The race is on for who will manufacture 1,000 mirrors for the European Extremely Large Telescope (E-ELT). Cranfield University in the UK has begun work on producing seven of the mirror segments for ‘the world's biggest eye on the sky' with the aid of high accuracy measurement systems from Hexagon Metrology. The current production is for prototype mirror segments. Once these are signed off Cranfield University, as part of an as yet un-named UK production company, will be able to bid for the manufacturing of more segments.

Built by the European Southern Observatory (ESO) the E-ELT, a ground-based telescope, will be 42m in diameter and made up of 1,000 hexagonal mirror segments, each 1.5m wide and just 5cm thick. The E-ELT is four to five times larger and will gather 15 times more light than the largest optical telescopes operating today.

The production and measurement challenges for this project are significant challenges for both Cranfield University and Hexagon Metrology. "Cranfield University is uniquely placed in the UK with the capability to undertake various stages of machining the mirror segments to the accuracy required", explains Professor Paul Shore, Head of the Cranfield University Precision Engineering. "We have developed Cranfield's BoX (Big OptiX), a specialised grinding and measurement system, at Cranfield for realising these mirrors."

Leitz PMM-F - For Optimum Measurement of the Mirrors

Located in Cranfield's Loxham Precision laboratory, which is sponsored by Hexagon Metrology, is a Leitz PMM-F 30.20.10 CMM. This Ultra High Accuracy (UHA) measuring system from Hexagon Metrology is used to verify the performance of the Cranfield BoX grinding machine and measures the mirror segments. With the Cranfield BoX and the Leitz PMM-F it is expected that each mirror segment can be ground within 20 hours.

After processing at Cranfield, the mirrors are sent for polishing. The segments are polished utilising error surface maps generated from the Leitz PMM-F. These identify high and low points for initial corrective polishing. The polished quality requirement is a surface roughness of 1-2 nanometres RMS (Root Mean Square) and form accuracy of 10 nanometres RMS.

Surface polishing - Closely watched by Leica Laser Trackers
To verify these extreme surface accuracies, an 8 metre optical test tower is used. To ensure accurate alignment of the tower a Leica Absolute Tracker AT901 from Hexagon Metrology is integrated into the tower structure. The laser tracker system measures the positioning of the tower's main optics during measurements, tracking any movement due to thermal effects which can be some microns over such a large structure.

"This is an exciting and highly technical process to be a part of", explains David Brown, General Manager of Hexagon Metrology Ltd, UK. "Astronomers tackle key questions that challenge our minds and our imagination. How did the planets form? Is life ubiquitous in the Universe? How did galaxies form? What are dark matter and dark energy? Hexagon Metrology is proud to be able to assist in answering some of these questions."