Mirror Lab and the Giant Magellan Telescope
Situated beneath the sports stadium of the University of Arizona is the Steward Observatory Mirror Lab, a facility where 8-meter class mirrors are made for some of the world’s largest telescopes.
The lab was set up in the 1980s by Roger Angel, an astronomer originally from St Helens in northwest England, who pioneered the spin-casting technique for thin mirrors. Instead of pouring a thick mirror, waiting months for it to cool then grinding away perhaps half of the total glass to generate the required curve, two key features help produce larger, lighter mirrors in a shorter timescale.
First, the mirror is cast in a mould containing hundreds of hexagonal ceramic cores. Pieces of borosilicate glass are placed on top of the cores. As the mould is slowly heated the glass melts and runs down the gaps between the cores, creating a honeycomb structure that is strong and stiff and much lighter than a solid one would be – 80% of the total volume is hollow.
Secondly the entire mould is rotated at 7 rpm, and the centrifugal force produces a curve on the mirror that is very close to the final required figure, hence polishing time is much reduced.
After making two mirrors for the Large Binocular Telescope on Mount Graham, Mirrorlab has been mainly occupied since 2005 with making eight mirrors for the Giant Magellan Telescope (GMT) – seven are in operation at any one time on the scope, with one spare to allow re-coating of the others in turn. The seven 8.4m mirrors will together be equivalent to a single mirror 24.5m in diameter which with the aid of adaptive optics should have a resolving power 10 times finer than the Hubble Space Telescope.
This visit took place in 2012 and at the time not all the funding was in place for the Giant Magellan Telescope (GMT). In 2015 a $20 million donation from Richard F Caris helped ease the funding issues, after which the mirror lab was renamed in his honour. As of late 2019 Roger Angel is still director of Mirror Lab.
Model of the GMT
This cardboard cutout model of the GMT shows the 7-mirror design and the human figure in the foreground indicates the overall scale of the finished instrument. Each of these mirrors is about the same size as those in the Very Large Telescope, which I visited in 2019 – see this post.
The six off-axis mirrors of the GMT have to be polished to a shape that is not symmetric about their axes, which complicates the process. Each reflects light onto its own secondary mirror which uses adaptive optics to correct the image wavefronts before reflecting them through the hole in the central primary to the waiting instruments below.
Printed on the model are the words “Expected completion 2019”. Reality has proved a little different: as of late 2019, two of the mirrors have been completed and delivered to Chile, a further three have been cast and are undergoing polishing, while the final three have yet to be cast. The plan is that the telescope will begin observing with only four mirrors, commissioning of the GMT is now scheduled for 2029.
Borosilicate Glass (aka Pyrex)
Pieces of the E6 glass from the Ohara corporation of Japan. Each mirror takes 20 tons of glass – the same as the mirror of the 5m Hale telescope when cast, though it had only 40% of the surface area. (Some 4½ tons of glass was subsequently removed from the Hale during grinding/polishing).
When placing the chunks in the mould, the lower quality pieces go in first as these will form the base of the glass blank and the honeycomb structure, while the highest quality pieces are placed on top as these will form the faceplate.
The oven takes 5 days to reach its peak temperature of about 1165°C. Over the next 4 days the temperature is reduced to 530°C, then over the next 40 days it is reduced to 450°C – this slow cooling allows the atoms in the glass to rearrange themselves, minimizing stresses in the finished blank. It will take another 66 days for the mirror to cool to room temperature. [2019 note – these were the timings in 2012 – they may be different now.]
After this the real work of polishing and figuring begins. The first mirror for the GMT took over 6 years to polish. The second was being cast at the time of our visit in May 2012.
Flipping an 8-meter mirror
When the mirror has completely cooled, and after a thorough inspection, a steel lifting fixture is glued to the front surface. This is used to turn the mirror upside-down and allow any remaining mould material to be removed. The bottom surface is then ground flat before the mirror is flipped over again so that polishing of the optical surface can begin.
Polishing is a slow process. As mentioned above, the first mirror (cast in 2005) took over 6 years to polish and figure.
Polishing laps
The final figuring of the mirror is done with small polishing laps like those seen on the table. They enable particular zones of the mirror to be targeted to achieve the exact required shape. The cross-hatch pattern cut in the laps is to enable the polish – a suspension of very fine particles of cerium oxide or similar – to flow out from between the surfaces.
It was sobering to see the size of these polishing laps for anyone who has ground their own mirror – the laps are bigger than many amateur-made mirrors!
The first 8-meter mirror for the GMT awaiting final testing before shipping to the site of the Giant Magellan Telescope at the Las Campanas observatory in Chile.
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