# The Extremely Large Telescope

Paolo Padovani and Michele Cirasuolo CONTACT Paolo Padovani. Email:
<EMAIL_ADDRESS>European Southern Observatory, Karl-Schwarzschild-Str. 2,
D-85748 Garching bei München, Germany

###### Abstract

Extremely large telescopes (ELTs) are considered worldwide to be one of the
highest priorities in ground-based astronomy. The European Southern
Observatory (ESO) is developing an ELT that will have a 39 m main mirror and
will be the largest visible and infrared light telescope in the world. The ELT
will be equipped with a lineup of cutting-edge instruments, designed to cover
a wide range of scientific possibilities. The leap forwards with the ELT can
lead to a paradigm shift in our perception of the Universe, much as Galileo’s
telescope did 400 years ago. We illustrate here the various components of the
ELT, including the dome and main structure, the five mirrors, and the
telescope systems. We then describe the ELT instrumentation and some of the
astronomical topics it will address. We then conclude by examining the
synergies with other astronomical facilities.

###### keywords:

ELT; Extremely Large Telescopes; science; astronomy; technology; telescopes;
solar system; exoplanets; stars; black holes; galaxies; cosmology; dark
matter; fundamental physics.

##  1 Introduction

The past decade has brought astronomical discoveries that have excited people
from all walks of life, from finding planets around Proxima Centauri, the
nearest star to the Sun, to the first image of a black hole, namely that at
the centre of M 87, a giant elliptical galaxy in the Virgo constellation. In
the next epoch of astronomy, the ELT111https://elt.eso.org/, with the other
two extremely large telescopes, the 24.5 m Giant Magellan
Telescope222https://giantmagellan.org/ and the Thirty Meter
Telescope333https://www.tmt.org, will tackle some of the biggest scientific
challenges of our time. The ELT will track down Earth-like planets around
other stars, and could become the first telescope to find evidence of life
outside of our Solar System. It will also probe the furthest reaches of the
cosmos, revealing the properties of the very earliest galaxies and the nature
of the dark Universe. On top of this, astronomers are also planning for the
unexpected – new and unforeseeable questions that will surely arise, given the
new capabilities of the ELT.

The ELT programme was approved in 2012 and green light for construction at
Cerro Armazones in Chile’s Atacama Desert was given at the end of 2014. From
building the immense telescope dome structure to casting of the mirrors, the
work on this wonder of modern engineering has been made possible thanks to the
spirit of collaboration. ESO has been working alongside a worldwide community
and dozens of Europe’s most cutting-edge companies to bring the ELT to
“technical first light” later this decade (currently planned for the end of
2028).

The decision on the ELT site was based on an extensive comparative
meteorological investigation, which lasted several years. The selection
process took several factors into account, with the “astronomical quality” of
the atmosphere, that is, the number of clear nights, infrared (IR) properties
(height, temperature, precipitable water vapour, etc.), seeing, atmospheric
turbulence profile, mean coherence length and time, outer scale length,
playing a crucial role. The selection also considered the operation and
scientific synergy with other major facilities, such as the Very Large
Telescope (VLT) at ESO’s Paranal Observatory and the Atacama Large
Millimeter/submillimeter Array (ALMA). Finally, a significant role was played
also by construction and logistical aspects, i.e. the impact of the location
on construction cost and schedule, considerations on where the observatory
staff would live, the closeness of power grids, water supply, political and
seismic stability, and so on.

Cerro Armazones’ location in the dry Chilean desert at high altitude (3046 m)
made it ideal for astronomical observations, with over 320 clear nights per
year and offering some of the darkest skies on Earth with virtually no light
pollution. The altitude of the site above sea level does not pose logistical
problems for operations, while meeting the science requirements for low
precipitable water vapour and low operating temperatures. The median seeing is
0.67 arcsecond at 500 nm with a median coherence time of 3.5 ms. The rainfall
in one year is of the order of 100 mm, with a median relative humidity value
of 15%. Moreover, it is located just 20 km away from Cerro Paranal, which
hosts ESO’s VLT and other telescopes.

##  2 Telescope Overview

The telescope and its interior structure will be housed in the the giant ELT
dome, which will provide protection from the extreme environment of Chile’s
Atacama Desert. The main structure of the telescope will hold its five mirrors
and optics, including the enormous 39 m primary mirror. The ELT will employ
sophisticated “adaptive optics” (AO) technologies to compensate for the
turbulence of the Earth’s atmosphere and to ensure its images are sharper than
those of any other telescope. It will also have other components such as a
prefocal station that works as the link between the telescope and its
instruments. Finally, it will have a modern control system to allow the user
to operate the telescope for science observations and maintenance activities.
The ELT will be part of ESO’s Paranal Observatory and will be operated from
the same control room as other ESO telescopes such as the VLT.

###  2.1 Dome and Main Structure

Figure 1: Artistic rendering of how the ELT will look like on top of Cerro
Armazones. Image credit: ESO.

The sensitive and delicate components of the ELT require special protection.
This is especially true in the rough desert environment, with potentially
damaging sand and wind, and in a country that is prone to earthquakes. The ELT
dome shelters the telescope in windy conditions up to 18 m/s during
observations and protects it and its delicate optics from the elements during
the day. It consists of a lower fixed part, the concrete pier, a rotating
upper part, and the enclosure, which is equipped with two motorised slit doors
that open laterally during observations. The latter has a thermally insulated
aluminium cladding, which enables thermal conditioning of the telescope
chamber during the day and limits radiative cooling during the night. This
rotating structure is also equipped with special cranes to handle operations
during the day when the slits are closed. The pier of the dome is surrounded
by an auxiliary building, with the instrument assembly area and the entrance
hall facing south, downstream of the prevailing wind. The various electrical,
thermal, and hydraulic plants used to operate the dome and the telescope are
distributed in different rooms of the building. The remaining rooms are used
for mirror segment storage, computer rooms, and to host a facility for mirror
coating. The anti-seismic concept of the ELT telescope and dome is based on
seismic isolators, which are placed below the telescope between the base layer
of the foundation and the rock. In case of an earthquake, the telescope and
the dome foundation can slide horizontally on those isolators by as much as 30
cm to compensate for the ground movements, minimising shocks to the telescope.
The telescope has been designed to survive the most powerful earthquakes ever
recorded in Chile.

The telescope needs to be able to track about the 1∘ zenithal avoidance locus
and preset to a new target within 5 minutes. This requires the enclosure to
accelerate and move at angular speeds of up to 2∘/s (the linear speed at the
outer diameter being approximately 5 km/h), which poses considerable
challenges to the motorisation system and its braking devices, and demands the
implementation of various safety provisions. The enclosure rotates
independently from the telescope structure, and sufficient mechanical
clearance is guaranteed to permit the complete movement of the telescope
within the open or closed dome. The dome design enables observations from
zenith down to 20∘ from the horizon. To avoid vibration during the rotation of
the enclosure, the rotating mechanism is, to the extent possible, structurally
decoupled from the dome concrete base. In addition, the possible propagation
of vibrations from the dome foundation to the telescope pier, via the seismic
isolators and the rock substrate, has been studied to ensure a smooth and
vibration-free movement and to guarantee the required precision of the
telescope tracking during observation.

