The Centre for Advanced Instrumentation (CfAI) develops state-of-the-art instruments that are used in fields as diverse as biophotonics and astronomical instrumentation. It is one of the major research groups in Durham University’s Physics Department, and is located at the NETPark Research Institute and at the Mountjoy Site in Durham city.
As a recent example of their work the CfAI led the development of the K-band Multi-Object Spectrograph (KMOS) recently delivered to the ESO Very Large Telescope in Chile. KMOS will provide researchers with a clearer picture of how galaxies evolved in the earliest stages of the universe. The project involved a complex optical system which required the manufacture of many, highly precise Aluminium mirror surfaces that were integrated into an array of robot arms that could be remotely positioned to capture light from these distant galaxies.
Many of the targets for KMOS are at infrared wavelengths, since light moves to longer wavelengths as the universe expands. As a result, the system needed to work at temperatures as low as -200C, to reduce the heat from the system that would interfere with the detection of the target galaxies. The mirror surfaces capturing the light were also required to be highly accurate to ensure the light was directed into the instrument with very high precision. Mirrors are typically made from glass materials but due to the challenges of making mirrors that would operate and maintain alignment at low temperatures CfAI decided on using Aluminium, a readily-available material which is fairly easy to machine using a process that involves diamond tools (diamond machining).
Paul Clark, Head of Engineering, said: ‘we don’t start with recycled pop cans. We use Aluminium manufactured by a special process that involves it being rapidly cooled. Aluminium contains a certain amount of impurities (e.g. Silicon and Iron) that provides its inherent strength without which it would be too mechanically soft for practical use, however the special manufacturing process ensures the “hard” impurities are small and very well distributed throughout the metal so as to avoid large, clumpy impurity sites. Having the impurities small and well distributed assists the diamond machining process enabling very smooth and accurate surfaces to be produced.’ In the last part of the process before being installed in the instrument the mirrors are gold-coated to provide extremely high reflectivity in the infrared.
KMOS will help scientists to discover more about the early years of galaxies, and is the result of several years of work by experts in the United Kingdom and Germany. By analysing the spectrum of each galaxy, researchers can glean important insights into the size and age of the universe, and how each individual galaxy is evolving.
Paul Clark said: ‘If you’ve got a galaxy that is rotating, the bit that’s coming towards you looks slightly bluer than the bit that’s turning away from you, so it’ll give you a measure of the speed of rotation of the galaxy. The spectrum will also give you lots of information about the types of chemical elements in there.’
KMOS is able to observe many different galaxies at the same time. It does this with the help of 24 robot arms, which can be maneuvered into different positions to capture individual points of light from one or more distant galaxies. It then breaks this light down using ‘image slicers’, which create a two-dimensional picture and enables researchers to conduct detailed spectral analysis of each individual system. This drastically reduces the time it takes to survey a large number of objects, enabling a study to be completed in months rather than years. The CfAI was responsible for the design and manufacture of the image slicers that were then integrated with the robot arms.
The CfAI has established facilities at NETPark where the ultra-high precision diamond machining and instrument development takes place. NETPark has become an important base for this specialised equipment and the expertise that enables the CfAI to support their work on these demanding and complex programmes.
Paul Clark explained, ‘One must have the equipment, and the legacy of the techniques and the know-how. You must also have experienced staff, who are able to get the best out of these machines. And that’s what we’ve been building up at NETPark for the last ten years.’
An average instrument project at the CfAI takes around five years, two to three of which are devoted to the design and review phase. The earliest step is to draw up the science requirements. A conceptual design is created on paper and CAD, and then reviewed.
Part of the brief for KMOS required the CfAI to create 1,200 diamond-machined mirrors. As this was new engineering territory, the CfAI was asked to create an engineering model with detailed designs. There is then a final review stage, in which the designs and plans are scrutinised before being cleared to go.
After the tricky process of producing the Aluminium mirrors to the correct shape, it has to be coated with gold, which is excellent at reflecting infrared light.
‘Getting the gold to stick to Aluminium is challenging’, said Paul Clark. ‘You put it into a vacuum deposition chamber, and you clean the surface with a plasma. You then insert an ingot of gold, and put a huge current through it so it all vaporises instantly and throws gold out everywhere. Everything in the chamber, including your mirror, gets coated with an extremely thin layer gold.’
‘Then that mirror has to keep its shape over 10 years of life, operating at very low temperatures. We immerse it in liquid nitrogen and bake it in an oven repeatedly to artificially age the Aluminium before diamond machining, otherwise the integrity of the component will likely degrade over time. When you’re diamond machining a mirror care has to be taken when mounting it on the machine so as not to induce distortions when it is bolted down. You might cut your surface perfectly, but when you take it off the machine it can have a different shape due to the mounting arrangement. It is very much preferred to have the component mounted on the machine in the same way as it will be mounted in the instrument, to reduce this possibility ’
Mirror surfaces were machined into the Aluminium at the CfAI’s precision optics facility at NETPark, using a diamond tool made out of industrial quality diamonds. These surfaces were tested and analysed to make sure they were the right size and shape. A Phase-Shifting Interferometer was used to evaluate the optical surface quality, while a White Light Interferometer was used to test roughness.
The surfaces were mounted in a housing and aligned with a filter wheel that allows users to select what wavelength of light reaches the mirror system. These were doweled into place once they were perfectly aligned. The housing is then aligned with “slit mirrors” which enable the system to pinpoint the exact area it wants to investigate and feed a specific beam of light through to the mirror surface. Alignment was carried out using specially-machined alignment cylinders, and each unit created was tested individually for characteristics such as image alignment and image quality.
Each of the 24 completed units, known as Integral Field Units, was attached to robotic arms created by the UK Astronomy Technology Centre in Edinburgh. The University of Oxford built the spectrometers that collect and interpret the light reflected by the mirrors into a signal that can be analysed by researchers.
The project’s German partners, University of Munich and the Max-Planck-Institute for Extraterrestrial Physics (MPIA), provided the detectors which record the light from the sources, and the electronics and software used to operate KMOS.
Late in 2012, KMOS was attached to the Very Large Telescope at the European Southern Observatory’s Paranal Observatory in Chile, and successfully tested.
So how can the techniques pioneered by the CfAI be applied elsewhere?
By adapting its Aluminium-machining techniques to Steel using an additive technology, Ultrasonic Assisted Machining, the Centre was able to manufacture precision steel moulds for use in injection-moulded plastic projects. This technology has been used in application areas such as improved ophthalmic lenses, a new generation of optical moulded components for use in automotive and indoor/outdoor LED lighting.
The Centre’s experience with the 1,200 mirror surfaces for KMOS also enabled it to create a smaller number of mirrors for the Near Infra Red Spectrograph, which will fly on the James Webb Space Telescope. This telescope will investigate the formation of the first galaxies from an orbit approximately 1.5 million kilometres from Earth.
Paul Clark commented, ‘Aluminium we have carefully machined at NETPark will end up in space – out beyond the orbit of the moon – at some point in the not-too-distant future. Engineering for space is about having the know-how and the expertise and the confidence that you won’t have to do as much prototyping to get it right. The requirements are different, but fundamentally the machining, metrology and coating processes are the same. You’re taking the lessons learned from past projects and it benefits future projects.’