High in Chile’s Cerro Armazones, a massive observatory that will one day serve as mankind’s most powerful eye on the universe is under construction.
Set to begin operation in 2025, the European Southern Observatory’s (ESO) Extremely Large Telescope (ELT) contains a litany of superlatives. It will collect 13-times more light than the largest single telescopes currently in use and conduct investigations in the nearultraviolet and mid-infrared spectral ranges. This power will make it possible, for example, to obtain images of rocky exoplanets and to directly measure the expansion of the universe.
Enabling these scientific advances is a groundbreaking optical system that features a primary mirror with a diameter of about 39-meters made of 798 hexagonal segments. Each 1.45-meter segment can be tuned with mechanical actuators from the rear, forming a complex active optics system that can compensate for deformation of the main mirror due to its weight, as well as the influences of temperature and wind.
The M1, as the primary mirror is known, along with the M2, M3, and M4, will rely on a substrate made from SCHOTT ZERODUR®, a highly technical glass-ceramic prized for its ability to remain dimensionally stable when temperatures change. For more than five decades, astronomers have relied on it to build some of mankind’s most important scientific instruments. The vast majority of telescopes with primary mirrors above 1.8m rely on it.
Developing Low CTE Materials
In material sciences, the degree to which something expands or contracts in response to heat or cold is referred to as its Coefficient of Thermal Expansion (CTE). For astronomers, low CTE is a prized property in optical mirror substrates, because any change in the shape and size of the mirror has a significant impact on the quality of the image that is amplified as telescopes get larger and distances observed get longer.
The Hale Telescope at Mount Palomar, built in 1948, relied on a borosilicate glass, which featured a CTE 3 × 10-6 per Kelvin. This was a significant improvement over other materials, such as window glass. Within a few short years of the construction of the Hale Telescope, astronomers turned to a quartz glass, which was an order of magnitude better. But the discovery of glass-ceramics greatly accelerated the feasibility of large telescopes.
Manmade glass-ceramics were discovered in the mid-1950’s, by accident, it turns out. They are inorganic, nonporous material with a key property — low CTE and resistance to thermal shock that is achieved by distributing crystalline structures within a glass matrix. When exposed to heat, the crystals contracted, and the glass matrix expanded, thus creating a novel material that did not change shape or size when the temperature changed, at least for a limited range of temperature. They were originally used in the cookware markets, but it was not long before various formulas emerged for other applications.
In the 1960’s, German scientists from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, faced with the prospect of falling behind the international competition in astronomical achievement, made a decision. The observatory at the University of Heidelberg contacted SCHOTT and asked the company to develop a mirror substrate for a large telescope. Following extensive tests, a team of developers led by the physicist Juergen Petzoldt began extensive tests. The result was a highly precise blend of crystallites 30 to 50 nanometer in size embedded inside a glass matrix of lithium, aluminum and silicon oxides.
In November of 1968, the Max Planck Institute for Astronomy (MPIA) in Heidelberg ordered 11 mirror blanks for what would become Europe’s largest telescope. Nearly 150 specially trained employees worked for nearly nine years to build casting molds, large melting tanks, and ceramization chambers before the order would be fulfilled. The results, however, broke records: a mirror substrate with a diameter of over 3.6 meters that was nearly 60 centimeters thick, used for the first time in observatories at Calar Alto in the south of Spain. Its CTE stood at 0.015 × 10-6per Kelvin, roughly 30 times better than quartz glass.
Today, ZERODUR is available in several expansion classes, some of which feature CTEs roughly twice as low as the substrate used at Calar Alto. To put its CTE into perspective, consider this: For the ELT’s M2, which measures approximately 14 square meters, a temperature change of one degree Celsius allows a maximum shape deviation of two nanometers. Over an area of 100 square kilometers, this is 14 millimeters.
In the following decades, planning for astronomical instruments focused on larger primary mirrors, and strategies to lower the weight of both primary and secondary mirrors. This became all the more important as planning for telescopes in the 8-meter class began.
