QMT Features: September 2010
Micron key to infinity
The race is on for who will manufacturer of 1,000 mirrors for the European Extremely Large Telescope project. Extreme manufacturing and measurement accuracies are the key.  By Brendan Coyne

Cranfield University in the UK has begun work on producing seven of the mirror segments for 'the world's biggest eye on the sky' - the European Extremely Large Telescope (E-ELT). A ground-based telescope, E-ELT will be 42m in diameter and made up of 1,000 hexagonal segments, each 1.5m wide and just 5cm thick.

At almost half the length of a soccer pitch in diameter, 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. Cranfield University is the only organisation in the UK with the capability to undertake various stages of the machining the mirror segments to the accuracy required.  Key to this claim is Cranfield’s BoX (Big OptiX). Designed and developed at Cranfield specifically for realising these mirrors, the BoX machine has the ability to grind large optics in a matter of hours.  Located in Cranfield’s Loxham Precision laboratory, sponsored by Hexagon Metrology, the BoX is currently dedicated (for 70% of its time) to grinding the first set of segments for the E-ELT.  Each of the 1.5m mirror segments has a complex shape off-axis ellipsoid. 

A super precise Leitz PMM-F CMM by Hexagon Metrology is used to verify the performance of the Cranfield BoX grinding machine and measures the mirror segments.  Professor Paul Shore, Head of the Cranfield University Precision Engineering, says, “What we’ve got in the Laboratory is an ultra precision grinding and metrology system made at Cranfield that grinds the part and performs some in-process measurement. The Leitz CMM is then used to validate the grinding and measurements done by the Cranfield machine.“

The first mirror segments Cranfield are working on are made of Zerodur, a ceramic glass with a very low thermal co-efficient of expansion. Other ultra low expansion materials may be used in the future.

The received blanks require approximately 1mm to be removed and a form accuracy  of 1 micron, peak to valley or, in RMS values, about 200 nanometres is demanded.  To maintain these accuracies, the Leitz CMM is a key tool, as Paul Shore explains, “We calibrate the CMM against a silicon carbide straight edge. This, in turn, is calibrated against an optical straight-edge located in the top of the Cranfield BoX grinding machine. A cross check correlation of the straight edges allows us to reduce the measurement uncertainty of the Leitz CMM. 

Cranfield written software takes data from the CMM to generate what we call a “synthetic interferogram”. We use conventional Leitz probes to acquire surface data in the form of points and high resolution scans. Measurements are generally taken unmanned overnight since the measurements take some time to complete. Measurement strategies must account for possible temperature effects. However the Loxham Lab has a stable environment with temperature controlled to better than +/- 0.5C and very good humidity control.“

Super accurate
After processing at Cranfield, the mirrors are sent to Technium OpTIC (Opto-electronics Technology and Incubation Centre) in North Wales, where they will be polished and measured by OpTIC based researchers from three UK universities, Cranfield, UCL and Glyndwr Universities. The segments are polished on Zeeko machines with a capacity of 1.6 metres.  Error surface maps generated from the Leitz CMM are sent with the mirrors. These identify high and low points for initial corrective polishing. The polished quality demand is to achieve a surface roughness of 1-2 nanometres RMS and form accuracy of 10 nanometres RMS.

To verify these extreme surface accuracies, an 8 metre optical test tower has been designed and built at OpTIC. The test tower is sited over the top of the Zeeko polishing machine, it employs laser interferometry techniques to generates an optical wave front that conforms to the demanded mirror segment surface. The measured difference between the actual mirror surface and the demanded optical surface shape of the spherical wave front provides a surface error map. This error map is used to generate a time-dwell derived tool path CNC program error using proprietary Zeeko software. The Zeeko machine then adaptively polishes the mirror surface to achieve the desired 10 nanometre RMS form accuracy.

To ensure correct alignment of the optical tower measurement system, a Leica Absolute Tracker AT-901 is integrated into the tower structure.  The Leica 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. 

Future development
To speed up the process even more and to add value to Cranfield’s activities, Paul Shore’s team are investigating a robotic lapping systems. ”The simple robot based process gives us a possible intermediate process between grinding and polishing. It could alleviate any bottleneck operations in the latter stage of mirror production,” explains Paul Shore.

The Cranfield team, together with Hexagon Metrology personnel are looking at a configuration using a standard Fanuc 5-axis robot fitted with a smoothing spindle. The air bearing spindle carries a small abrasive ring which moves across a previously ground mirror surface. A portable Leica Absolute Tracker AT901-Long Range has been used to check the robot system’s motional accuracy capability as it moves the spindle across the mirror surface.

One area of research is thermal stability and dimensional movement of robots over long periods of time. Cranfield researcher Andy Eve explains the process: “The Leica Absolute Tracker AT901 can be used to measure in 3 degrees of movement using a reflector technology or in 6 degrees of freedom using a robot specific device, the Leica T-Mac. The investigation is also to discover how much temperature shift is occurring on the robot during the lapping cycles. We are looking for repeatability of results to consider error compensation practices. It is an important investigation because it potentially adds a lot of value to our E-ELT mirror program.““

Another development at Cranfield is how to complete the process with a final machining phase using a Reactive Atom Plasma Technology (RAPT) machine. This process will allow final figuring of the mirror surfaces using a non contact plasma etch method. This RAP process is interesting says Shore as “it has a very high material removal rate and with nanometre levels of material removal dexterity used in combination with the abrasive process it offers even high production rates for the future”.

Competitive edge
The European Southern Observatory, ESO, have placed two contracts for 7-off prototype mirror segments - one with SAGEM, a large French organisation and the other with a UK consortium, headed up by OpTIC Glyndwr in North Wales.  When the prototypes are signed off, both SAGEM and the as yet un-named UK production company, potentially others, will bid for manufacturing quantities - around 1,000 mirror segments in total.

What are the chances of the order (or part of the order) going to the UK consortium?   A team from ESO arrived at Cranfield in July 2010 to look at the BoX and Leitz machine. “In particular, they were interested in how we were using the CMM and how we dealt with the measurement data as most CMMs don’t measure optical surfaces. The Cranfield code allows us to generate the synthetic interferograms using the Leitz CMM. We think ESO were happy with our technical process and the data demonstrated to them.” says Paul Shore

“The issue here is now a production engineering one.  To give you an idea - we started about grinding this very first 1.46m mirror segment five days ago and it will be finished in four more days. During this time we have validated our measurement approach. We know that with some attention production engineering issues, the Cranfield BoX grinding machine will grind each mirror segment within 20 hours. To our knowledge that is ten times faster than our competitors. I am very confident that the BoX grinding technique is sorted. We expect to take a number of orders in the near future for making more segments.”l

email: paul.shore@cranfield.ac.uk www.cranfield.ac.uk
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