Euclid Definition Study Report

Abstract

Euclid is a space-based survey mission from the European Space Agency
designed to understand the origin of the Universe's accelerating expansion. It
will use cosmological probes to investigate the nature of dark energy, dark
matter and gravity by tracking their observational signatures on the geometry
of the universe and on the cosmic history of structure formation. The mission
is optimised for two independent primary cosmological probes: Weak
gravitational Lensing (WL) and Baryonic Acoustic Oscillations (BAO). The Euclid
payload consists of a 1.2 m Korsch telescope designed to provide a large field
of view. It carries two instruments with a common field-of-view of ~0.54 deg2:
the visual imager (VIS) and the near infrared instrument (NISP) which contains
a slitless spectrometer and a three bands photometer. The Euclid wide survey
will cover 15,000 deg2 of the extragalactic sky and is complemented by two 20
deg2 deep fields. For WL, Euclid measures the shapes of 30-40 resolved galaxies
per arcmin2 in one broad visible R+I+Z band (550-920 nm). The photometric
redshifts for these galaxies reach a precision of dz/(1+z) < 0.05. They are
derived from three additional Euclid NIR bands (Y, J, H in the range 0.92-2.0
micron), complemented by ground based photometry in visible bands derived from
public data or through engaged collaborations. The BAO are determined from a
spectroscopic survey with a redshift accuracy dz/(1+z) =0.001. The slitless
spectrometer, with spectral resolution ~250, predominantly detects Ha emission
line galaxies. Euclid is a Medium Class mission of the ESA Cosmic Vision
2015-2025 programme, with a foreseen launch date in 2019. This report (also
known as the Euclid Red Book) describes the outcome of the Phase A study.

Full text

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ESA/SRE(2011)12
July 2011
Definition Study Report
Euclid
Mapping the geometry
of the dark Universe
European Space Agency

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Euclid

Mapping the geometry of the dark Universe







Definition Study Report
ESA/SRE(2011)12


J uly 2011
September 2011 (Revision 1)

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On the front cover: composite of a fragment from Raphael’s fresco “The School of Athens” in the Stanza
della Segnatura of the Vatican Palace depicting the Greek mathematician Euclid of Alexandria, a simulation
of the cosmic web by Springel et al, and an image of Abell 1689; the composition is made by Remy van
Haarlem (ESA/ESTEC).

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Euclid Mission Summary

Main Scientific Objectives
Understand the nature of Dark Energy and Dark Matter by:

Reach a dark energy FoM > 400 using only weak lensing and galaxy clustering; this roughly corresponds to
1 sigma errors on wp and wa of 0.02 and 0.1, respectively.

Measure γ, the exponent of the growth factor, with a 1 sigma precision of < 0.02, sufficient to distinguish
General Relativity and a wide range of modified-gravity theories

Test the Cold Dark Matter paradigm for hierarchical structure formation, and measure the sum of the
neutrino masses with a 1 sigma precision better than 0.03eV.

Constrain ns, the spectral index of primordial power spectrum, to percent accuracy when combined with
Planck, and to probe inflation models by measuring the non-Gaussianity of initial conditions parameterised
by fNL to a 1 sigma precision of ~2.
SURVEYS
Area (deg2)
Wide Survey 15,000 (required)
20,000 (goal)
Deep Survey 40
Description
Step and stare with 4 dither pointings per step.
In at least 2 patches of > 10 deg2
2 magnitudes deeper than wide survey
PAYLOAD
Telescope
Instrument
Field-of-View
Capability
1.2 m Korsch, 3 mirror anastigmat, f=24.5 m
VIS NISP
0.787×0.709 deg2
Visual Imaging
0.763×0.722 deg2
NIR Imaging Photometry