The rotating enclosure, once fully equipped and finished, will have a height
and a diameter of 80 and 88 m respectively, a mass of around 6100 tons, and it
will rotate on 36 stationary trolleys mounted above the pier at a height of 11
m from the ground (Figure 1). The structure itself consists of a round girder
with a special track at its bottom that rests on the wheels of the trolleys.
The primary structure of the enclosure is completed by three structural arches
resting on the girder, one at each side of the slit and one at the back. The
enclosure structure, which will be bolted together on site, is closed by a
number of secondary beams which will permit the assembly of the insulated
aluminium cladding. A complex series of accesses inside the structure and the
slit doors allows engineers to reach all mechanisms of the doors, ventilation
louvers, and installed equipment. The enclosure’s slit doors will move on
three rails each, one at the round girder and two at the top. The doors
provide an aperture of 41 m when open, and their system motors have sufficient
redundancy to ensure the doors can be closed when needed, in all conditions.
In addition, the doors will be equipped with latching mechanisms to achieve
structural continuity and with special inflatable seals to guarantee
environmental tightness when closed.

###  2.2 Mirrors

The ELT will have a pioneering optical design with five mirrors, all having
different shapes, sizes, and roles but working together seamlessly. The
primary, M1, is the most spectacular: a giant 39 m concave mirror that will
collect light from the night sky and reflect it to the secondary mirror. The
convex M2, the largest secondary mirror ever employed on a telescope, will
hang above M1 and will reflect light back down to M3, which in turn will relay
it to an adaptive flat mirror, M4, above it. This will adjust its shape a
thousand times a second to correct for distortions caused by atmospheric
turbulence, before sending the light to M5, a flat tiltable mirror that will
stabilise the image and send it to the ELT instruments on one of the two
Nasmyth platforms. Figure 2 shows the optical path of the telescope with the
five mirrors.

Figure 2: The novel 5-mirror optical system of the ELT. Before reaching the
science instruments the light is first reflected from the telescope’s giant
concave 39 m segmented primary mirror (M1), it then bounces off two further
4-metre-class mirrors, one convex (M2) and one concave (M3). The final two
mirrors (M4 and M5) form a built-in AO system to allow extremely sharp images
to be formed at the final focal plane. The two Nasmyth platforms to the left
and to the right will host the science instruments and the pre-focal stations
(Section 2.3.2). Image credit: ESO.

This design provides an unvignetted field of view (FoV) with a diameter of 10
arcminutes on the sky, or about 80 square arcminutes (i.e. $\sim 1/9$ of the
area of the full moon). Thanks to the combined activation of M4 and M5, it
will have the capability to correct for atmospheric turbulence as well as the
vibration of the telescope structure induced by its movement and the wind.
This is crucial to allow the ELT to reach its diffraction limit, which is
$\sim$ 8 milli-arcseconds (mas) in the J-band ($\lambda\sim 1.2\umu$m) and
$\sim$ 14 mas in the K-band ($\lambda\sim 2\umu$m), thereby providing images
15 times sharper than the Hubble Space Telescope (HST).

####  2.2.1 M1

The primary mirror, M1, at 39.3 m in diameter will be the largest optical/IR
mirror ever built (Figure 2). The mirror is segmented, being made of 798
quasi-hexagonal mirrors, each of which is about 1.45 m in size (corner-to-
corner), only 50 mm thick and weighs 250 kg. The full M1 has a six-fold
symmetry, as there are six identical sectors of 133 segments each. In each
sector all 133 segments are different from one another in shape and optical
prescription; in other words there are 133 different segment types. In order
to facilitate recoating there will be a seventh sector with 133 segments, i.e.
one for each segment type. This adds up to a grand total of 931 segments. To
achieve the required scientific performance, M1 needs to be maintained in
position and be phased to an accuracy of tens of nm, i.e. 10,000 times thinner
than a human hair, across its entire 39 m diameter. This is extremely
challenging, as the full structure will be moving constantly during an
observation, and will be affected by wind and thermal changes. To achieve
this, each segment, made of the low-expansion ceramic material Zerodur©444This
special material is not sensitive to thermal fluctuations thanks to its very
low thermal expansion coefficient. This means that the form and the shape of
the mirrors will not change significantly with temperature during
observations. It is also extremely resistant, can be polished to the required
finishing level, and has been used in telescope mirrors for decades. (from
SCHOTT), is supported on a 27-point (three 9 points) whiffletree, which is a
mechanism to evenly distribute the support across the back of the segment
using 27 points of contact across its surface (Figure 3). Each whiffletree
includes a primary tripod and three secondary tripods (inner and outer
secondary tripods), connected by flexures in a frictionless design. The axial
support carries the loads between the segment and the moving frame along the
segment local normal z axis. The load can be adjusted via warping harnesses so
as to slightly change the shape of the mirror to compensate for optical
aberrations induced by gravity and thermal effects. Moreover, each segment
assembly can be moved in height and tip/tilt relative to the structure by
using three positioning actuators (PACTs). These three actuators can move
independently with an accuracy of 2 nm with a maximum excursion (or stroke) of
10 mm to reposition the segment to its desired position, thereby compensating
for the deflections of the underlying telescope structure and maintain the
perfect co-alignment of all the segments, effectively creating a giant
monolithic mirror. In addition, the actuators need to provide the resolution
and accuracy necessary for phasing the primary mirror and holding it against
perturbations. These latter are dominated by the effect of wind across the
front surface of the primary mirror and by possible vibrations arising from
machinery either directly under the segments or transmitted through the
structure to the primary segments. The co-alignment is done through two “edge
sensors” (Figure 4) on each side of each hexagonal segment that constantly
measure piston, gap and shear with respect to the adjacent segment to
nanometre accuracy and provide the necessary information to the control system
to activate the PACTs, allowing the segments to work together to form a
perfect imaging system. The edge sensors are non-contact devices which measure
relative segment positions at the border between adjacent segment. They are
made of two major components: 1. a sensing head, consisting of two parts,
called by the generic terms “transmitter” and “receiver”; 2. a control
electronic, including all the necessary cabling, providing the piston, gap and
shear measurements, as well as diagnostic data. The “transmitter” and the
“receiver” are located on the adjacent edges of two neighbouring segments and
are facing each other. There are six “transmitters” and six “receivers” per
segment, except for the segments located at the inner and outer contours of
the M1 mirror. All in all, M1 features 798 segments, almost 2500 PACTs and
about 9000 edge sensors (4500 pairs), not including the seventh sector and the
spares. More details on the M1 mirror can be found in [1].

Figure 3: A dummy M1 segment assembly in the ESO Integration Lab in Garching,
Germany, with position actuators (in black), whiffletree (above them), and two
test edge sensors (Figure 4) just below the segment. Image credit: ESO. Figure
4: The M1 edge sensors, which are designed and manufactured by the FAMES
consortium (comprising Fogale Nanotech in France and Micro-Epsilon in
Germany), are the most precise edge sensors ever designed for a telescope. Two
on each side of each segment, the sensors can detect when the segments move
out of position, even if only by a millionth of a millimetre. This photo shows
an edge sensor test model. Image credit: FAMES (Fogale Nanotech, Micro-
Epsilon).

