The two ELT prefocal stations, which are designed and manufactured by IDOM, are large structures measuring over 9 metres tall and each standing on supporting platforms, located on opposite sides of the telescope main structure. Working at the interface between the telescope’s five-mirror system and its instruments, they have a triple function: control the telescope mirror alignment (including the precise pointing on-sky of the telescope structure), ensure the segmented main mirror continues to act as one, and distribute light to the telescope instruments.
Sensing starlight to check alignment
The ELT is extremely sensitive to very small changes in environmental conditions, which will shift the mirrors out of alignment, degrading observations. Gravity can pull the mirrors out of shape as the telescope moves. In addition, small changes in temperature cause the telescope’s metal structure to expand slightly by hundreds of micrometres, again shifting the positions of the various ELT mirrors.
The PFS-A and PFS-B analyse the light from ‘guide stars’ — natural, bright stars close to the object of study in the sky. The stations, which do not work simultaneously, will monitor up to three guide stars during observations, assisted by its three sensor arms positioned with accuracies of a few hundred micrometres. Information from the guide stars is used to actively position the mirrors and telescope main structure to control the telescope’s alignment during observations — a process known as active optics. This ensures that light remains properly focused and that the telescope remains pointed accurately at the target throughout observations.
798 segments acting as one mirror
An additional function of the PFS is to ensure that the hundreds of segments of the main mirror (M1) continue to act as one giant mirror. To achieve this, the relative positions of the 798 mirror segments must be accurate to tens of nanometres. However, the positions of the segments drift over time due to changes in temperature and gravity loads so they need to be corrected periodically. The PFS will regularly control the shape of M1 using the guide star measurements to maintain its optimal shape.
Distributing light
Finally, the PFS is also used to distribute the light collected by the telescope to the various scientific instruments and auxiliary equipment, depending on which system is being used at any given moment. The platforms where the PFS A and B stand will host multiple scientific instruments, specialising in analysing the light collected by the telescope to answer our questions about the Universe. The PFS will direct the light to the select scientific instrument using a large flat mirror. It is also the last component before the light arrives at the telescope focus, giving the station its name.
A successful collaboration
Following two years of work by engineers at IDOM and at ESO on the complex PFS-A, the teams are now very close to successfully completing the design activities and will soon begin manufacturing this crucial telescope component. In addition, ESO, through its Director General Xavier Barcons, and IDOM, through their president Luis Rodríguez, have now signed a contract for the Spanish company to develop and supply the second prefocal station for the ELT.
ESO’s ELT, made possible by high-tech engineering solutions and many talented people, will answer the biggest astronomical questions of our time, from investigating the Universe’s history to finding and studying Earth-like planets outside our Solar System. The telescope will sit atop Cerro Armazones in the Chilean Atacama Desert and will start operations later this decade.
