Measuring 1,000 Mirrors for the European Extremely Large Telescope

High-precision inspection challenges for the world’s biggest ELT

Published: Thursday, January 13, 2011 - 05:00

The race is on for who will manufacture 1,000 mirrors for the European Extremely Large Telescope (E-ELT). Cranfield University in the United Kingdom 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-unnamed 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 42 m in diameter and made up of 1,000 hexagonal mirror segments, each 1.5 m (4.9 ft) wide and just 5 cm (1.9 in.) 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 for both Cranfield University and Hexagon Metrology. “Cranfield University is uniquely placed in the United Kingdom with the capability to undertake various stages of machining the mirror segments to the accuracy required,” says Paul Shore, professor and head of Cranfield University Precision Engineering. “We have developed Cranfield’s Big OptiX (BoX), a specialized grinding and measurement system, at Cranfield for realizing these mirrors.”

Verifying grinding and size

Located in Cranfield’s Loxham Precision Laboratory is a Leitz PMM-F 30.20.10 coordinate measuring machine (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.

Polishing

After processing at Cranfield, the mirrors are sent for polishing. The segments are polished using 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 nanometers root mean square (RMS) and a form accuracy of 10 nanometers RMS.

To verify these extreme surface accuracies, an 8 m (26.2 ft) optical test tower is used. To ensure accurate alignment of the tower, a Leica Absolute Tracker AT901 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,” says 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.”

 

About The Author

Hexagon Manufacturing Intelligence

Hexagon Manufacturing Intelligence helps industrial manufacturers develop the disruptive technologies of today and the life-changing products of tomorrow. As a leading metrology and manufacturing solution specialist, our expertise in sensing, thinking and acting – the collection, analysis and active use of measurement data – gives our customers the confidence to increase production speed and accelerate productivity while enhancing product quality.

Hexagon Manufacturing Intelligence is part of Hexagon, a leading global provider of information technologies that drive quality and productivity across geospatial and industrial enterprise applications.