Y (920-
1146nm),
24 mag
5σ point
source
NIR Spectroscopy
Wavelength range 550– 900 nm J (1146-1372
nm)
24 mag
5σ point
source
H (1372-
2000nm)
24 mag
5σ point
source
1100-2000 nm
Sensitivity 24.5 mag
10σ extended source
3 10-16 erg cm-2 s-1
3.5σ unresolved line
flux
Detector
Technology
Pixel Size
Spectral resolution
36 arrays
4k×4k CCD
0.1 arcsec
16 arrays
2k×2k NIR sensitive HgCdTe detectors
0.3 arcsec 0.3 arcsec
R=250
SPACECRAFT
Launcher
Orbit
Pointing
Soyuz ST-2.1 B from Kourou
Large Sun-Earth Lagrange point 2 (SEL2), free insertion orbit
25 mas relative pointing error over one dither duration
30 arcsec absolute pointing error
Step and stare, 4 dither frames per field, VIS and NISP common FoV = 0.54 deg2
7 years
4 hours per day contact, more than one ground station to cope with seasonal visibility
variations;
maximum science data rate of 850 Gbit/day downlink in K band (26GHz), steerable HGA
Budgets and Performance 
Mass (kg)
TAS
897
786
148
Adapter mass/ Harness and PDCU losses power 70
Total (including margin)
Observation mode
Lifetime
Operations
Communications

industry
Payload Module
Service Module
Propellant
Nominal Power (W)
TAS
410
647

65
1368
Astrium
696
835
232
90
2160
Astrium
496
692

108
1690

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Foreword
Euclid is a Medium Class mission of the ESA Cosmic Vision 2015-2025 programme, and competes for one
of the two foreseen launch slots in 2017 and 2018. This report (the Euclid Red Book) describes the outcome
of the Phase A study, which started with the industrial kick-off in July 2010 and ended with the completion
of the Preliminary Requirement Review in July 2011. Phase A is the first part of the Mission Definition
Phase in which the mission is prepared for the Implementation Phase where an industrial prime contractor
will lead the system development and the Euclid Mission Consortium will provide elements of payload and
science ground segment.
The Assessment Phase of Euclid, which preceded Phase A, showed that the Euclid science and mission
concept are feasible and could meet the M Class mission requirements. However, the study also showed that
a serious optimisation of the mission was necessary to stay within the programmatic constraints with
sufficient margin. In particular, the payload mass had to be reduced to create margin for a Soyuz launcher,
and a lower number of detectors was recommended, since procurement of many detectors has a high
schedule risk beyond the foreseen launch in 2018.
The Euclid study team considered several options to optimise the mission, but only a few cases could be
investigated in detail. The most viable solution was the merging of the near-infrared photometric and
spectroscopic channels into one instrument with a single focal plane array and with a reduced number of
near-infrared detectors. This concept provides a comfortable system mass margin due to a payload mass
reduction as a result of a more compact payload design. The optimisation comes with a price: the near-
infrared spectroscopic and photometric observations are now done sequentially instead of simultaneously,
thereby significantly increasing the observation time for a single field.
A team of 6 scientists, the Euclid Optimisation Advisory Team (EOAT), was selected to provide re-
commendations on the set of mission parameters that can meet the Euclid science objectives and top level
requirements, taking into account the optimised payload. It was decided that a payload configuration with 16
NIR detectors for NISP, would best meet the Euclid science requirements. The alternative configuration with
a smaller FoV and consequently fewer detectors was rejected as baseline because of its inability to meet the
sufficient survey area during the nominal mission.
The new payload and mission concept was used to prepare the invitation to tender (ITT) for two competing
Definition Phase studies by two independent industrial consortia. Two industrial contractors were selected:
Thales Alenia Space Italy (Turin) and Astrium GmbH Germany (Friedrichshafen). Both industrial teams
already performed the Assessment Phase studies, and carry a lot of expertise about the Euclid mission.
In June 2010 the Euclid Science Management Plan was accepted by the Science Programme Committee. The
science management plan provides the programmatic framework for the issue of the Euclid Announcement
of Opportunity for a single consortium - the Euclid Mission Consortium - to provide elements of the payload
and the science ground segment. A European wide consortium consisting of 7 lead countries submitted their
proposal in October 2010 and was selected by the Science Programme Committee in February 2011.
The Euclid Science Management Plan was updated in June 2011 and reflects the latest programmatic
developments as well as the data release scheme for Euclid.
This report provides a description of the Euclid mission as emerged from the phase A study. Section 1 gives
the executive summary. The science case is provided in Section 2. The first level science requirements and
the top level instrument and mission requirements are provided in Section 3. Payload and mission are
described in Sections 4 and 5, respectively. An assessment of the system and instruments performance in
view of the science requirements is provided in Section 6. The description of the ground segment is given in
Section 7, and the management and programmatics are described in Section 8.
The Euclid Study Team, July 2011

