Industry pushes science

The Antarctic Plateau is considered the most inhospitable inhabited place on Earth. At Dome C, within the Australian Antarctic Territory, mid-winter minimum temperatures drop to -81°C and access to and from the outside world becomes impossible outside the very small window of 12 weeks, during which time the sun rises above the horizon.

It is sobering to consider that CO² freezes before this temperature is reached and mild steel undergoes changes which effectively swap stiffness with ductility, enabling a builder’s hammer to penetrate a 200 L drum without a great deal of effort. Even mid-summer temperatures don’t rise above -25°.

For an astronomical telescope, the intense cold, high altitude (3250 m above sea level) and stable atmospheric conditions at Dome C combine to promise sensitivities and image quality that surpass those found anywhere else on the planet and perhaps even matching the powerful Hubble Space Telescope. Most importantly, an Antarctic telescope can out-perform a similar telescope at a temperate location that is several times the diameter and therefore much more expensive.

Ironically, the very conditions that make the site ideal for astronomy also make it one of the most difficult to execute. Besides the effect of prevailing temperature on common materials, precluding the utilisation of temperate engineering solutions to astronomical requirements, the site offers many engineering challenges as a result of the surprising rate of temperature change with height (1°C/m); the regular presence of ‘diamond dust’ (very small ice particles) that permeates any small aperture and seals and settles on all surfaces; the remoteness and small window of access; altitude; the lack of solid foundation (the Antarctic Plateau is essentially a great dome of moving ice — (3000 m deep at Dome C); and the constant icing of all surfaces as they lose heat to the night sky and encourage ice formation.

Sydney-based automation consultancy Fast Automation was seconded to the project to advise on a controls system, a remote interface in Australia and the most appropriate and effective data transfer system for a remote control 2 m class telescope to be deployed to Dome C.

David Askew, director of Fast Automation, was the lead controls consultant and considered the project amongst the most unusual his company has encountered. “We have been involved in a number of critical systems but none with as many constraints as this. The system has to physically get there and then survive the most extreme environmental conditions imaginable. It must also be self-powered, operate remotely, tolerate compounded failure scenarios and then deal with large volumes of data in a location that has limited communications at best. We quickly realised that the technical issues were going to require many unique and innovative solutions. To compound the difficulties, as mechanical engineering peeled back each issue, another group of issues emerged, each highlighting more constraints. The volume of control ultimately required was exceptional.

“Primarily, science groups have very difficult applications and typically construct their own low-level controllers, but it was very clear that the use of industrial controls would provide a far superior solution in terms of reliability, redundancy, cost, the volume of control and the monitoring requirements.

“The design lent itself very nicely to the use of PLCs and decentralised control solutions due to the large volume of I/O and drive handling. The primary control involves a lot of interlocks and sequencing to bring components to temperature. Obviously, the system generally has a lot more control on it for monitoring and protection purposes, given its inaccessibility. As the preferred solution to each issue was identified, the control requirement grew to enable the solution. The housing of components, for example, requires environmental control, which in turn requires more control!”

A difficult constraint was the need to maintain the primary mirror temperature constant (despite the fact that the surrounding air may vary by 2°C across its diameter when at maximum angle) and to have less than 0.5° variation between the mirror surface and the local ambient temperature to maintain optical performance to requirements (the refractive index of air varies with temperature). This represented a major challenge to the team and resulted in the exploration of innovative applications of the latest materials and manufacturing techniques.

The impact of wind — even if very low speed — added to the temperature element, as wind turbulence could introduce colder air from below the telescope, rapidly changing its optical capability. A closed loop-controlled wind deflection system to maintain an optimised fluid flow was suggested.

Another general aspect of telescope control is tracking and guiding it with accuracy in the order of nanometres. This is achieved by using drive control and image sensing, then using ‘guide stars’ (known stars from patterns in the sky) and locking onto them. As the world rotates, maintaining a still image becomes a very high-resolution motion control application. The design complexity arises from the mechanical restraints. For example, at certain angles the truss may exhibit unusual twisting and bending which would cause large distortions in the image.

Fast Automation Pty Ltd
www.fast-automation.com