Optical corrector for WEAVE multi-object spectrograph

19th October 2020
Alex Lynn

The Instituto de Astrofísica de Canarias (IAC) and the Isaac Newton Group of Telescopes (ING) have accepted delivery from SENER Aeroespacial of the Prime Focus Corrector (PFC) optical corrector for the WEAVE Multi-object spectrograph of the William Herschel Telescope (WHT), located on the island of La Palma (Canary Islands, Spain). 

With WEAVE, the WHT is upgrading its technology with an instrument capable of observing 1,000 different celestial objects at once thanks to the 1,000 fibres positioned in its focal plane.

SENER Aeroespacial was responsible for the detailed design, procurement of components and mechanical integration and verification of the WEAVE optical corrector. This is a complex opto-mechanical system, weighing 1,700kg, which provides an optically corrected two degree diameter field of view (FOV) and is mounted in the prime focus area of the WHT.

The PFC structure consists of six lenses manufactured in New Zealand by KiwiStarOptics, including: two fixed lenses, one of which, measuring 1,100 mm diameter and weighing 230kg, is one of the largest lenses used to date in astronomy, these fixed lenses are placed inside the INVAR mounts with a flexible compensation ring; two doublets with two lenses each that move using a rotation system to compensate for atmospheric dispersion; an external steel housing with clean protection and an access point for maintenance purposes; and rotatory actuators and sensors for the rotating elements, with their control electronics.

The opto-mechanical integration and alignment of the PFC was carried out in a clean room for each lens-mount assembly and for the integrated system as a whole.

SENER Aeroespacial's Head of Science, Joan Manel Casalta, described this project as ‘a remarkable breakthrough for SENER Aeroespacial in the design, integration and verification of highly complex optomechanical systems, which puts us at the forefront of this technology and will allow us to undertake further opto-mechanical projects for large scientific infrastructures’.

In May, due to the COVID-19 pandemic, the team of SENER Aeroespacial had to conduct the tests of the PFC in a clean room at SENER Aeroespacial online with IAC and ING, which proved challenging, although the instrument was accepted remotely and then shipped to La Palma. In June, the device was received by ING/IAC, and it was integrated into the WEAVE instrument in the telescope by ING over the summer.

As reported by ING on its website, in September they installed a small acquisition camera and a ShackHartmann camera behind the PFC to assess the quality of the images it provided. And on September 11th, ING celebrated the first light of the WEAVE corrector after obtaining images of Vega with the acquisition camera.

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