C. An expanded description of the work being done on the European Extremely Large
Telescope (E-ELT). By Sinead Donaghy.
The telescope has always been an important tool for scientists, allowing astronomers to observe
distant objects. It seems to represent the way in which our perspective of the Universe has changed
drastically over the last 400 years, with the rise of science showing us just how insignificant we are.
William Herschel said, on the discovery of Uranus in 1781, “I have looked further into space than
ever human being did before me”1. The telescope has constantly pushed science forward and
contributed to great leaps in the understanding of the universe. However it has also been of great
interest to engineers: the method of using mirrors and lenses to magnify and focus on distant objects
has been continuously refined, from the crude refractive telescope to Galileo’s reflective telescope
and further still to the complex and immensely powerful telescopes we have today2. The European
Extremely Large Telescope (E-ELT) is currently in construction and will be the largest telescope in
the world at 400m, collecting 13 times more light than existing optical telescopes3.
The aim of the project is to build a more effective telescope which will ultimately benefit the
scientific community. Astronomy and cosmology are very wide and varied fields so the telescope will
support numerous projects, and as the technology advances even more questions will be raised about
the universe we live in. One major project that the E-ELT will be used for will be finding and
studying habitable zones around stars where there could be planets that could sustain life. This could
be important because it could confirm whether there is life on other planets, and it could also be the
start of establishing a human civilization on another planet. Whilst observation of exoplanets has been
going on since 1995, the E-ELT will advance the field greatly in that it can directly observe planets as
opposed to using the radial velocity technique. The radial velocity technique involves looking at the
spectra of stars. A planet’s orbit around a star will affect the speed at which the star moves away from
the Earth, which is shown in the Doppler shift in the spectra of the star. However this method has
flaws in that it can only be used if certain conditions are met. For example, the star that the planet is
orbiting must have a sufficient number of absorption lines (which excludes a lot of hotter, younger
stars) and those absorption lines must be narrow (which also excludes younger stars as they show too
much rotational broadening). The star must also be very stable, as magnetic fields and stellar activity
can produce false readings4. Also, not a lot of information can be gained from radial velocity alone, as
it can only tell us the minimum mass of the planet. Whilst the E-ELT will probably continue to use
this method to a certain extent (especially in combination with other methods such as the transit
method), they will mostly use direct imaging which will be possible due to the adaptive optics in the
telescope. The main challenge for direct imaging is separating the light from the planet from the light
of the star and it requires a very high resolution telescope5. It is important that we are able to
accurately observe exoplanets because we could send humans to live there in the future. Knowing the
mass of the planet, its orbit, the temperature of its closest stars and its atmosphere is vital in order to
assess its suitability.
1
Constance Anne Lubbock Ed., The Herschel Chronicle: the Life-story of William Herschel and his Sister,
Caroline Herschel (New York, Cambridge University Press, 2013) page 336
2
Ryan Tweny, “How the telescope changed our minds” last modified 10/02/08,
www.wired.com/2008/10/how-the-telesco/
3
“The European Extremely Large Telescope (“E-ELT”) project” last modified 12/02/15,
www.eso.org/sci/facilities/eelt/
4
Christopher Keller “Exoplanet Detection with radial velocities” Astronomical Data Analysis 2011:Lecture 6
(2011) slide 24, 25
5
E-ELT Science Office, “An Expanded View of the Universe” European Southern Observatory page 16-18
1
The E-ELT could also be part of the effort to understand dark energy and explain why the universe is
expanding at the rate it is. Redshift is an indicator of the expansion of the universe, and the E-ELT
will be able to measure it much more accurately and therefore measure the effect of dark energy. The
size of the E-ELT allows it to collect much higher resolution data and this has a number of
applications and is the main appeal of the project. One project this will be important in is the nuclear
dating of stars. This will tell scientists much more about the life cycle of stars which will in turn give
them a greater understanding of the processes taking place within galaxies and how that shapes their
evolution6. Nuclear dating or nucleocosmochronometry is very useful for distant stars whose ages
cannot be determined any other way and it uses the idea of nuclear decay to work out the age of a star
by measuring the abundance of long-lived radioactive isotopes such as 232Th (mean lifetime 20.3
billion years) and 238U (mean lifetime 6.5 billion years)7. This method has disadvantages, such as
high uncertainties due to systematic errors, however the use of the E-ELT will overcome one of the
major disadvantages of this method being limited to brighter stars as very high resolution spectra must
be used. Use of this method will allow the ages of many stars to be calculated to help us understand
how the Milky Way has changed and evolved. These are just a few of the projects that will benefit
from using the E-ELT, there are many more that will progress science and help scientists to
understand our universe even more.
The E-ELT is being built because its design
will be of benefit to science. It is different
from other telescopes firstly in terms of scale:
it will have a 39m primary mirror, which its
designers claim will “provide images 16 times
sharper than the Hubble Space Telescope”.
