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Boldly going where no refurb has gone before

Cover story: Jodrell Bank

Severe corrosion has led to a worldfamous scientific landmark having to undergo a life-saving refurbishment.

Diarmaid Fleming reports, pictures by David Jones.

Deep in the Cheshire countryside lies a jewel of British scientific achievement, the Lovell Telescope at Jodrell Bank Observatory. The centre, now run by the University of Manchester, was established in 1946 by radar pioneer Sir Bernard Lovell, whose discoveries played a crucial role in helping the Allies win World War II. At its centre, the giant 76m diameter radio telescope is, however, far from a relic of a bygone age: a refurbishment project will soon provide the telescope with new power, providing a window to the farthest reaches of outer space.

While some of the brightest brains in British science occupy themselves with mathematical interpretations of the messages coming via radio waves from space to the observatory, an engineering task no less complex is occupying the minds of engineers from AEA Technology and Shal Engineers working on the refurbishment. Were it not for their ingenuity, the telescope could have been lost for ever due to severe structural corrosion.

The original telescope, shaped like a giant satellite dish, was built between 1952 and 1956. A new reflector surface was installed in 1968 directly above the existing one, mounted on an array of backing angles.

The additional 960t loading provided by the 336 new reflector panels required a new load carrying wheel girder, supported on four upthrust units on wheel bogies which run on a circular rail track to allow for horizontal movement, while the dish can also rotate on a horizontal axis.

But the addition of the new surface was to cause problems later. 'In the early 1980s, staff at the observatory noticed that there was signs of corrosion with rust appearing on the reflector, ' says Jodrell Bank Observatory chief engineer Tony Battilana.

'The reflector plates were made of ungalvanised mild steel, 12 to each panel, ' adds AEA Technology materials consultant Martin Kingham. 'These were fixed to mild steel angle frames with a parabolic contour, and secured at 150mm centres with plug welds. Corrosion was occurring in the reflector plates in the zone above and immediately beside the backing angles.'

Patch repairs were carried out in 1991, but it soon became clear this was not a long term solution.

A complete replacement of the reflector surface was needed.

Inside the vast bowl-like dome, more like a gigantic steel arena than a telescope, evidence of the structure's distress is clear. In places, corrosion has eaten through the plates. Aside from the obvious risk to the structure, corrosion also affected the telescope's accuracy.

'The rust pushing up the plates meant the reflector surface no longer had the correct parabolic alignment, which diminished the power of the telescope, ' says Battilana.

But no solution would be easy.

All corrosion activity would have to be removed, while the new arrangement would have to provide a robust rust-free structure which would not require constant maintenance. Access would also be a major difficulty: the rotation axis of the of the reflector stands 60m above the ground and can only be reached by a series of small lifts designed for carrying people rather than chunks of material. Failure to find a solution would have meant the demise of Lovell's creation, and with the meagre funding available for scientific research in the UK the likelihood of a replacement would have to be questioned.

The team, along with Observatory engineers working under Battilana and telescope engineer John Haggis, came up with a solution to the corrosion problem. Telescope and transmitter repair specialist Shal Engineers won the £2M contract, which is funded by a joint grant from the Department of Trade & Industry and the Wellcome Trust.

'We needed to use replacement materials with protective coatings and ensure the new components were compatible and would not lead to the presence of galvanic cells and crevice corrosion, ' says Kingham.

Existing reflector plates are removed from the panel frames by chiselling, while the top of the supporting angles are then ground down. 'Removing the plates is tricky in itself, because you have to find the location of the puddle welds underneath the plates, ' says Shal Engineers director Steven Hardwick.

The angle frames which provide the support 'skeleton' behind the reflector surface are then degreased, shot blasted, degreased again and cleaned with detergent wash before being treated with Temaprime - a zinc phosphate and micaceous iron oxide-based compound.

Once this is completed, mastic is applied by trowel to the upper surface of the angle frame, which provides a compressible bed for the new reflector plates, and closes the gap which can trap water and hasten corrosion between the angle and plates.

Carbon steel with 18-22 microns thickness of galvanising was chosen as the new plate material, fixed with zinc coated 'Tek' self-drilling threaded fasteners. A final coat of alkyd protective paint is applied to the plates with special care taken to avoid moisture ingress around the head of the fasteners.

Inside the dish, the painstaking nature of Shal Engineering's work is evident. All treatment to each of the 4,032 plates and angles underneath is by hand, but work which began in March is ahead of programme. With a sharp wind howling outside, Hardwick says weather conditions have been one of the most difficult aspects of the job.

'Because you can't walk up the side of the dish as the telescope's geometry steepens, everyone has to be trained in special rope climbing techniques. But it is an extremely interesting job, and we are proud to be helping restore this historic structure, ' he says.

Around 60% of the plating will be completed by December, before the telescope is turned for its next peep into space scheduled for the end of the year.

The remainder will be completed next year, giving the telescope a more accurately profiled reflecting surface, and around four times its current power of resolution.

'It may look like an icon of 1950s science, but it plays a vital role, and is being restored not out of sentiment, but for scientific reasons, ' says Battilana.

The truth is out there

The Lovell telescope is the third largest fully steerable telescope in the world, and can see right to the farthest reaches of the universe.

'We do get people coming along asking for a look - we have to tell them it's not that sort of telescope, ' jokes Battilana.

The telescope receives radio waves rather than optical waves, emitted by distant objects in space and interpreted at Jodrell Bank to give details of the objects being viewed. With radio waves 100,000 times longer than visible light, a radio telescope would need to be 100,000 larger than an optical one to get the same resolution. While the Hubble Space Telescope is 2m in diameter, an equivalent radio telescope would therefore have a diameter of 200km.

Using a technique known as interferometry however, astronomers can link distant telescopes electronically, enabling them to imitate a telescope whose diameter is equivalent to the distance between them by combining the information provided by each.

The Lovell Telescope is one of the six telescopes across Britain linked together to form the MERLIN system, giving an incredible equivalent diameter of 217km. MERLIN'S information is interpreted at Jodrell Bank, and can provide a resolution comparable to images from Hubble.

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