Astronomical instrumentation

Abstract:

With 1300 movements, 160 sensors and a Ground Layer Adaptive Optics capability, MOSAIC, currently in the preliminary design phase, will be a large and complex instrument. At its very heart, the control system will monitor its health in real time and allow any instrumental reconfiguration for calibration, maintenance and science operations. MOSAIC instrument control is based on the ELT (Extremely Large Telescope) instrument Framework developed by ESO. The Assembly, Integration and Test (AIT) phase of the MOSAIC instrument will involve a large number of partners around Europe. The three main integration facilities will be Dwingeloo (Netherlands) for the visible spectrograph, Madrid (Spain), for the two near infrared spectrographs, and Marseille (France), for the front-end part of the instrument. The general instrument validation site in Europe is not identified yet and there is a risk that no suitable facility is found. To mitigate this risk, a preparation of a distributed AIT concept is carried on. It consists in testing the deployment of the main elements of MOSAIC control (Programmable Logic Controllers, Detector Workstations and Instrument Workstations) over the three main sites mentioned above, interconnected with each other via a secured Ethernet connection. In this paper, we present the benefits and risks of this novel AIT approach, the selection of a network remote access principle and preliminary results obtained in simulation and with hardware in the loop, between different testing sites in Europe.

Abstract:

MOSAIC is the multi-object spectrograph and IFU for the ESO ELT, currently in its design phase leading up to PDR. The GLAO subsystem is a key component to improve coupling of flux into the fibre apertures over the 7.8arcmin field of view. The apertures are positioned by 261 robot positioners which are attached to a > 1.5m Focal Plate that is mounted onto the Instrument Core Structure barrel which rotates for sky tracking. The four MOSAIC GLAO sensing units are also mounted on the barrel, and this entire assembly flexes under gravity and temperature changes. The ELT itself is expected to flex and its mirror positions will deviate from nominal locations leading to focal surface aberrations and distortions. Each GLAO unit then has three measurement goals: pupil plane wavefronts from LGS and NGS, focal surface distortions based on measurements from NGS, and Focal Plate distortions from artificial sources. All measurements pass to the AO Control Module which computes the fast control signal for the ELT M4 deformable mirror from LGS and NGS, while slower offsets allow the ELT to compensate focal surface distortions on-demand, and the instrument control updates knowledge of aperture positioning geometry from NGS and the artificial sources. The NGS WFS architecture combines high-speed tip/tilt (> 10Hz) from the ELT and low-order modes (< 10Hz) from NGS WFSs. The GLAO NGS WFSs enable the > 99% sky coverage of the instrument and have three measurement roles: telescope control modes, ELT focus surface distortions, and correcting LGS NCPA WFE due to their finite conjugate. The GLAO LGS WFSs use sky tracking LGS to measure the high-order WFE modes. The third GLAO modules are the metrology cameras which monitor the positions of the focal plate artificial sources, with one set per GLAO unit.