European Extremely Large Telescope: the future is already here!
To go farther and farther towards the origins of the Universe,
studying ever fainter objects, ever larger telescopes are needed. This decade will see the
rise of real giants with mirrors with a diameter of almost 40 meters that will use the most refined
technologies to get images of the sky sharper and sharper, much better than those given to us so far
by the Hubble Space Telescope. The decision of the European Southern Observatory (ESO) to start
with the construction of its European Extremely Large Telescope, with a diameter of 39 meters,
has kicked off the race to the super telescopes and the Americans do not want to watch. They have
under construction the Giant Magellan Telescope (diameter 25 m) in Chile and plan the Thirty
Meter Telescope (diameter 30 m) to be put on top of Mauna Kea, in Hawaii. However, it will be the
European telescope to cross the finish line first: its first light is in fact expected in 2025!
When the 2.5-meter Hooker Telescope, then the largest in the world, was
installed at Mount Wilson Observatory and looked up at the sky for the first time,
it was the year 1917. No one knew what wonders would be able to reveal, but within a decade,
thanks to this instrument astronomer Edwin Hubble was able to demonstrate the existence
of other galaxies beyond ours and to discover the expansion of the universe.
History repeated itself in 1949, when the 5-meter telescope of Mount Palomar
took the prize of the largest and revealed to the world the distance and the real nature of Quasars,
or that of supermassive black holes that accumulate matter at the centers of galaxies.
For that time, it was pure science fiction! In the '90s there was a further technological
leap, and a handful of telescopes, such as the four of the Very Large Telescope of ESO,
the Japanese Subaru and the two of Keck in Hawaii arrived at 8-10 meters in diameter. Which, with
the essential help of the Hubble Space Telescope gave us the most amazing cosmological discovery
of the last decades: the universe is not only expanding, but the expansion is even accelerating!
Well, have you noticed? Every 30-40 years the size of the top telescopes tends to double. So that
we should now expect to soon have a generation of reflectors about 20 meters in diameter available.
Right? But no! In the next five
years we will see at least three telescopes with diameters of several tens of meters appear on the
world astronomical stage, up to four times larger than the largest instruments in existence today!
The most giant of all is the one that the European Southern Observatory is building on
Cerro Amazones, in the Chilean Atacama Desert. It will measure 39 meters in diameter and will
be called ELT, or Extremely Large Telescope. With that diameter, ELT will be able to devote
itself to the direct detection of extrasolar planets and in the most favorable cases also
to the spectral analysis of their atmospheres. In addition, ELT's suite of instruments will
allow astronomers to probe the early stages of the formation of planetary systems and detect water
and organic molecules in protoplanetary disks around forming stars. Thus, the ELT will answer
fundamental questions regarding the formation and evolution of planets.
The site where the Observatory has been under construction since 2014,
namely Cerro Armazones, located only 23 km away from the Very Large Telescopes, was chosen in
2010 based on a careful comparative analysis that lasted a few years. In order to select the most
suitable location, several parameters were taken into account. First of all the "astronomical
quality" of the atmosphere, such as the number of clear nights, the stability of the atmosphere and
the amount of water vapor. Cerro Armazones is in fact a 3060 meters high mountain that offers
ideal conditions for astronomical observations as it can count on 320 clear nights per year.
The Chilean government has donated to ESO a substantial piece of land contiguous to the
Paranal Observatory property that contains Cerro Armazones, in order to ensure its
protection from possible man-made disturbances, such as mining activities and light pollution.
An absolutely necessary protection, given the enormous sensitivity of such an instrument.
The ELT was in fact designed to carry out detailed studies on the so-called "population III stars",
the first to form shortly after the Big Bang and to give birth to the first galaxies. Objects then
of infinitesimal brightness, which to be revealed need an absolute absence of "background noise".
One of the more specific goals of ELT will also be to make a direct measurement
of the acceleration of the expansion of the Universe, and also to look for possible variations
in the fundamental physical constants over time. An unequivocal detection of such variations
would have far-reaching consequences for our understanding of the general laws of physics.
. In front of the great
amount of mysteries that such an instrument seems to be able to solve, one would immediately ask:
why did we wait so long before equipping ourselves with such large telescopes?
As tempting as the scientific prospects just outlined may be, the desire for ever larger
telescopes can only be satisfied by overcoming the limits that are unavoidable on Earth.
"BE sure to join the InsaneCuriosity Channel...
Click on the bell, you will help us to make products of ever higher quality!"
Without claiming to be completely exhaustive, one must first consider the enormous difficulties
encountered when trying to shape a large mirror. It is necessary in fact that the shape of the
mirror is very precise, just think what happened to Hubble when an error of 2.3 thousandths
of a millimeter on the edge of the primary mirror, found in 1990 just put into orbit,
caused a deterioration of the images of almost ten times in terms of spherical aberration.
Beyond the precision hurdle, the quality of a mirror is also challenged by its own weight. The
force of gravity, in fact, tends continuously to deform mirrors and flex telescope mounts.
These two problems are closely related to each other, since, just to withstand its own weight,
the thickness of a mirror must increase with its diameter, making it increasingly difficult
to build a suitable mount to hold it. Moreover, in this way, the weight of the mount itself will
increase, so it will be more and more subject to the problems of bending... The telescopes
of Mount Palomar (5 meters in diameter) and of the Observatory of Mount Pastukhov in Russia
(6 meters), represent the insurmountable limit for the traditional construction technology of the
so-called "single mirror telescopes" whose primary mirror consists of a single block of glass.