Secondly, it will have five main mirrors which
will increase the quality of the images. The
telescope will use Nasmyth foci, like many
other large telescopes, in order to increase
their mobility. It means that the main structure
can be mounted on a permanent platform but
the Nasmyth focal surface can be attached on
an altazimuth mount that needs only a small
Figure 1
range of movement to cover the whole sky8. In
the case of the E-ELT, it is the fifth mirror that directs the light to the Nasmyth focal point. Figure 19
shows the direction of light as it enters the telescope. As it enters, it is reflected by the primary mirror
(M1) towards the secondary mirror (M2). The convex secondary mirror reflects the light through a
hole in the quaternary mirror (M4) and towards the tertiary mirror (M3) which is located in the centre
of the primary mirror, within the 11.1m obstruction. The tertiary mirror reflects the light into the
adaptive optics system which is made up of the quaternary and fifth mirror. These mirrors can change
their shape in order to correct distortion caused by the Earth’s atmosphere, which can be measured
using a very bright reference star. This means that telescopes that use this system can produce images
that are almost as sharp as those taken in space. The quaternary mirror is inclined at a 7.74 degree
6
E-ELT Science Office, “An Expanded View of the Universe” page 16-18
David Soderblom, “The ages of stars” Annual Reviews of Astronomy and Astrophysics pages 10-11
8
David Darling “Nasmyth Focus” The Encyclopaedia of Science, www.daviddarling.info/encyclopedia/N/
Nasmyth_focus.html
9
Figure 1: Nasmyth configuration, “E-ELT Optical Design” last modified 16/06/11,
www.eso.org/sci/facilities/eelt/telescope/design/
7
2
angle and directs the light to the fifth mirror which does the final image correction and then reflects
the light towards the Nasmyth focus. An alternative to the Nasmyth focus would be a Coude focus
(see figure 4). They are similar, but a Coude focus is at a fixed point which differs from the Nasmyth
focus which has a focal point that is separate from the main telescope but which moves with it. Coude
focuses give a narrower field of view10.
The concave primary mirror (M1) has been
focused on a lot in the media due to its huge
diameter of 39m and a radius of curvature of
69m. As mentioned before, it has an 11.1m
obstruction in the centre (where the tertiary
mirror is). It is made up of 798 hexagonal
segments that are grouped into six sectors
each with 133 segment ‘families’- see figure
211. There is a 7th segment in each family that
Figure 3 can be used when one of the segments needs
to be recoated. The segments’ coating is what
makes them reflective, and they have to be recoated every 18 months therefore there must be constant
rotation of segments with 1 or 2 being recoated every day. Each segment is made of polished glass (or
glass-ceramic substrate) and has to have a surface roughness of less than 2 nanometres, making it an
extremely sophisticated piece of engineering12. When all of the 789 segments are put together,
position actuators allow each segment to be moved slightly to change the shape of the entire mirror, in
order to correct errors: it is the position actuators that provide the high resolution of the final image.
The main causes of error are the wind and vibrations from all the machinery in the telescope. There
will be three actuators under each segment, controlling three planes of movement. The actuators have
two position stages, the first is a Voice Coil Actuator (VCA) which uses a magnetic field and a coil of
wire to produce a force proportional to the current supplied. This uses the Lorentz Force principle:
F=q(E+v×B) which determines the size of the force produced by
the VCA13. This design is suitable to for very accurate movements
in short travel ranges. The second stage is based on a Brushless DC
Motor (BLDC). A DC motor typically uses permanent, stationary
magnets (the stator) and a spinning armature (the rotor). The
amature contains an electromagnet which when spinning, induces a
magnetic field. Brushes continuously change the polarity of the
electromagnet in order to keep it spinning. However in a BLDC,
which are more efficient, the magnets are the rotor and the
electromagnet is the stator. A computer then charges the
Figure 2
electromagnet, giving superior control and smoother movement
compared to mechanical brushes14. The BLDC contrasts the VCA in that it has a large stroke range
and can move the segment closer to the required position15. A gear box is attached to the motor to
10
“The Coude Focus” retrieved 11/02/15, mthamilton.ucolick.org/public/tele_inst/3m/coude.html
Figure 2: Primary Mirror Segmentation Pattern, M. Cayrel, “E-ELT Optomechanics: overview” European
Southern Observatory (2012), page 2
12
M. Cayrel, “E-ELT Optomechanics: overview” page 2, 3
13
Bill Black, Mike Lopez, Anthony Morcos, “Basics of Voice Coil Actuators” page 1
14
Marshall Brain “How does a brushless electric motor work?” last modified 15/12/08,
electronics.howstuffworks.com/brushless-motor.htm
11
3
reduce the power consumption which is important as if a lot of power is used, a lot of heat will be
dissipated directly under the mirror segments. Ideally, the power consumption of each position
actuator will be around 0.6 W16. The prototype currently being developed and tested (see figure 317) is
promising in terms of both accuracy and power usage.
