Telescopes to hunt for origin of gravitational waves assembled in Groningen

At the SRON Groningen workshop, scientists and technicians have assembled three survey telescopes to look for the optical source of gravitational waves. The telescopes will watch the sky above Chile to catch any brief flash caused by the fusing of black holes or neutron stars.

In a sense, it is like putting together a highly sophisticated IKEA product. The parts for three 60-centimetre telescopes were delivered to the lab in Groningen in three large crates last autumn. ‘They are designed by the Radboud University Nijmegen and Astron’, explains University of Groningen Project Manager Joost Adema. The telescopes were supposed to be assembled at the Astron workshop in Dwingeloo, but space and manpower was lacking. ‘So, we stepped in’, explains Adema.

After assembly, the so-called BlackGEM telescopes will be shipped to La Paranal in Chile, a site run by the European Southern Observatory. There, they will survey millions of stars and wait for a new light dot to appear. The aim is to catch the afterglow of events that generate gravitational waves. The waves are ripples in space, caused by huge gravitational events like the merging of two black holes or neutron stars. These waves are monitored by the American LIGO and European VIRGO detectors, which have so far seen seven gravitational wave events.

Good fortune

‘By combining both detectors, it is possible to get a rough estimate of where a gravitational wave originated’, Adema explains. The three telescopes should be able to catch some of the light emitted by a merger and thus pinpoint the origin, enabling larger telescopes to perform follow-up measurements across the electromagnetic spectrum. ‘By some good fortune, astronomers did find the optical counterpart of the third gravitational wave ever detected on 17 August 2017’, says Adema.

The merger was that of two neutron stars, which resulted in a phenomenon called a ‘kilo nova’. This was believed to produce all sorts of heavy elements (elements with an atomic number greater than 92). And indeed, a study of the afterglow of the kilo nova revealed the presence of gold and platinum, confirming a long-held hypothesis on the origin of these heavy elements.

BlackGEM should increase the chances of catching more of these events. It will scan the heavens regularly, looking out for any changes. This is done using fairly simple 60-centimetre mirror telescopes, with some intricate optics to produce sharp images. The CCD that catches the light is pretty large, measuring 10,500 by 10,500 pixels. ‘Both ESA and NASA have shown interest in these telescopes, as they want to map all the space debris circling around the Earth’, says Adema. There are lots of bits and pieces from rockets and satellites in orbit, which could be dangerous for space missions.

Hiccups

The telescopes will sit on seven metre-high poles and operate fully automatically. A special, resilient coating, developed by the Dutch technology innovation organization TNO, has been applied to the mirrors to keep them stable and clean. ‘Otherwise, operators would have to clean the dust off the mirrors at regular intervals.’ Three telescopes are not very many, given that the entire sky must be monitored. ‘The original plan was to install 15 of them, but the funding wasn’t there.’ Each telescope costs around € 750,000.

Over the past few months, SRON technician Mariëlle Bekema carried out most of the assembly and recorded the assembly procedures. And, just like assembling an IKEA closet, she encountered a few hiccups. For example, all of the holes had already been drilled – but sometimes not exactly right. ‘We also had to assemble the electronics’, says Bekema, ‘However, one actuator was missing, so we had to wait for that.’

A crucial point was the optical test, to see if the optical system was properly aligned. This was done by hauling an assembled telescope through large doors to a narrow strip between the lab building and a fairly busy street, at the corner of the Zernikelaan and Zernikeplein. Busses appear to drive straight at you, before taking a left turn. On that spot, the Pole star can be seen and can be used to test the optical system.

The first two tests went flawlessly. However, the third and final telescope failed: something was wrong. With the help of Rik ter Horst, who works in the Optical and Infrared group of the Netherlands Research School for Astronomy (NOVA) in Dwingeloo, it was found that the mirror had a slight fault: the shape of one section was off by a few hundred nanometres. Instead of sending the mirror back to its producer, Ter Horst – who is an extremely accomplished grinder of optical instruments – decided to slightly deform the mirror and thus correct the image. This saved weeks, if not months, of delay.

Gold

The combined skills and knowledge in both engineering and optics of SRON Space Research, the University of Groningen Kapteyn Institute and NOVA thus meant that the three telescopes were ready to be shipped to Chile at the end of February. It will take some time before they are fully operational. ‘Ideally, we would have liked them to be up and running at the start of the third operational run of LIGO, which is in a few weeks’, says Adema. The BlackGEM telescopes will probably be a bit late, but after installation in April, they will be on the lookout for newly-formed gold.