In addition to the problems posed by the fusion of increasingly large mirrors and the closely
related effect of gravity, there is a third major problem for a telescope built on Earth, that of
the atmosphere. Very briefly, suffice it to say that the turbulence brought by an ever-moving
atmosphere perturbs the image of a star, deforming it as it happens, for example, when we observe an
object behind a candle flame. The deterioration of the image caused by this type of perturbation,
unlike the problems caused by gravity, is totally unpredictable, and therefore it is
even more difficult to think of remedying it. This is one of the main reasons why designers
place large Astronomical Observatories on mountain tops, where the atmosphere
above them is less thick and more rarefied. All these problems raise the question if it is
advantageous to build larger and larger optical telescopes on the ground or if vice versa it is
not the case to move them into space. From there in fact they operate in a regime of weightlessness
(or better, microgravity) and also the problem of the atmosphere would be solved at the root.
Thinking about the Hubble telescope, which has been orbiting around our planet for a long time
and that continuously gives us spectacular images, it would seem that this is the way to follow,
but experience has made us understand that there are also serious disadvantages. Substantially in
fact it is very difficult and expensive to place and maintain operational in space large mirrors
with their measuring instruments, and also their operational life is very short. These difficulties
have made competitive again the observation from ground because in the meantime it has
become possible to build larger and larger telescopes thanks to two new techniques that
allow to overcome the problems due to the weight and the presence of the atmosphere.
These solutions are called "active optics" and "adaptive optics", they are already used
today on all modern telescopes, not necessarily of large size, and will be essential to make feasible
and fruitful the realization of the future mastodons of observational astronomy.
The technique of active optics is used to correct in real time any deformation of the
parabolic surface of the primary mirror, and it succeeds thanks to a number of "actuators",
in practice hydraulic pistons controlled by computers placed under the mirror,
able to maintain each area of the mirror in its ideal shape even in the presence of mechanical
distortions or sudden changes in temperature. At the heart of the adaptive optics system
is a thin, flexible mirror that is also computer-controlled. Basically,
you target a fairly bright reference star located near the object you want to study.
The computer analyzes the star's light to measure how atmospheric turbulence distorts
its theoretical diffraction shape; it then tells the control system how to adjust the mirror's
shape to correct the image in real time. Because atmospheric turbulence is constantly changing,
such systems can alter the shape of the mirror up to a thousand times per second. And if,
as is often the case, no bright enough reference star is nearby, astronomers can simply project
powerful laser beams into Earth's upper atmosphere and thus create a reference light source. This is
why the ELT will have eight lasers with which it can generate artificial stars anywhere in the sky.
In short, the real answer to "why didn't we do this sooner" is that we actually did it as soon as
we had the computing power of ever more powerful computers, ever more efficient imaging sensors,
and the availability of ever lighter and higher-performance materials.
A whole series of substantial and concomitant progresses that only now are putting us in
a position to increase the diameter of the telescopes of this generation
by at least four times compared to the previous one, and not only by two as
the old law of "doubling" wanted instead. But back to the Extremely Large Telescope.
The first projects, dating back to 1997, included even the construction of a
telescope with a diameter of 100 meters: the famous OWL (Overwhelmingly Large Telescope),
then resized by budget problems up to 42 meters in 2006 and finally to 39.3 meters in 2011.
Too large to be made from a single block of glass, the ELT mirror was composed as a mosaic of
798 hexagonal mirrors of about 1.45 meters in diameter, 5 centimeters thick and weighing 250
kg each. The shape of each hexagon is controlled and corrected by 3 actuators capable of ensuring
a deviation from the theoretical shape of just ten nanometers (one nanometer is
equal to one millionth of a millimeter!). The space between the hexagons is 4 mm.
A composite mirror like the eye of a fly, which although reduced to 40% of what was
originally fabled, will still be able to collect 100 million times more light than the human eye,
256 times more light than the Hubble Space Telescope and 13 times more light than the largest
of the currently operating optical telescopes. The ELT is designed to have a five-mirror
Nasmyth-type optical scheme. The 39-meter primary mirror, M1, will collect light from the night
sky and reflect it back onto the M2 secondary mirror, the realization of which was a major
challenge for the manufacturer because this mirror is convex in shape and also very large;
with a diameter of 4.2 meters and a weight of 3.5 tons, it is in fact the largest secondary
mirror ever used on a telescope and the largest convex mirror ever produced. Just think that it
is as large as the primary mirror of the Herschel Telescope, the second largest telescope in Europe!
However, M2 will collect light from M1 and reflect it back to M3, which in turn will transmit
it to the thin flat mirror of the adaptive optics system (M4), located even higher up.
This fourth mirror, by changing its surface shape a thousand times per second, will correct the
distortions caused by atmospheric turbulence, before sending the light to M5, a flat mirror
that will stabilize the image and send it to the imaging instruments placed on the focal plane.
The focal ratio of the primary (given by the ratio of focal length to diameter) is f/0.87,
which means that the mirror will focus the image at a distance of 34.5 meters, where it will be
collected by the M2 mirror, magnified and sorted along an optical path as long as 684 meters!
The total field of view at the Nasmyth focus will be 10 arcminutes, which means that the
telescope will be able to frame a field as large as one third of the angular diameter of the Moon:
four times larger than that of Hubble. The telescope, with its entire altazimuth mount,
will be contained in a giant 86-meter-diameter dome, nearly 74 meters high above the ground.
The dome will have a total mass of about 5000 tons,
while the telescope with the mount will weigh about 3700 tons.
As mentioned at the beginning, this extraordinary and revolutionary structure should be ready for
2025, while more delayed seem to be the projects of the Giant Magellan Telescope of 24.5 meters,
under construction in Las Campanas, also in Chile, and the Thirty Meter Telescope, the 30 meters
to be built on top of Mauna Kea, in Hawaii and currently blocked by the protests of the natives.
Other giant telescopes are however in planning. And this is a good thing,
in the sign of a healthy competition on the road of astronomical and technological knowledge.