WEAVE project: manufacturing optical components for Europe’s second largest telescope

The KiwiStar Optics team successfully completed a technically challenging, multi-year project to manufacture, polish and test six lenses (up to 1.1m diameter) required for the William Herschel Telescope’s new prime focus corrector.


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The Project

WHT Enhanced Area Velocity Explorer (WEAVE) is a new multi-object facility for spectroscopy. WEAVE has been constructed to house the 4.2m William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos, on the island of La Palma, in Spain’s Canary Islands.

WEAVE will allow simultaneous observations of up to 1,000 targets, across a 2-degree field of view. The facility is designed to provide high-quality spectra over a wide wavelength range, to complement large-scale imaging surveys from the European Space Agency’s (ESA’s) Gaia satellite and the European Low Frequency Array telescope (LOFAR).

WEAVE will operate for a minimum of five years to conduct large-scale surveys of more than 10 million astronomical objects, including stars in the Milky Way, and extragalactic sources out to the extremes of the observable universe.

A crucial function of this facility is installing a new Prime Focus Corrector (PFC) for the WHT, made up of a set of six lenses. One of these lenses is 1.1m in diameter, making it one of the largest in astronomy. The PFC has also been combined with four wedged elements for atmospheric dispersion correction, and two aspheres. The light collected by the telescope’s primary mirror and transmitted through the PFC will be fed into approximately 950 optical fibres at the focal surface.

KiwiStar Optics’ team of scientists, engineers and master opticians led the multi-year project of manufacturing, polishing and testing the six lenses for the PFC.

The Challenges

Six glass blanks were delivered to KiwiStar Optics. With only one blank per lens there were no spares, therefore no room for error during the polishing process.

In addition to the challenge of no spare blanks, KiwiStar also tackled significant technical challenges. The large size of the lenses, specific characteristics and the high-quality performance required meant that the manufacturing, polishing and optical testing of the lenses had to be carefully managed.

The largest lens weighed approximately 230kg and the lenses also had steep curvature on the convex surfaces, adding to the challenges.

Due to the large scale of the lenses, KiwiStar had to build a bespoke polishing machine.

Gavin Dalton, WEAVE Principal Investigator and University of Oxford Professor of Astrophysics, says it is particularly impressive how the bespoke polishing machine handles the heavy-duty lenses.

“As well as taking a long time to polish the large-scale lenses it’s an incredibly difficult thing to handle a piece of high-precision glass that’s that heavy. You’ve got to look after it and you’ve got to have confidence in how you deal with it.” Due to the size of the lenses, KiwiStar had to extend the platform of the CMM to support them during the measurement process.

The CMM data was used to mill, or polish, the optical surfaces plane-parallel to each other and perpendicular-cylinder, to within the specified tolerance.

The blanks were mounted on the milling machine, and the concave surfaces were milled first. They were then lapped with different grit sizes, down to 5um. The wedge, central thickness, and radius were checked on the CMM after each step. For the two aspherical lenses, the concave surface was lapped to the best-fit sphere before being aspherised.

The lenses were then turned over and the convex surfaces milled and lapped. During this stage, the central thickness and wedge geometries were tightly controlled.

The concave surfaces were then polished with grit size between 1 and 3um, depending on how hard or scratch-resistant the glass was. The concave surfaces were then tested optically and protected with tape for the polishing and testing of the convex surfaces.

To work around the lenses’ large dimensions, KiwiStar developed a dedicated test tower and lens support system, to measure the optical surface, and to check for any irregularities on the concave surfaces, as well as the transmitted wavefront of each lens.

KiwiStar’s ‘can-do’ attitude

Professor Dalton says KiwiStar’s adaptability was a key reason they won the contract.

“When we were looking for someone to build the corrector, the response we got from KiwiStar was: ‘Well, we don’t currently have a machine big enough for this, but we could build one.’ That was a very encouraging attitude.”

Price is a really important factor when considering contract bids for a project like WEAVE. Other aspects such as ingenuity, expertise and effective communication, are equally important, he says.

“The thing that jumped out at us was the attention to detail that KiwiStar paid to the manufacturing and testing of the lenses,” Professor Dalton says.

“They were upfront that some of the testing was going to be very difficult, but they were also able to offer a very clever way around it.

“That was probably the match winner, and in the end that process worked extremely well.”

KiwiStar produced and tested the lenses within the allocated time and budget and met all our expectations, Professor Dalton says.

People and effective communication make the difference

“Initially, the thing we were most concerned about with this particular contract was how the communication would work. It’s one thing to have a vendor who’s in a different country, but it’s another thing altogether if they’re on the other side of the world,” Professor Dalton says.

“At the beginning of the project, I thought we would probably have to come to New Zealand regularly, to talk to KiwiStar about the project’s progress. But as it turned out, we came down for a kick-off, spent four days in Wellington and we were happy.”

“After that, we set up a fortnightly video conference. Because the communication system was so efficient and transparent, the conclusion was we didn’t have to travel back.”

“The fact that I didn’t have to get on a plane and travel down there [to Wellington] every two months, just to check up on people, was a real bonus for us.”

“We had a really good working relationship with KiwiStar. There are always issues to resolve when you are polishing these lenses but KiwiStar had a really positive attitude to any challenges. They always communicated if there were any issues and discussed what they could do about it and how they could solve the problem.”

Professor Dalton said this was a really positive experience, compared to some other companies he had worked with in the past.

“In other projects I have been involved with, companies have not wanted to talk about any issues that come up, so KiwiStar’s approach was a refreshing and positive change.”

“We’re very happy with what KiwiStar have delivered. It’s nice to have a supplier who you’re happy to recommend to others. A successful project is all about the people, and KiwiStar have great people.”