Cerro Armazones, a mountain
in Chile, has been selected as
the site for the E-ELT. Figure
418 is a map of the area,
labelling the Paranal
observatory site as well as the
E-ELT. The site selection
process has been going on for
years, and several sites have
been considered, including
Figure 4
Roque de los Muchachos in
Canary Islands, Cerro Tolonchar, Cerro Ventarrones and Vizcachas in Chile19. Mountains are an
obvious choice when building a ground based telescope as their high altitude helps to minimise the
distortion caused by the atmosphere. In addition, mountains are often far away from towns and cities
so there is less light pollution. The parameters that were to measured were “Optical turbulence, wind
velocity, outer scale, seeing, isoplanatic angle, coherence time, extinction, dust, cloud cover,
humidity, precipitable water vapor, sky emission, sky darkness, light pollution, soil properties and
seismicity”20. A number of instruments were used to measure all of these, for example the G-SCIDAR
technique. SCIDAR stands for SCIntillation Detection And Ranging and the technique involves
imaging the scintillation patterns of double stars using very low exposure. Average autocorrelation
function can be calculated from that, using a computer algorithm, has peaks which provide
information about the various turbulent layers. Generalised- SCIDAR differs from classical in that it
is able to tell us about layers closer to the ground and the optical atmospheric turbulence, including
the wind profile. This can in turn give us information about the parameters: seeing, coherence lengths
and time of the wave fronts and isoplanatic angles21. There are three main layers of turbulence that
can affect the quality of images a telescope produces: the free atmosphere, the atmospheric boundary
layer and turbulence around the telescope dome. G-SCIDAR was used at a number of the proposed
sites where there were already existing observatories with the correct infrastructure. In addition to the
issues regarding the quality of the images that could be procured at that site, the Site Selection
Advisory Committee also had to think about the construction cost and schedule as certain sites will be
easier to access and closer to where the parts of the telescope will be produced therefore the transport
costs will be less and it will be quicker to build. Also, although the site would ideally avoid light
15
A. Jimenez et al. “Design of a prototype position actuator for the primary mirror segments of the European
Extremely Large Telescope” Ground based and airborne telescopes (2010) page 1
16
M. Dimmler et al. “E-ELT M1 Test Facility” Ground based and airborne telescopes (SPIE, 2012) Page 5
17
Figure 3: M1 Position Actuator Prototype, M. Cayrel, “E-ELT Optomechanics: overview” page 5
18
Figure 4: artists impression of the land around E-ELT and VLT “ESO and Chile sign agreement on E-ELT”
retrieved 14/02/15, www.eso.org/public/news/eso1139/
19
“E-ELT site” last modified 15/07/11, www.eso.org/sci/facilities/eelt/site/index.html
20
J. Vernin, C. Mun˜oz-Tun˜on , M. Sarazin “E-ELT site characterization status” European Southern
Observatory (2004) page 3
21
H. Ramió et al. “Cute-SCIDAR at Paranal for E-ELT Site Characterisation” Telescopes and Instrumentation
(2008) page 29
4
pollution be being built away from civilisation, but where observatory staff will live should be
considered, as well as access to water and electricity. The telescope could require in excess of 10MW
of power so there must be grids capable of providing that22.
The E-ELT will have very sophisticated instruments. As the telescope is still in the process of being
designed and built, the instruments that are eventually used will probably be much more advanced
than those being considered at the moment because of the fast pace of scientific and technological
advancement. The procurement of the main instruments will begin in 201523. The ELT-PCS is one of
the instruments that will require very new technology. It consists of a planetary camera which will be
used to take high quality images, specifically of exoplanets like Earth that could be considered for
terraforming and human habitation in the future. It also has a spectrograph which is an instrument
used to separate and measure the different wavelengths in Electromagnetic radiation. The relative
amounts of each type of EM radiation can then be determined and that information analysed. Most
exoplanets would be detected using methods such as radial velocity, but the PCS will allow them to
be observed using visible light or infrared wavelengths in order to learn more about an exoplanet,
such as if it is rocky or if it has water24. It is one of seven instruments that will be in the E-ELT, six of
which have already been determined with space for one more instrument whose purpose will be
determined closer to the time of construction.
The E-ELT is expected to be operational in 2024, a mere 9 years away, and its impact of the scientific
community will undoubtedly be very significant. From observing exoplanets to dark matter, the uses
of this telescope are vast. It’s incredibly complex design is a testimony to the creativity, innovation
and far-reaching impact of modern engineering. The E-ELT is at the forefront of science and
engineering, with technology such as adaptive and active optics, cryogenic bearings, real time
processors and planetary cameras. Whilst the advancement of science is the telescope’s priority,
society and industry will also benefit from the spin-off technologies. The E-ELT and other projects
like it are pushing science forward and the progress that has already been made is incredible.
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6