####  2.2.2 M2 and M3

The ELT’s M2 mirror, with its 4.25 m diameter, will be the largest secondary
mirror ever used on a telescope, and the largest convex mirror ever produced.
For comparison, the secondary mirrors on the 8 m VLTs are just over 1 metre in
diameter. M2 is larger than the primary mirror of ESO’s VISTA telescope and
indeed the primary mirrors of many other telescopes that are operating today.
There is also the added challenge that M2 will hang upside-down over the 39 m
M1 mirror, about 60 m above the ground, held in mid-air by its support
structure (called the M2 cell) and anchored to the telescope main structure.
The M3 mirror is similarly large and complex, with its 4 m diameter. The
mirrors alone weigh more than 3 tonnes each but with the cell and structure
the overall weight of each assembly is about 12 tonnes. Both M2 and M3 are
produced by the German company SCHOTT and, as is the case for M1, are also
made of ZERODUR©.

The M2 mirror is a convex 4.25 m F/1.1 thin meniscus, about 100 mm thick, with
an 800 mm central hole. Its optical surface shape is very aspheric, with a
departure from a sphere that is close to 2 mm. The size, convexity, aperture
ratio and asphericity make this mirror extremely difficult to polish and test.
The M3 mirror is a concave 4.0 m F/2.6 thin meniscus, about 100 mm thick, with
a 30 mm central hole. Its optical surface shape is mildly aspheric, with a
departure from a sphere of only about 30 $\umu$m. Besides its 4 m size, the M3
mirror is easier to manufacture and test compared to the M2, and the required
M3 mirror production and metrology processes are more common. More details on
the M2 and M3 mirrors can be found in [2].

####  2.2.3 M4

Figure 5: A rendering of M4, the main adaptive mirror of the ELT. Image
credit: ESO.

M4 (Figure 5) is the main adaptive mirror of the telescope, meaning that its
surface can be deformed to correct for atmospheric turbulence, as well as for
the fast vibration of the telescope structure induced by its motion and the
wind, and is one of the most challenging and exciting ELT components. More
than 5,000 actuators will be used to change the shape of the mirror up to
1,000 times per second. In combination with the M5 mirror, M4 forms the core
of the AO of the ELT (Section 2.3.1). With a diameter of 2.4 m, M4 will be the
largest deformable mirror ever built. By comparison, current adaptive mirrors
are just over 1 m in diameter (e.g., the 1.1 m M2 adaptive secondary on the
VLT UT4 telescope).

M4 consists of six ultra-thin segments, or petals, made of Zerodur©, like the
M1 segments (Section 2.2.1). Because the M4 petals are so thin and need to be
deformed to incredible accuracy, they require a very stable structure to
support them: a reference body with attached magnets that support the mirror
and adjust its shape. Initial studies have demonstrated that to achieve this
highly demanding level of performance only specialist materials can be used
for the mirror, either silicon carbide (SiC) or ultra-low-expansion glass
machined in a special way to make it extremely lightweight. The French company
Safran Reosc has developed a design for the M4 mirror using Boostec® SiC. This
material is well known for its high stiffness (stiffer than steel, carbon
fibre or beryllium) and low density, properties that make the mirror very
lightweight. SiC has been used for many space telescopes and the Herschel
Space Observatory primary mirror is a good example of its technological
feasibility.

Adaptive mirror technology was translated into an industrial astronomical
product more than two decades ago by the Italian companies Microgate s.r.l and
ADS, internationally known under the consortium name of AdOptica. Many 8 m
telescopes now have a metre-scale adaptive mirror. The same technology is now
being adapted to serve the ELT to produce a mirror with an area five times
larger. The M4 mirror uses the same principle as a loudspeaker: the mirror is
made of a very thin shell levitating 100 $\umu$m away from its reference
surface and acting like a membrane, which deforms under the effect of about
5,000 voice coil actuators. These are a type of direct drive linear motor and
the name “voice coil” comes from one of its first historical applications,
vibrating the paper cone of a loudspeaker, consisting of a permanent magnetic
field assembly and a coil assembly. The current flowing through the coil
assembly interacts with the permanent magnetic field and generates a force
that can be reversed by changing the polarity of the current. Depending on the
current injected into the coil the mirror can be pushed or pulled up to a
distance of 90 $\umu$m from its mean position. With the help of a very fast
and precise set of capacitive sensors and amplifiers that are co-located with
the voice coil actuators, the mirror’s position is measured 70,000 times per
second to an accuracy of a few tens of nm with the actuators being driven up
to 1,000 times per second. More details on the M4 mirror can be found in [3].

####  2.2.4 M5

M5 is the “field stabilisation” unit of the telescope. This means that the
mirror is moving in a rigid way (tip-tilting) to steer the image and correct
for vibrations of the telescope structure induced by its motion and by the
wind, as well as some of the atmospheric turbulence. This is achieved by the
M5 cell which is composed of three piezo actuators. It supports and moves the
M5 mirror up to 10 times per second. M5 is a flat, elliptical mirror with a
diameter of 2.2 m on the minor axis and 2.7 m on the major axis. The M5 mirror
assembly needs to be very stiff and at the same time very light (less than 500
kg in total) to allow its cell to move it fast enough whilst remaining flat
when moving. The French company Safran Reosc is manufacturing the M5 mirror
assembly using Boostec® SiC, as for the M4 mirror, while the Spanish company
SENER Aeroespacial is responsible for the M5 cell. More details on the M5
mirror can be found in [4].

###  2.3 Telescope Systems

####  2.3.1 AO

Turbulence in the Earth’s atmosphere causes the stars to twinkle in a way that
blurs the finest details of the cosmos. Observing directly from space can
avoid this atmospheric blurring effect, but the high cost of building,
launching, and operating space telescopes compared to using ground-based
facilities limits the size and scope of the telescopes we can place off-Earth.
Astronomers have turned to a method called adaptive optics. Sophisticated,
deformable mirrors controlled by computers can correct in real-time for the
distortion caused by the turbulence of the Earth’s atmosphere, making the
images obtained almost as sharp as (or, in the case of the ELT, sharper than)
those taken in space. AO allows the corrected optical system to observe finer
details of much fainter astronomical objects than is otherwise possible from
the ground. AO requires a fairly bright reference star that is close to the
object under study. This reference star is used to measure the blurring caused
by the local atmosphere so that the deformable mirror can correct for it.
Since suitable stars are not available everywhere in the night sky,
astronomers can create artificial stars instead by shining a powerful laser
beam into the Earth’s upper atmosphere. Thanks to these laser guide stars, of
which the ELT will have up to eight, almost the entire sky can now be observed
with adaptive optics.

There are various flavours of AO. Some of them are: 1. the classical Single
Conjugated Adaptive Optics (SCAO) mode, where the turbulence corrected for is
that in a cylinder projection of the telescope aperture towards a guide star.
SCAO benefits from high performance, but across a small field of view; 2. the
Laser Tomography Adaptive Optics (LTAO) mode, which is a technique similar to
executing a computerized tomography scan of the atmosphere, as it allows the
telescope to reconstruct the turbulence above it layer by layer. It uses laser
guide stars instead of a natural one and implements several off-axis wavefront
sensors; 3. the Multi-Conjugate Adaptive Optics (MCAO) mode, which uses
multiple off-axis wavefront sensors to measure the turbulence in several
directions. These measurements are fed to a tomographic reconstructor (using
both multiple lasers and stars) that estimates the 3D turbulence, which is in
turn projected onto the deformable planes to generate the control commands.
This mode offers medium-to-high performance across a medium FoV.

