Building the impossible

Willem Jellema graduated with honours in Applied Physics from the University of Groningen in 1998. His next step constituted a major contribution to the design and construction of HIFI, a vital instrument for the Herschel far-infrared space telescope. He will now defend his PhD thesis on the optical design and calibration of HIFI. His work has also inspired a new startup company.

As is often the case in astronomy, HIFI required capabilities far beyond anything in existence. It needed to analyse light in the Terahertz range, which was uncharted territory. That meant that there were no design tools, nor testing equipment.

SRON Netherlands Institute for Space Research took up the challenge and led an international consortium of 25 institutes in 12 countries in the project to design and build HIFI. The Groningen branch of SRON, which is closely associated with the University of Groningen Kapteyn Astronomical Institute, took the lead in the development, assembly and verification testing of the instrument. This is where Jellema comes in.

‘But when the wavelength is in the same range as the components in your optical system, you need to treat the light as a wave.’ This is called quasi-optics, a regime that lies between conventional optics and radio techniques.

Jellema compares the light to the ripples in a pond when you throw in a stone. ‘The wavefront travels through the pond, and is affected by obstacles or reflections.’ His task was to design HIFI so that the waves would pass from the 3.5-metre mirror to the detectors which were about 1 μm² (a hundredth of the thickness of a human hair).

The first step was to find suitable software to simulate these quasi-optics. At the time this was not readily available and validated. ‘This took us around two years. But we weren’t satisfied with the simulation alone: we wanted an experimental method to verify the system.’ The problem was that terahertz ‘light’ is not visible.

So a probe was designed which sent a well-defined wave through the system towards the detector. ‘We could effectively measure the time it took to pass through the optics by taking direct measurements of the phase of the wavefront. Then we varied the location of the source just a bit and measured the effect on the phase.’ This allowed Jellema and his colleagues to measure how the optical path affected the passage of the wave, and detect any distortions.

Combining the software simulations with the measurements of the probe allowed them to design and verify the optical system for HIFI. ‘That is how we could check all the components before they were integrated into the HIFI system’, says Jellema. ‘For example, we discovered a flaw in the lenses made for HIFI, which was due to a fault in the production process. The manufacturer was able to correct this before we put them in.’

They also gained such a thorough understanding of the system that it was possible to predict with great precision the optical performance of the entire system of the Herschel telescope and the HIFI detectors. This was very important, as the full system could not be tested before the launch. ‘And once it was launched, there was no way to repair it, as Herschel would be 1.5 million kilometres from Earth.’ In the end, everything worked perfectly. Herschel was launched in 2009 and collected data until the cooling system was exhausted in 2013.

The research needed to build and validate HIFI became a PhD project, back in 2004. Jellema will now defend his thesis on 1 May. It took a while to complete: ‘Building HIFI always took priority, and after the launch, the astronomical community needed a detailed description of the optical specifications of the instrument, so we worked on making that available.’

Jellema’s work allows astronomers to calibrate the astronomical data and extrapolate back from the detector readouts to the flux and temperature scale of the original source. ‘That is a great help in the interpretation of the data.’ Furthermore, the knowledge garnered from the HIFI project can be used in other terahertz systems.

Terahertz radiation can penetrate objects that are opaque to visible light. Jellema shows a picture of a block of white plastic. A terahertz scan reveals it contains several embedded objects. ‘It enables you to look through plastic, and also through envelopes or clothing.’ Indeed, terahertz scanners are already used at major airports to check for hidden objects under the passengers’ clothes. ‘But our technology provides a much better resolution than those scanners and can it provide three-dimensional imaging capability.’

The potential uses of terahertz technology has inspired a startup company, led by SRON scientist Professor Andrey Baryshev and SRON technology transfer officer Alena Belitskaya. There is a wide range of potential applications of the terahertz scanners, from airport security to checking the integrity of composite materials. Jellema: ‘I’m the co-inventor on a number of patent applications, but I enjoy being an instrument scientist, so I’m staying at SRON.’ There are plenty of other impossible instruments to build for astrophysics, earth atmosphere and exo-planetary research.