This is how the mirror of the largest telescope in the world is moved

The largest optical telescope on Earth has a mirror with a diameter of 39 meters. It has just begun being sent to the Cerro Armazones Observatory in Chile. How is it done?

In observational astronomy, the principle “the larger the mirror,” the better the accuracy of the observations is applied. Of course, technologies may change, but while maintaining specific ways of building optical devices, size matters. That’s why the Webb Space Telescope provides much higher resolution images than the Hubble Telescope at its peak, although it’s difficult to compare them directly because they cover different wavelengths. However, there is also Hubble’s collaboration with Webb. However, if we limit ourselves to visible, near-infrared and ultraviolet light recorded by ground-based telescopes, what is the maximum potential we have today?

Limitations of ground-based telescopes and what we can do

The largest optical telescopes on Earth today have a primary mirror with a diameter of about 9-11 meters (GTC, HET, Salt, Keck). We also have some 6-8 meter telescopes (VLT, Magellan, Subaru, Gemini) and these are the most powerful instruments we have available. Their capabilities compared to similar sizes, i.e. telescopes of the 5-6 meter class of 50 years ago, are much greater than the difference in the size of the mirrors would indicate, thanks to the use of adaptive optics, which eliminates the influence of the greatest astronomers. Enemy, Earth’s atmosphere. However, no matter how effective this optics is, we will not be able to overcome some obstacles. The first is that certain wavelengths are blocked by the Earth’s atmosphere and cannot be bypassed. That’s why we put telescopes in Earth orbit (Hubble, Chandra) and even at much greater distances (Webb, Kepler, Spitzer).


Lunar eclipse over Paranal Observatory in Chile. Telescopes with the world’s largest single mirrors with a diameter of 8.2 meters operate here. (Image: European Southern Observatory)

However, the second limitation, the accuracy of the observations in cooperation with adaptive optics, can be overcome. We just need much larger mirrors in telescopes. You can see how much you can earn even with amateur tools. Jupiter observed through binoculars with 20x magnification looks much more interesting than in the case of 7x binoculars, which are often purchased. However, these 20x binoculars must be placed on a tripod, otherwise good observations will be lost. It is similar in the world of large telescopes. The larger it is, the more difficult it is to install and the more complex the mirror structure and supports.

The largest mirror in the history of astronomy has a diameter of 39 meters

In the case of the largest telescope currently under construction, the ELT (Extremely Large Telescope), the diameter of the primary mirror reaches 39 meters (visualized in the text input image). It is an area similar to the size of two tennis courts, or even larger, including the so-called platforms, that is, the space surrounding each court on which players can move.


Portions of an ELT mirror awaiting processing. (Image: European Southern Observatory)


And ELT mirror parts that are already cut and ready for grinding. (Image: European Southern Observatory)

A mirror of this size cannot be made from a monolithic piece of glass. The largest pieces of monolithic glass, with a diameter of 8.4 metres, will be used to build another large telescope, the Large Magellan Telescope. In addition, they are round. In contrast, the ELT uses classic hexagonal strips, in the same shape as the Webb, which when installed next to each other, will create the equivalent of a single mirror. The ELT’s primary mirror will contain 931 of these parts, of which 798 will permanently form the primary mirror (the telescope will have five more, including the largest secondary mirror with a diameter of 4.2 metres). The rest will be needed as spare parts, among other things, when the main mirror parts will be periodically recoated with a light-reflecting layer, which is done in every telescope.


The mirror pieces packed in this way will travel 10,000 kilometers to Chile. (Image: European Southern Observatory)

One hexagon mirror piece is cut from a round piece of SCHOTT Zerodur glass about 1.5 meters in diameter. The thickness of the polished piece is only 5 cm, and the weight, including the support supporting the active optics, is 250 kg. This is a large component, but much easier to pack and transport than a single 39 meter mirror. The mirrors are cast at SCHOTT in Mainz, Germany, then polished in France at Safran Reosc. In addition, the German company Physik built actuators to control the position of each piece, and the German-French consortium FAMES built sensors to control this position.


Now ordinary trucks are enough to transport the ELT mirror piece by piece. (Image: European Southern Observatory)

In the case of existing 6-8 meter class telescopes (the larger ones are already segmented) and monolithic mirrors, the entire transportation was a very tedious procedure, often requiring the construction of new roads in the desert in Chile or wherever such a telescope was located. Being built. However, the vehicles that carried the mirrors up the hill were moving very slowly, and not much faster than the Kennedy Center rocket transporter.

The ELT primary mirror chips are manufactured using a technology that can be called the 10nm process, as this is the maximum permissible deviation from a perfectly smooth surface. This is less than a thousandth of the thickness of a human hair.

The ELT telescope parts, packed in separate boxes, must also be transported carefully, but this will be much easier. Moreover, the loss of a single part during transportation would not be as costly as in the case of damage to the homogeneous mirror, which would then have to be completely replaced or accept a deterioration in image quality.


A Christmas gift for the astronomers and engineers working on building the ELT telescope in Chile. (Image: European Southern Observatory)

The first parts of the ELT mirror are on their way to Chile

In mid-2023, the European Southern Observatory announced that the ELT was already 50% ready. This half of the work wasn’t fully visible in the photos, but now that the first parts of the massive mirror have set off on their journey from France to Chile, it feels closer to finishing construction than to just beginning. The Danish company DSV, also known as a logistics service provider also in Poland, is responsible for the transportation.


Current status of ELT telescope construction. (Image: European Southern Observatory)

The ELT telescope mirror is shipped to Chile in small packages. First, I started a batch of 18 packages. More are already waiting for transfer. Unfortunately, the parcels are too large to fit in the parcel locker.

There are actually 18 segments on the trip to Chile, which is quite an interesting number as would be required to build the Webb Telescope’s mirror. However, there are more sectors ready. At the beginning of November, there were 100 of them, and now there are dozens more. Currently, the production rate of mirror chips is about 4 units per week, and soon it will reach 5 units. At this rate, it would take about 3.5 years to produce all the missing chips. So, first light for the ELT shouldn’t be expected before 2028, and assuming there may be construction issues along the way, it will also take some time to fit those hundreds of parts together nicely, even later.


Illustration showing the significant jump in resolution in near-infrared observations made by ELT using the proposed MICADO instrument, compared to Hubble (WF3 camera) and Webb (NIRCam camera). Examples of three star fields of different densities show that only the ELT will discover their true nature. (Image: European Southern Observatory)

However, once the ELT is built, the resolution of the images it will produce will surpass what we are currently looking at with the Webb telescope. These will of course be images at other wavelengths, but perhaps we will finally see the first images of exoplanetary disks, although this will also be a major challenge for ELT.

Source: ESO, information. king

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