From the largest adaptive mirror ever built to advanced control systems, the
ELT will have some of the most sophisticated technologies ever employed on a
telescope to correct for the blurring effects of the Earth’s atmosphere. M4
and M5 are at the heart of adaptive optics corrections on ESO’s ELT (see
Sects. 2.2.3 and 2.2.4).

####  2.3.2 Prefocal Stations

Figure 6: Computer rendering of the first ELT prefocal station (PFS-A) on top
of its supporting platform. Image credit: ESO/L. Calçada, IDOM.

The ELT requires not only sophisticated mirrors and advanced science
instruments, but also a tight link between these two components. This link
takes the form of two massive structures, the prefocal stations (PFS-A and
PFS-B), which stand over 12 m high and sit on the platforms, called Nasmyth
platforms, on either side of the ELT’s giant tube structure (Figs. 6 and 2).
The name comes from the fact that the prefocal stations are the last
components in the telescope’s light path, right before the light comes to a
focus. One of the most important functions of a prefocal station is to
distribute light to the appropriate science instrument, which will then
capture and analyse it. The instruments themselves are also on the two Nasmyth
platforms, and the light is distributed to them via flat mirrors on the
prefocal stations. Another main function of the PFS is sensing star light to
control the telescope mirror alignment when observing. The PFS observes stars
close to the target, known as guide stars, to check that the ELT’s mirrors are
aligned and that the telescope is correctly pointing at the target. The PFS
also hosts a phasing and diagnostic station (PDS), which is equipped with
special sensors used for testing the telescope before and during operation. As
the name indicates, the PDS is used for general diagnostics, and to ensure
that the giant primary mirror of the ELT (composed of 798 segments) maintains
an ideal shape and acts as one giant mirror, a process called phasing.

####  2.3.3 Controls

The many advanced mechanical and optical components, or subsystems, of the
ELT, are operated by means of a control system. This complex system integrates
the various components of the telescope and works as the user interface to the
ELT. It provides coordinated, safe operation of the subsystems as a single
system to perform science observations and support engineering and maintenance
activities. The telescope control systems are also key in helping the ELT
obtain sharp images. The complex primary mirror control system monitors and
corrects the positions of its 798 segments, which are distorted by the effects
of gravity and temperature, so it continues acting as one mirror (Section
2.2.1). The M4 (Section 2.2.3) and M5 (Section 2.2.4) mirror control systems
are important for compensating for distortions in the images caused by
atmospheric turbulence and wind, an important part of the ELT’s adaptive
optics.

Overall, the control system manages around 15,000 actuators and over 25,000
sensors distributed throughout the telescope and dome. The communication
infrastructure allows the majority of the computing units to be hosted in the
computer room in the auxiliary building of the dome. This reduces vibration
and heat pollution in the telescope area and provides a controlled data centre
environment for the hardware. The control system itself is a software-
intensive system of systems, which presents a complex challenge in terms of
algorithms and computational performance, organisation of the overall system,
and its behaviour and interactions.

##  3 Instruments

The suite of instruments planned for the ELT includes cameras, to capture
images, and spectrographs, which disperse light into its component colours.
Four of the instruments, HARMONI, MICADO, MORFEO, and METIS, the first
generation, will start to operate at or shortly after ELT technical first
light while an additional two, ANDES and MOSAIC, will start operations at a
later stage. Throughout the telescope’s lifetime, other instruments will be
installed on the telescope to study the Universe in ever more detail, one of
them being the Planetary Camera and Spectrograph (PCS: [5]). This will be
dedicated to detecting and characterising nearby exoplanets with sizes from
sub-Neptune to Earth-size in the neighbourhood of the Sun through a
combination of eXtreme Adaptive Optics (XAO), coronagraphy, and spectroscopy.
Figure 7 summarizes the main features of ELT instruments.

Figure 7: A summary of the main features of the ELT instruments. PDR stands
for Preliminary Design Review while FDR stands for Final Design Review. Image
credit: ESO.

###  3.1 HARMONI

The High Angular Resolution Monolithic Optical and Near-infrared Integral
field spectrograph (HARMONI: [6]) is the visible and near-infrared (NIR),
adaptive-optics-assisted, integral field spectrograph for the ELT. It will
have both an SCAO mode (using a single bright natural guide star) and an LTAO
mode (using multiple laser guide stars: Section 2.3.1), to correct for
atmospheric turbulence, providing near diffraction-limited hyper-spectral
imaging. With its 3D spectroscopic capabilities and a variety of spatial and
spectral settings, HARMONI will be a very powerful and versatile first-
generation instrument. HARMONI will use an Integral Field Unit (IFU) with an
image slicer to divide a single contiguous FoV into many spatial pixels
(called spaxels). The signal from each spaxel will be fed into a spectrograph
that generates a spectrum for each one of them. Four spaxel scales (from 4 mas
to 60 mas), resulting in different FoVs, will be available, with smaller
spaxels corresponding to a smaller FoV but a higher spatial resolution. A
range of spectral resolution settings (3000, 7000, 18000) will also be
available in all spatial scales covering wavelengths between 0.47 and 2.45
$\umu$m, though not simultaneously.

###  3.2 MICADO

The Multi-AO Imaging Camera for Deep Observations (MICADO: [7]) will make the
most of the giant ELT’s full resolution potential. It will take images at an
unprecedented depth, thanks to the combination of the ELT’s large primary
mirror and the correction that will be provided by the MORFEO (Section 3.3) AO
system. The instrument will go far beyond the capabilities of the best
observatories that we have today, including HST, and its sensitivity will be
comparable to that of the James Webb Space Telescope (JWST) but with six times
the resolution.

The design of MICADO was driven by a desire for high sensitivity, resolution,
astrometric accuracy, and wide wavelength coverage spectroscopy. Offering
almost an arcminute-squared FoV with 4 mas pixels to sample fully the
diffraction limit of the ELT, MICADO takes advantage of the wide-field
correction and uniform point spread function offered by the MCAO module,
MORFEO. A second, finer, plate scale (1.5 mas) will be used to help MICADO
reach challenging requirements on astrometric precision (50 $\umu$mas). In
addition to a large number of broad and narrow band filters for imaging (up to
30) covering the 0.8-2.45 $\umu$m wavelength range, MICADO also offers a wide-
band spectroscopic mode. In single-slit spectroscopy, MICADO covers this
wavelength range with a resolving power of 20,000. In common with the other
early ELT instruments, it will have a coronographic capability to block
starlight so that dimmer objects close to these stars, exoplanets, for
example, can be seen more easily. A second adaptive-optics mode, SCAO, will be
provided by MICADO and MORFEO. This mode is particularly suited to
observations with the coronagraph, as it will provide excellent image quality
over a smaller FoV.

