one Laser Guide Star (LGS) on each Unit Telescope (UT). This first requires an upgrade of the infrastructure of each of the UTs, which was made over 18 months in 2022 and 2023. The addition of the LGS system requires the implementation of multiple components on several areas of the telescope. These areas include the top ring, the centerpiece, a new platform under the Nasmyth platform, and in the basement. The system includes pointing and aircraft avoidance cameras, the laser projector, laser and electronics cabinets, a heat exchanger, and pumps. As none of the UTs were developed with the infrastructure needed to carry such a system, the same upgrade is made for each of the three UTs. This upgrade consists of a full adaptation of the centerpiece, an upgrade of one of the altitude cable wraps, the installation of a platform under the Nasmyth platform, and the implementation of a cooling circuit running from the basement to the new sub-Nasmyth platform via the azimuth cable wrap. This upgrade requires two missions per telescope, for a total of 30 nights out of operation per telescope. The centerpiece activity also requires the removal of the M1 mirror and cell. The activities were therefore coordinated with the regular recoating of the M1 to minimize the number of nights out of operation. The upgrade required approximately 7.5 staff years of work and 36 missions from Europe to Chile, with around 60 people participating in at least one of the seven missions.
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