Why space telescope mirror is most complex ever built

Why the most ambitious telescope to leave our planet will be launched with deliberately bumpy mirrors that are out of focus on purpose.

“You will not be touching anything.” It is clear from the tone of the technician’s voice that this is a command, rather than a suggestion, and that it requires a response. “No,” I reply emphatically, “I will not be touching anything.”

I am dressed in a white smock, hairnet, gloves and overshoes; my audio recorder swabbed down with wipes and my notebook dusted. I am as clean as I am ever likely to be.

All this preparation has made me slightly paranoid and for good reason. I am about to enter the large clean room at Ball Aerospace in Boulder, Colorado where the mirrors for the $8.7 billion James Webb Space Telescope (JWST) are being built and tested. These will enable us to look back in time 13.6 billion years to the immediate aftermath of the Big Bang. They will be precise enough to capture single photons. And the slightest speck of dust or greasy fingerprint could ruin them.

The room is around the size of an American football pitch and pumped full of filtered air. Within it are several tent-like structures – clean rooms within the clean room. On tables inside these, sit some of the 18 individual hexagonal mirrors that make up the massive 21ft (6.5m) diameter primary mirror of this new space telescope.

“On that table is a mirror segment,” explains Allison Barto, JWST Program Manager at Ball, as we peer through a window into one of the super-clean clean rooms. “Each of these is made of solid beryllium and then we put a bunch of stuff on the backside to allow us to move it around in space.”

Beryllium is a metal chosen for its light weight, strength and durability under a wide range of temperatures, and the “stuff” she is referring to is a collection of extremely precise actuators that allow the individual 5ft (1.5m) diameter mirrors to be precisely adjusted, relative to each other, to act together as one single mirror. “A piece of paper is about 100,000 nanometres thick,” says Barto. “We need to move those mirrors about five nanometres at a time.” It’s like adjusting the wing mirrors on a car. The actuators allow the mirrors to be tipped and tilted, only these ones can also adjust the curvature of the surface.

The fact that the mirrors can be tweaked so precisely means that, like the Hubble space telescope, the JWST will be launched into space with its mirror out of focus. Only this time, unlike when Hubble was launched, this will be deliberate.

“The mirror unfolds like a drop-leaf table and, when we first deploy, these 18 mirrors are going to be all over the place,” Barto explains. “So, we will have to find the 18 fuzzy spots in our images and from those fuzzy images figure out where to move each of these mirrors to have them line up into a single well-focused image.” That laborious and painstaking process is expected to take at least six months.

One shot only

Currently due for launch in 2018, the JWST will be stationed 1 million miles (1.5 million km) from Earth – some five times further away from us than the Moon. At that distance, servicing missions, to correct errors or fix failed components, are not an option. This puts additional pressure on the teams of scientists and engineers around the world who are working on this international space telescope, to get it right. And, for the mirrors, that means a level of precision that is difficult to get your head around.

Barto explains proudly that if the length and width of the 18-part mirror was increased so that it covered the US, and its height increased proportionally, there wouldn’t be a mountain or valley higher than two inches (5cm).

As if achieving such a pristine surface was not hard enough, there’s an additional complication. The telescope will be operating in deep space, at a temperature of –243C (–405F). That’s only 30C (86F) above absolute zero, the lowest possible physical temperature. All materials, including beryllium, expand and contract with heat and cold and, although the element is extremely stable at low temperatures, the mirrors are being constructed at room temperature. Fortunately, engineers have come up with an ingenious solution to make sure the mirrors are the right shape.

“What we have to do is polish them pretty well,” explains Barto, “then we cool them down to 30 Kelvin (-243C) and look at how they change.” By taking extremely accurate measurements of any deformities, they can see how the mirrors warp. “Then we warm them up again and polish the inverse of those bumps – so if there is any lump when it’s cold, we polish in a little valley.”

So at room temperature, the mirror is slightly imperfect but at -243C, it is pristine. The upshot is, the telescope will be launched into space with deliberately bumpy mirrors and out of focus optics.

The progress of the JWST has been slow and costly. Led by Nasa, the European Space Agency and Canada, the project was started more than ten years ago. With the final price tag still to be determined, and some estimating it at around $10 billion, only last year there was still talk in Congress of withdrawing Nasa’s funding for the mission. Finally though, this amazing engineering endeavour is starting to come together. The key instruments for the telescope have been tested and delivered, the structure completed and the (folding) tennis-court sized heat shield finalised.

Next year marks the start of a gruelling four-year testing and integration process at Nasa centres in Washington and Houston. “We know we have only one chance to get it right,” says Barto.

I ask her if she will miss the mirror when the final sections are shipped to Nasa at the end of this year. “We love building it,” she says. “We’ve put in all this energy and time. But we’d love to see it leave!”

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