###  3.3 MORFEO

The Multiconjugate adaptive Optics Relay For ELT Observations (MORFEO,
formerly known as MAORY: [8]) will help compensate for the distortion of light
caused by turbulence in the Earth’s atmosphere which makes astronomical images
blurry. MORFEO will not make observations itself; rather, it will enable other
instruments, such as MICADO (Section 3.2) in the first instance, to take
exceptional images. MORFEO will use both M4 and two other deformable mirrors
and other state-of-the-art systems to correct for different layers of
turbulence high above the ELT. To measure atmospheric turbulence, MORFEO will
observe three natural guide stars around the scientific FoV of the camera
(MICADO) but it will also use six laser guide stars to reach the demanding
image quality required for MICADO’s ambitious science goals. The distortion of
light by atmospheric turbulence will be measured by wavefront sensors using
newly developed detectors capable of reading images hundreds of times per
second with low noise. MORFEO will use six large mirrors including the two
extra post-focal deformable mirrors to deliver light from the focal plane of
the telescope to the focal plane of MICADO. By tilting one of those mirrors,
it will be possible to steer the light beam towards MICADO or a second client
instrument.

###  3.4 METIS

The Mid-infrared ELT Imager and Spectrograph (METIS: [9]) will provide the ELT
with a unique window to the mid-infrared (MIR: $3-13~{}\umu$m) band. Its SCAO
system will enable high contrast imaging and IFU spectroscopy at the
diffraction limit of the ELT. The instrument consists of two separate units,
one for the imager and another one for the spectrograph. METIS will provide:
1. imaging in the L/M ($3-5~{}\umu$m) band, together with medium resolution
($R\sim 1,000$) slit spectroscopy as well as coronagraphy for high contrast
imaging; 2. imaging in the N ($7.5-13.5~{}\umu$m) band, with low resolution
($R\sim$ a few hundred) slit spectroscopy as well as coronagraphy for high
contrast imaging; 3. high resolution ($R\sim 100,000$) IFU spectroscopy in the
L/M band, including a mode with extended instantaneous wavelength coverage
(assisted by a coronagraph).

###  3.5 ANDES

The ArmazoNes high Dispersion Echelle Spectrograph (ANDES, formerly known as
HIRES: [10]) baseline design is that of a modular fibre-fed cross dispersed
echelle spectrograph with three ultra-stable spectral arms, visual and NIR,
providing a simultaneous spectral range of $0.4-1.8~{}\umu$m at a spectral
resolving power $R\sim 100,000$ for a single object. ANDES will also include
an IFU mode fed by an SCAO module to correct for the blurring effect of
turbulence in the atmosphere. ANDES will separate light from the ELT mirrors
into the wavelength channels using dichroic filters. Each wavelength channel
interfaces with several fibre bundles that feed the corresponding spectrograph
module (visual and NIR).

###  3.6 MOSAIC

The Multi-Object Spectrograph (MOSAIC: [11]) is a versatile multi-object
spectrograph that will use the widest possible FoV provided by the ELT. It
will have three operating modes covering observations in the visible and IR
bands for more than one hundred sources simultaneously. Namely: 1. a high
multiplex mode in the visible (HMM-VIS), with simultaneous integrated-light
observations of almost 200 objects, with the possibility of using both medium
and high spectral resolving power ($R\sim 5,000$ and 20,000); 2. a high
multiplex mode in the NIR (HMM-NIR), with simultaneous integrated-light
observations of 80 objects (goal: 100) each of them with dedicated sky fibres
to subtract the strong sky background present in the NIR. This mode will cover
simultaneously the $0.8-1.8~{}\umu$m wavelength range with the possibility to
observe in medium and high spectral resolving power; 3. a high-definition mode
(HDM), with simultaneous observations of eight IFUs (goal: 10 IFUs) deployed
within a $\sim 40$ arcminute patrol field. Each IFU will cover a 2.5 arcsecond
hexagon with $\sim 200$ mas spaxels, sharing the same spectrograph as for the
HMM-NIR, with the same wavelength coverage and spectral resolving power.

##  4 Science

The ELT will tackle the biggest scientific challenges of our time, and will
aim for a number of notable firsts, including tracking down Earth-like planets
around other stars in the habitable zones where life could exist, which is one
of the Holy Grails of modern observational astronomy. It will also make
fundamental contributions to cosmology by probing the nature of dark matter
and dark energy. Other key science areas include the study of stars in our
Galaxy and beyond, black holes, evolution of distant galaxies, up to the very
first galaxies in the so-called “Dark Ages”, the earliest epoch of the
Universe, which started 380,000 years after the Big Bang, and watching the
Universe expand in real time. On top of this astronomers are also planning for
the unexpected: new and unforeseeable questions will surely arise, given the
capabilities of the ELT.

###  4.1 Solar System

Although the most detailed information regarding objects in our solar system
has generally come from in situ (or close-by) interplanetary missions (to,
e.g., Venus, Mars, Jupiter, Saturn, Pluto and Charon, comet 67P/Churyumov-
Gerasimenko), ground-based and space observatories have complemented these
studies with new discoveries, increasing their scientific return. The ELT will
be no exception, as it will peer through planetary atmospheres, study volcanic
activity and watery plumes on the moons of Jupiter and Saturn, and probe the
origin of asteroids and comets. By taking repeated high-resolution images and
spectra of planets and moons with evolving surfaces and atmospheres, the
telescope will be capable of assembling a unique atlas of hundreds of Solar
System objects.

As the ELT will have a much larger sensitivity and resolution than the current
generation of large telescopes, it will be particularly useful for studying
the faintest objects in the Solar System, including those in the outer System,
Neptune, Uranus and outer asteroids and comets, many of which have not been
explored in detail with space missions. Furthermore, objects in the Asteroid
Belt between Mars and Jupiter and the Kuiper Belt beyond Neptune provide a
window into the properties of the protoplanetary disc from which Earth and the
other planets formed 4.5 billion years ago. Using the ELT to explore these
objects, we will gain a better insight into the creation and evolution of the
Solar System.

###  4.2 Exoplanets

Figure 8: Effective temperature of the host star (or brown dwarf) estimated
via the Stefan-Boltzman law vs. the distance to the system in parsecs. The
thermal background limit of METIS for reasonable integration times sets the
largest distance to which Earth-like planets can be detected in the liquid-
water zone. As the equilibrium temperature of a planet increases with stellar
luminosity, the habitable zone as bounded by the diffraction limit can be
surveyed to greater distances around more luminous stars. The maximum contrast
limits a search for Earth-like planets in the liquid-water zone to stars with
$M<2-3~{}M_{\odot}$, where $M_{\odot}$ is one solar mass. The METIS IFU can
offer improved contrast at high spectral resolution enabling detection of
unique planets like those around Proxima Centauri. The second generation
ELT/PCS instrument will be more sensitive and will study liquid-water zone
planets in scattered light. It will have some overlap with a NASA future Great
Observatory recently dubbed the Habitable Worlds Observatory (HWO). The colour
of the points codes temperature from blue (hot) to red (cool). The size of the
circles are related to the solid angle of the source, i.e. its surface area
divided by its distance. Image credit: Ignas Snellen and the METIS Science
Team.

