ELT Prefocal Station

Chile

Located on each of the Nasmyth platforms on both sides of ELT’s tube structure, the two Prefocal Stations are identical systems with a footprint of 4.5m x 5 m and almost 9 m height. They are the last opto-mechanical component before the light from the giant telescope comes to a focus. The Prefocal stations define the optical axis for all the subsequent instruments and contain themselves critical systems for the correct function of the ELT.

The mission of the Prefocal Station is twofold. On the one hand, this high precision optical/mechanical device propagates the light collected on the telescope into science instruments and other test equipment. On the other hand, by means of three sensor arms, it picks and adapts the light from up to three the guide stars for its use in the Acquisition, Guiding & Wavefront Sensing.

The M6C and M6N mirrors, polished to an accuracy of some 30nm, are used to deliver the light collected by the telescope to the different instruments. Each of the mirrors is installed on high precision mechanisms allowing to position them with high accuracy while maintaining a stringent stability over one hour despite of temperature changes, telescope induced movements and other effects. The mirrors are moved independently with translational and rotational movements in order to redirect the light to the different telescope foci (A1, A2 and Coudé) or to simply remove the mirrors from the field of view.

Each of the Sensor Arms is equipped with three Optics Boxes where the focal imaging and wavefront sensing cameras are installed. Since the information from the focal imaging detectors is used to control the telescope to render a steady image and the information from the wavefront sensing detectors is key to apply the adaptive optics technologies to obtain a sharp image, the Prefocal station is a critical ELT subsystem. The information gathered by the sensor arms is also used to control the active optics system that keeps the telescope’s optics aligned and produces optimum image quality despite the constantly-changing effects of wind and other disturbances on the telescope.

The contract covers the design, fabrication, assembly, integration and verification of these two components for the ELT.

The Extremely Large Telescope is an astronomical observatory and the world’s largest optical/near-infrared extremely large telescope now under construction. Part of the European Southern Observatory (ESO) agency, it is located on top of Cerro Armazones in the Atacama Desert of northern Chile.

Artistic render of the ELT Prefocal Station A. /  Images courtesy of ESO/L. Calçada, IDOM 

Astronomy & Space : Precision Instruments