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The contract for the second ELT Prefocal Station is awarded to IDOM as the design for the first is close to approval.

As the Final Design Review of the first Prefocal Station (PFS-A) that will operate in the Extremely Large Telescope (ELT) is close to completion, ESO has awarded IDOM the supply of the second Prefocal Station (PFS-B) which will be integrated in the opposite Instruments Platform. This is a very important milestone in the development of this major system of the giant telescope.

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 systems before the focus and are located on each of the Nasmyth Platforms. 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 Prefocal Stations are crucial systems of the ELT. On the one hand they distribute the telescope beam among the different scientific instruments. On the other, they are in charge of collecting delicate optical information to correct the pointing of the telescope and to compensate the image degradation induced by atmospheric turbulence (adaptive optics). On top of that they are also used for verifying the relative positions of the M1 mirror segments. The Prefocal Stations are thus vital for proper focusing and image quality of the ELT Telescope.

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 an accuracy of tenths of microradians while maintaining a stability of tenths of microradians 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.

Additionally, a portion of the light collected by the telescope, corresponding to three bright stars close to the object of interest (Guide Stars), is picked up by three Sensor Arms, processed and used to correct the pointing of the telescope and to compensate the blurring effect induced by the atmospheric turbulence.

The Sensor Arms can be commanded to different radial and azimuthal positions with an accuracy below 300 microns in order to adapt to multiple possible star trajectories. By means of the Optic Boxes installed on all the Sensor Arms, they are continuously monitoring the quality of the telescope pointing such that the required corrections can be commanded on the telescope mirrors and on the telescope drives to ensure that it is properly focused and points to the target throughout the observations.

The same light collected by the Sensor Arms is also used to determine how the atmospheric turbulence is blurring the image (seeing) and to command the deformable mirror (M4) which uses front end adaptive optics technologies to compensate the blurring. The correct operation of the telescope adaptive optics is crucial to reach the resolving power corresponding to its large aperture.

Finally, the 798 hexagonal segments that compose the ELT primary mirror are also regularly aligned (phased) with the feedback provided by the Prefocal Stations to make sure that they behave as one single mirror, compensating the drift over time caused by changes in temperature and gravity loads.

Since the contract was signed in 2018, IDOM has been working closely with ESO in the detail definition of the ELT Prefocal Station. Once the design is completed, IDOM will start the production of the components and the assembly, integration and verification of the first unit included in the scope of the original contract (PFS-A) and the second unit which has been just awarded (PFS-B).

November 13, 2020

Science & Technology

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