A simulation model for a telescope earthquake analysis: the VST primary mirror safety system

Abstract

The VST (VLT Survey Telescope) is a wide field survey telescope going to be commissioned at Cerro Paranal (Chile). Due to the geological nature of the area, telescopes in Chile can be submitted to unpredictable and sometimes severe earthquake conditions. The commonly used response spectrum analysis has proven to be not suitable to evaluate the behavior of the VST telescope during an earthquake, due to the high number of devices with a nonlinear force-displacement characteristic in the primary mirror cell. Therefore, a nonlinear transient analysis based on a mixed Finite ElementSimulink approach has been carried out. A linear Finite Element (FE) model of the telescope has been developed and exported to Simulink, using a space state model representation. Then all nonlinear devices have been properly modeled and artificial accelerograms based on seismic history of the site have been applied. The paper focuses on the Simulink model, while giving an overview of the whole procedure.

Software and control system for the VLT Survey Telescope

“…The VST active optics software supports different possible operating modes. The common part is the actuation system, composed by a Stewart platform which moves the secondary mirror and four rings of force actuators which modify the shape of the primary, which is also protected by a safety system [26] from possible damages caused by earthquakes. The operating modes differs in the command part, which can be an online computation of corrections coming from one of the two wavefront sensors installed in the telescope and camera, or an interpolation of a look-up table containing calibrated values, with a dependence on the zenith angle and on the temperature.…”

Removing static aberrations from the active optics system of a wide-field telescope

“…The camera provides an alternative curvature wavefront sensor and a guiding system based on four additional CCDs at the outer edge of the mosaic. Further details on VST subsystems are discussed in [18][19][20][21][22][23][24][25][26].…”

The tracking control system of the VLT Survey Telescope

“…Further details on the telescope subsystems are discussed in Ref. 13 for the image quality error budget, [14][15][16][17][18][19] for the primary mirror support and safety system, [20][21][22][23] for the secondary mirror support system, 24,25 for the control simulations, [26][27][28] for the preliminary activities on axes control, 29,30 for the adapter-rotator and the atmospheric dispersion corrector, 31 and for the alignment.…”