The first exoplanet orbiting a solar-type star (51 Pegasi) was discovered in
1995 by Michel Mayor and Didier Queloz, who received the Nobel Prize in
Physics in 2019 for this
breakthrough555https://www.eso.org/public/announcements/ann19049/. Since then,
almost 6,000 planets (at the time of this writing) have been found orbiting
stars other than our Sun, from super Jupiters to Earth and Mars-size planets.

Most exoplanets are detected indirectly by the radial velocity method, which
detects planets by the wobble they produce on their parent star as they orbit
it. Another method used by astronomers is the transit technique, which detects
the drop in brightness of the host star when a planet passes in front of it.
However, these detections are indirect and very limited in the information
they can provide about the planet itself. In addition, very few direct
observations of planets have been made.

The ELT will revolutionise the study of planets outside our Solar System, as
it will allow us to obtain direct images of some of these systems. Thanks to
the very large collecting power of the ELT, we will also be able to detect and
characterise the fingerprints of transiting planets’ atmospheres as they pass
in front of their parent star. Moreover, the ELT will allow us to observe and
characterise exoplanets in habitable zones, i.e. in the regions where the
distance from the central star is such that liquid water can exist on the
planet’s surface (Figure 8). It could even become the first telescope to find
evidence of extraterrestrial life on these planets.

###  4.3 Stars

Figure 9: Comparison of how crowded stellar fields would appear when observed
by the HST (left), the JWST (centre), and MICADO (right). The bottom row
matches the stellar density at 4-5 $R_{\rm eff}$ (the radius which encloses
half the light) for NGC 4472 in the Virgo Cluster (i.e. at the edge of the
galaxy) and represents the limit of JWST resolution. The top row corresponds
to 2 $R_{\rm eff}$ in the same galaxy (i.e. closer to the centre) and many
individual stars can still be measured by MICADO. Each panel is 1 arcsecond
across. $\mu$ is the surface brightness in magnitudes per square arcsecond.
These simulations were performed with ScopeSim. Image credit: ESO/MICADO
consortium (see also [7]).

Stars with different characteristics including their evolutionary state,
chemical composition, and mass, emit nearly all the visible light in the
Universe. However, we can resolve and study thoroughly individual stars in
stellar populations only in nearby systems. Similarly, only for these
populations can we characterise properly their formation, evolution, and end
products.

With the ELT, astronomers will be able to study star-forming regions in
unsurpassed detail at distances ten times larger than is possible today
(Figure 9). By enabling a closer look at how stars are born and evolve, the
ELT will help us make substantial progress in the study of the early phases of
star formation. Astronomers will use the ELT to measure the ratios of
different chemical elements within stars, which will enable them to precisely
measure their ages and chemical evolution. Comparing this against the stars’
mass will help us better understand the lifecycle of different types of stars.
With its extraordinary collecting power, the ELT will also be able to observe
faint brown dwarfs, which trace the transition between stars and giant
planets. The ELT will bring us closer to a full understanding of the evolution
of planets and stars ranging from Jupiter mass to tens, and probably hundreds,
of solar masses. Enormous stars end their lives violently in supernova
explosions that spew out chemical elements that influence the formation of
later stars and planets. Supernovae are some of the most luminous events in
the Universe, meaning they can be seen at great distances and used as
signposts of the Universe’s evolution. The very biggest stars create
$\gamma$-ray bursts when they explode; as the most energetic objects in the
Universe these are also some of the most distant objects ever observed. The
collecting power of the ELT will allow us to use them as distant lighthouses,
shining through billions of years of evolution and taking us into the largely
unknown early epoch of the Universe’s Dark Ages.

Using the ELT, for the first time, astronomers will be able to look beyond our
Galactic neighbourhood to investigate individual stars in more distant
galaxies and trace their history back to the very early Universe. As well as
unravelling the complex formation and evolution of different types of
galaxies, this will also allow us to compare our neighbourhood with other
parts of the Universe.

###  4.4 Black Holes

Figure 10: Black hole mass vs. angular size distance. Spatial resolution
assumes the diffraction limit case and scales with the telescope diameter. The
ELT (red) line is for $\theta_{\rm res}=10$ mas (a value intermediate between
the two wavelength-dependent extremes), the 8 m - VLT (blue) one is for
$\theta_{\rm res}=50$ mas (ERIS), while the 6.5 m JWST (black) one is for
$\theta_{\rm res}=60$ mas. All lines correspond to the value required to
resolve the so-called “sphere of influence”. The regions of parameter space
within reach of the different facilities is the area to the left of the lines.
For illustration, we show the location of the SMBH in M 87, a giant elliptical
galaxy in the Virgo constellation whose nucleus contains a black hole, the
first ever to be directly imaged [12]. Sgr A∗, the SMBH at the Galactic Centre
of the Milky Way, at a distance of $\sim 8$ kpc, is off-scale to the left by
two orders of magnitude. Image credit: ESO.

Black holes are extremely common throughout the Universe. Indeed, a
supermassive black hole (SMBH) lies at the centre of almost every large
galaxy, including the Milky Way, while some of the less massive black holes
are thought to form when massive stars reach the end of their lives. Since
these objects are black, i.e, they do not directly emit nor reflect light,
astronomers rely mostly on indirect observations to spot their presence and
study them. For example, they can infer much about a black hole by tracking
the movements of stars and gas around it, something the ELT will excel at.

The centre of the Milky Way is a unique laboratory for exploring gravity
around the closest SMBH, Sagittarius A∗, with a mass $\sim 4\times
10^{6}M_{\odot}$. The ELT will enable astronomers to build on research done
with ESO telescopes on the Galactic Centre, which was recognised with the 2020
Nobel Prize in Physics666https://www.eso.org/public/news/eso2017/. A dense
cluster of stars surrounds the SMBH, and the ELT will enable astronomers to
study the behaviour of these stars in their strange environment with a level
of detail and quality that we cannot reach with smaller telescopes. The
Galactic Centre also provides us with a place to study the accretion of matter
onto SMBHs, as well as to better understand the relationship between their
activity and star formation at the centre of galaxies. ELT’s MICADO will excel
when compared with GRAVITY, the VLT interferometric instrument, which has
already delivered several important results on the Galactic Centre [e.g. 13].
While GRAVITY in fact offers a higher resolution, MICADO will have a much
higher sensitivity and larger FoV, making it possible to track many more stars
orbiting Sagittarius A∗ than currently possible. MICADO will deliver high-
quality images with 12 mas resolution with a superb sensitivity over a FoV
view tens of arcsecond across.

Another open question awaiting the advent of the ELT is the existence and
demographics of intermediate mass ($100-10,000M_{\odot}$) black holes. These
black holes represent a link currently missing between stellar-mass black
holes and SMBHs, and they could serve as seeds in the early Universe for the
formation of the SMBHs that we see today. The ELT will be able to accurately
measure the 3D velocities of stars in massive star clusters and dwarf
galaxies, where these intermediate mass black holes are thought to reside,
allowing astronomers to find out more about them.