Casting and Ceramization Process
The production process for ZERODUR is similar to that of other glass-ceramics. It relies on two steps — the melting of glass and the subsequent ceramization. Because of the large size of ZERODUR blanks, each can take several months and a high degree of precision.
In the initial step, raw materials are heated to 1600°C to obtain a homogeneous melt. Once poured into an insulated mold, the temperature is allowed to drop to approximately 900°C over a period of several hours. To cool the blank to room temperature, the mold is placed into an annealing oven which slowly lowers the heat.
At this point, the mirror blank is glass. What follows is a ceramization process, in which the material is continuously tempered to achieve controlled nucleation and growth of the crystal phase. The process is tightly controlled to create high-quartz nano-crystallites of 30 nm to 50 nm, which comprise approximately 70 to 78 percent of the material’s weight. The nano-crystallites contract when they are subjected to heat, whereas the glass itself expands.
While low CTE is a key property of ZERODUR overall, the homogeneity of a blank’s CTE is also important. The precision of the melt and ceramization process are both tightly controlled to eliminate the risk of bubbles and striae and even distribution of crystalline structures.
As ZERODUR blanks have gotten larger over the years, there has been an increased demand for lightweight blanks. The substrate that was used in Calar Alto weighed approximately 27 tons when poured into a mold. Obviously, much of the material is removed during polishing and shaping the mirror. The expense of those raw materials are costs nonetheless that impact the financial viability of large observatory proposals.
To lower the weight of ZERODUR blanks, a team of scientists and engineers developed a spin-casting technique to produce on meniscus-shaped blanks. To compensate for the blank’s lack of stiffness, the astronomers developed the active optics system. These two techniques were used in the ESO’s New Technology Telescope, the first to employ active optics that could compensate for any “flex” in the mirror due to its weight.
The four 8.2-meter meniscus-shaped ZERODUR blanks used in the Very Large Telescope (VLT) represent one of the material’s most significant milestones. An entirely new, greenfield production facility was required for this process. The blanks, delivered between 1993 and 1996, used 70 tons of raw materials each, poured into spin casting machinery that weighed an additional 80 tons.
Other strategies have been deployed to lower the weight on ZERODUR blanks by using a honeycomb structure. This has been applied in space-based satellite mirror mounts and was used for the Stratospheric Observatory for Infrared Astronomy (SOFIA), an infrared telescope on-board a jumbo jet.
The honeycomb enabled SOFIA’s developers to reduce the weight of the telescope enough so that it could be practically deployed on an aircraft. While a large solid mirror in a similar telescope would have weighed 3.8 tons, ZERODUR was configured by Safran Reosc into a honeycomb shape, reducing the mirror’s weight to only 850 kilograms.
The ELT is expected to achieve first light in 2025, but production of the substrates is well underway. Components for the segmented fourth mirror (M4) were delivered in 2016. The M2, the world’s largest convex mirror, was delivered in 2019. In January 2020, the tertiary mirror blank was delivered to Safran Reosc after 15 months of fabrication.
The final steps of fabrication have been a painstaking process. At a state-of-the-art 5-axis CNC machining center, the tertiary mirror substrate was ground down to its final weight of 3,200 kilograms by performing several machining steps. The back of the mirror material was etched with hydrofluoric acid to remove micro-cracks that occur during grinding. A perfect material surface is important to ensure an optimal bonding of the mirror underside with pads or actuators that fix and align the mirror precisely. Polishing is expected to take several months.
The use of ZERODUR is not limited to astronomical applications. Several terrestrial industries also require low-CTE materials. ZERODUR is used, for example, in microlithography techniques and the printing of integrated circuits because it allows for the precise positioning of wafers. It is also used in ring laser gyroscopes that aid aircraft and submarine navigation, and as a substrate for the measuring standards of instruments.
For more than 50 years, ZERODUR has served the scientific community whenever there is a need for levels of precision that push the physical limits of material science. As the world gets more complex, as manufacturing tolerances become more demanding, and as scientific inquiry pushes further, that track record will continue.