The ELT will also study the role SMBHs play in the formation and evolution of
galaxies and structures in the Universe. The centres of most galaxies harbour
SMBHs with masses $>10^{6}M_{\odot}$. SMBHs will be characterised out to large
distances with the ELT, also through the resolution of the so-called “sphere
of influence”, i.e. the region where black holes directly influence the
motions of stars and gas through their gravity (Figure 10). For example, the
volume within which the sphere of influence of a $10^{9}M_{\odot}$ SMBH will
be resolved will increase by a factor of 75 as compared to what can be done
now with the VLT, leading to a comparable increase in the number of accessible
SMBHs. This will allow us to trace the build-up of supermassive central
objects in galaxies when the Universe was as young as a quarter of its present
age.

Figure 11: Simulated observations of absorption due to O I, C II, and Si II at
$z\sim 7$ using 8 m VLT+X-shooter (top) and ANDES (bottom). The thin red line
at the top of each panel shows the predicted model spectrum generated from a
cosmological simulation. The signals have been smoothed by the instrumental
resolutions. Signal-to-noise ratios have been calculated assuming a 25 (5)
hour integration with X-Shooter (ANDES) on a $z=7$ source with J${}_{\rm
AB}=20$, similar to the quasar ULAS J1120+0641. No signal is detected in the
X-shooter spectrum. In contrast, numerous narrow lines are detected with
ANDES. Image credit: Reproduced from ref. [14], with permission.

###  4.5 Galaxies

Figure 12: A simulation of how a galaxy in the early universe would look like
when observed with a VLT instrument and HARMONI with three different spaxel
scales and LTAO. Red is H$\alpha$, green is the V band, and blue is the B
band. Image credit: ESO/HARMONI consortium.

Galaxy formation and evolution is driven by a number of mechanisms and a very
complex interplay between them. These include the hierarchical merging of dark
matter halos, the accretion and cooling of gas, and the gravitational
fragmentation and the formation of molecular clouds. Star formation,
nucleosynthesis, metal-enriched outflows driven by stellar winds, supernova
explosions and energetic output from accretion onto SMBHs also influence
galaxy formation and evolution.

Thanks to the large number of recent deep surveys, it has become easier to
derive galaxy properties such as luminosities, sizes, star formation rates,
masses, and stellar masses, as well as their redshift evolution. We now need
more detailed spatially and spectrally resolved studies of individual galaxies
to improve our understanding of their underlying physical processes. A
comprehensive picture of galaxy formation should also describe a galaxy’s
internal structure, for example, how stellar populations and dust are
distributed or what properties its central region has. Current facilities,
however, are not able to describe these internal properties in detail in
distant galaxies as they cannot reach the required small spatial scales, i.e.
that of giant molecular clouds; the ELT will enable us to do just this.

HST has extended our view of evolving galaxies from the optical to the NIR,
probing the emission from many stars in galaxies at high redshift and
resolving the shapes and colours of galaxies. At the same time, IFUs, such as
those at ESO’s VLT, have routinely mapped the motion of ionised gas and the
physical conditions of galaxies. The ELT will allow astronomers to observe
fainter and more distant galaxies, up to when they were first building up
their stellar populations and gas reservoirs, to improve substantially our
current picture of galaxy evolution. The ELT will also study in great detail
the gas in the intergalactic medium as revealed by the numerous absorption
lines that are seen in the spectra of quasars (Figure 11).

Characterising galaxies with a wide range of masses, star formation
activities, and surroundings requires the giant leap in spatial resolution
uniquely provided by the ELT, with its 39 m main mirror and diffraction limit
of 14 mas ($\sim$ 100pc at $z>1$) in the K-band. Both imaging and IFU (with
MICADO and HARMONI) will enable views of galaxies across cosmic times
respectively $\sim$ 15 and $\sim$ 6 times sharper than possible with HST and
JWST. HARMONI will overcome present limitations by tracing structures just a
few tens of parsecs wide, and providing detailed studies by exploiting
spectroscopic information with high angular and spectral resolution and
sensitivity. Figure 12 shows a simulation of an Antennae-like galaxy at $z\sim
2$ seen with a VLT instrument and with HARMONI.

###  4.6 Cosmology and Dark Matter

Standard probes of the expansion of the Universe include weak gravitational
lensing and the signature that light imprinted shortly after the Big Bang on
today’s distribution of galaxies, and type Ia supernovae. The ELT will
contribute to current efforts to measure the acceleration of the Universe by
characterising high-redshift Type Ia supernovae identified by JWST and other
survey facilities. But the ELT will also map the expansion history of the
Universe using a whole different method, namely by watching the Universe
expand in real time.

The current standard model of cosmology is the so-called Lambda-CDM model,
wherein the Universe today is flat and consists of approximately 5% baryons,
$\sim 25$% cold dark matter, and $\sim 70$% dark energy. This implies that the
Universe is not only expanding but also accelerating, due to the effects of a
force never seen in the laboratory, which among other things works as an
“anti-gravity” by providing negative pressure. This requires entirely new
physics, and consequently has sparked intense interest in mapping the
expansion history of the Universe with high precision.

Extracting information about the Universe’s expansion from standard probes
relies on assumptions about the curvature of space, depends on the adopted
cosmological model, and can only estimate the average expansion history over
long time periods. A model-independent approach that measures the expansion
rate directly was proposed as early as the 1960s [15], but limitations in
technology have meant that astronomers have not been able to make such a
measurement in practice. This approach is known as redshift drift, and is a
method that offers a truly independent and unique approach to exploring the
expansion history of the Universe. The redshift of the spectra of distant
objects is an indication of the expansion of the Universe, but the change in
this redshift over time is a measure of the change in the rate of expansion
and depends on the underlying geometry: different Universes will display
different drifts. For example, if the Lambda-CDM model is correct the drift
will switch from positive to negative at $z\approx 2$ but if there is no dark
energy the drift will always be negative [16].

However, the estimated size of this redshift drift over a decade is only about
10 cm/s. Such a signal is about 10-20 times smaller than today’s large
telescopes can measure in such distant galaxies. The huge light-collecting
area of the ELT, coupled with new developments in quantum optics to record
ultra-stable spectra, means that this amazing measurement will lie within
reach of the ELT. All current methods to measure the expansion and shape of
the Universe have to rely on various intermediate steps, as in the case of
type Ia supernovae. Astronomers will use the redshift drift method with the
ELT’s ANDES instrument to determine the details of the accelerating expansion
of the Universe and its shape directly and in real time, bypassing in one go
all the intermediate steps, which are currently required to do this and
thereby allowing us to quantify the nature of the dark energy responsible for
the acceleration. Once a first epoch of observations is made, the redshift
drift signal grows linearly with time. Hence, in the very long run (on the
timescale of many decades) the redshift drift may well overtake the ability of
other methods to constrain the expansion history of the Universe.

Apart from the fundamental conceptual importance of directly observing the
Universe’s expansion, the redshift drift provides a new and crucial
consistency test of the assumptions of our theories of cosmology. Two other
such tests that will be significantly improved by the ELT are measurements of
the temperature of the cosmic microwave background radiation and the
primordial abundances of light elements.

Astrophysical evidence for dark matter halos around galaxies first emerged in
the 1930s with studies of galaxy rotation curves, which plot the velocities of
stars and gas against their distance from the centre of their host galaxy.
Such studies still play an important role today. They require high-resolution
observations of kinematics from the inner parts of galaxies to constrain the
distribution of baryonic, or visible, mass. At higher redshifts, rotation
curves are not currently resolved enough to constrain the fraction of dark
matter. However, using data from the SINFONI and KMOS instruments on ESO’s
VLT, recent studies tried to do this in the inner regions of distant galaxies
using both seeing limited and AO-assisted observations, as well as stacking
several images together to reduce noise [17, 18]. Their results point towards
a low fraction of dark matter, a finding that is supported by the observation
of decreasing rotation curves. However, these results remain controversial
because the signal-to-noise ratio is low, and the spatial resolution is rather
coarse.

ELT’s HARMONI will be the first instrument able to reach the spatial
resolution needed to disentangle the visible matter distribution from the dark
matter distribution for high redshift galaxies. Mass model distributions will
provide the measurements of dark halo central density and core radius for
galaxies in a redshift range that is not reachable with 8-10 m class
telescopes. Astronomers will be able to use the ELT to measure the shape of
dark matter halos as a function of galaxy mass assembly history, cosmology,
and environments. Detailed simulations show that HARMONI will be able to study
spatially resolved kinematics with sufficient details and signal to noise
ratio to perform mass models and recover the shape of dark matter halos down
to stellar masses of $10^{9}M_{\odot}$ at $z\sim 1.4$ and $10^{9.5}M_{\odot}$
at $z\sim 2.7$ using the H$\alpha$ line in just 2 hours of exposure. A multi-
object spectrograph like the ELT’s MOSAIC instrument will allow us to expand
the statistical sample of galaxies, as well as extending studies of rotation
curves and dark matter profiles.

###  4.7 Fundamental Physics

A fundamental constant of nature is any quantity whose value cannot be
calculated using a given physical theory but can only be determined
experimentally. Examples include the fine-structure constant
$\alpha=e^{2}/(2\epsilon_{0}hc)$, where $e$ is the elementary charge,
$\epsilon_{0}$ and $h$ are the electric and Planck constants, and $c$ is the
speed of light (all of them also fundamental constants), which quantifies the
strength of the electromagnetic interaction between elementary charged
particles, the proton-to-electron mass ratio, $\umu=m_{\rm p}/m_{\rm e}$, and
the gravitational (or Newton’s) constant $G$. Nothing prevents us from
thinking that the laws of nature, which depend on these constants, may vary
with time and location in the Universe.

Astronomical observations can probe much longer timescales than Earth-based
laboratory experiments and are therefore much more sensitive to possible time
variations in the fundamental constants. By exploring the spectra of extremely
bright distant quasars, for example, such possible variations can be probed
over a large fraction of the history of the Universe [see, e.g. 19, and
references therein]. However, making these observations is very challenging,
as it involves measuring the relative wavelength shifts of pairs of absorption
lines whose wavelengths have different sensitivity to the fundamental
constants, meaning that the wavelength calibration must be extremely accurate.

The ELT’s ANDES ultra-stable high-resolution spectrograph will essentially
remove the wavelength calibration uncertainties that plague current
measurements and will vastly improve the constraints on the stability of
fundamental constants. A discovery that the fundamental constants do vary
across space and/or time would have huge implications. It would mean, for
example, that Einstein’s equivalence principle, which states that it is
impossible to tell the difference between the effects of gravity and the
effects of acceleration, because gravity is a fictitious force, due to the
local geometry of spacetime, does not hold.

##  5 ELT Synergies

Astronomy is currently experiencing a golden age, thanks to a combination of
ground-based observatories and space telescopes that allow us to study the
Universe in all electromagnetic bands, including also gravitational waves,
neutrinos, and cosmic rays. The ELT will seamlessly integrate with a multitude
of astronomical facilities covering the whole electromagnetic spectrum, which
will include, besides the other two ELTs, ESO’s VLTs, and ALMA, the Square
Kilometre Array (SKA)777https://www.skatelescope.org/the-ska-project/ in the
radio, (hopefully still) JWST888https://webb.nasa.gov/ in the MIR,
Euclid999https://www.cosmos.esa.int/web/euclid (launched on July 1, 2023) in
the visible and NIR, the Vera C. Rubin Observatory (previously referred to as
the Large Synoptic Survey Telescope [LSST])101010https://www.lsst.org/ in the
visible, PLATO111111https://www.cosmos.esa.int/web/plato in the visible, the
Cherenkov Telescope Array (CTA)121212https://www.cta-observatory.org/ in the
$\gamma$-rays, and many more.

##  6 Conclusions

The ELT will be a revolutionary telescope and will be transforming Astronomy
by excelling in collecting power and angular resolution, and by being an AO
telescope and thereby providing space-like image sharpness. Located on Cerro
Armazones in Chile, close to ESO’s VLTs, it will also be the largest and
likely the first of the ELTs, with its 39 m diameter and planned first light
by the end of 2028.

It will be an amazingly complex machine, with its huge dome, five mirrors,
elaborate telescope systems and operations, and sophisticated instrumentation.
It will have extremely exciting and unique scientific capabilities covering
the whole of Astronomy, with a very strong community involvement and
significant synergies with many other astrophysical facilities. The ELT will
open brand new windows on our Universe. Historically, such new windows caused
revolutions in the way we understand Nature. We are ready for the next one.

Please see the ELT Webpage (https://elt.eso.org/) for updated news and
developments.

## Acknowledgement(s)

Making the ELT a reality is only being possible thanks to a collaboration
between a large number of different people in an enormous variety of roles,
ranging from scientists and engineers to project managers and technicians, at
ESO and across the ESO member states. The key responsibilities of some of the
team members are shown at https://elt.eso.org/about/team/. This paper would
not have been possible without them. There are many contractors and
institutions working for the ELT dealing with, e.g., the dome and the main
structure, the five mirrors, the instruments, the roads, software writing,
consultancy, etc. A complete list can be found here:
https://elt.eso.org/about/industrial/. We thank Richard Ellis and Olivier
Hainaut for many useful comments, Michael Meyer for producing Figure 8, and
Alessandro Marconi and Valentina D’Odorico for providing us with Figure 11.

## Disclosure statement

No potential conflict of interest was reported by the author(s).

## Notes on contributor(s)

Paolo Padovani
Paolo Padovani is Full Astronomer in the ELT Science office at ESO. After
getting his PhD at the University of Padova, Italy, in 1989, he worked at the
Space Telescope Science Institute (STScI), Baltimore, USA, at ESO, at the II
University of Rome, Italy, and again at STScI for the European Space Agency.
He finally (re-)joined ESO in 2003. He has authored a number of ELT
requirement documents and is currently the overall coordinator of the ELT
Working Groups, which bring together expertise from within ESO, the Instrument
Consortia, and the wider Community. He works on Active Galactic Nuclei over
the whole electromagnetic spectrum and has been recently expanding his
research interests to Neutrino Astronomy.

Michele Cirasuolo
Michele Cirasuolo is the ELT Programme Scientist at ESO and is responsible for
the scientific leadership of the telescope and its instrumentation. He has got
a long experience with astronomical instrumentation and has been instrument
scientist for the KMOS instrument and is principal investigator of the multi-
object spectrograph MOONS for the VLT in Chile. With a PhD in astrophysics and
two decades of active research, he has published extensively in the field of
galaxy formation and evolution across cosmic time.

## References

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