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Authorship and Acknowledgements
Euclid Red Book Editorial Team
R. Laureijs ESA/ESTEC, The Netherlands (Chair; ESA Study Team)
J. Amiaux CEA /IRFU Saclay, France
S. Arduini IAP UPMC Paris, France
J.-L. Auguères CEA/IRFU Saclay, France
J. Brinchmann Leiden Univ., The Netherlands
R. Cole UCL-MSSL, London, UK
M. Cropper UCL-MSSL, London, UK (ECB, EST member))
C. Dabin CNES, Toulouse, France
L. Duvet ESA/ESTEC, The Netherlands (ESA Study Team)
A. Ealet CPPM and LAM, Marseille, France
B. Garilli INAF IASFMI, Milan, Italy
P. Gondoin ESA/ESTEC, The Netherlands (ESA Study Team)
L. Guzzo INAF OA Brera, Italy
J. Hoar ESA/ESAC, Spain (ESA Study Team)
H. Hoekstra Leiden Univ., The Netherlands
R. Holmes MPIA, Heidelberg, Germany
T. Kitching ROE, Edinburgh, UK
T. Maciaszek CNES, Toulouse, France
Y. Mellier IAP UPMC, Paris, France (ECL)
F. Pasian INAF OA Trieste, Italy
W. Percival Univ. Portsmouth, UK
J. Rhodes JPL, Pasadena CA, USA (ECB)
G. Saavedra Criado ESA/ESTEC, The Netherlands (ESA Study Team)
M. Sauvage CEA/IRFU Saclay, France
R. Scaramella INAF OA Roma, Italy (ECB)
L. Valenziano INAF IASFBO Bologna, Italy
S. Warren Imperial College London, UK
ESA Study Team
L. Duvet
P. Gondoin
J. Hoar
R. Laureijs
G. Saavedra Criado









Study Payload Manager
Study Manager
Science Operations
Study Scientist
System Engineer
Euclid Consortium Board (ECB)
R. Bender
F. Castander
A. Cimatti
M. Cropper
O. Le Fèvre
H. Kurki-Suonio
M. Levi
P. Lilje
Y. Mellier
G. Meylan
R. Nichol
K. Pedersen
V. Popa
R. Rebolo Lopez
J. Rhodes
H.-W. Rix
H. Rottgering
R. Scaramella
W. Zeilinger
























MPE Garching and USM Muenchen, Germany
IEEC Barcelona, Spain (EST member)
Univ. of Bologna, Italy (EST member)
UCL-MSSL London, UK (EST member)
LAM Marseille, France
Univ. Helsinki, Finland
LBL Berkeley CA, USA
ITA Univ. Oslo, Norway
IAP UMPC Paris, France (ECL)
EPFL Lausanne, Switzerland
Univ. Portsmouth, UK
SSC Univ. Copenhagen, Denmark
ISS Bucharest, Romania
IAC, Spain
JPL, Pasadena CA, USA
MPIA, Heidelberg, Germany (EST member)
Leiden Univ, The Netherlands
INAF OA Roma, Italy
IfA Univ. Wien, Austria

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Other Euclid Tiger Team (TT) contributors (not in previous lists)
F. Grupp
P. Hudelot
R. Massey
M. Meneghetti
L. Miller
S. Paltani
S. Paulin-Henriksson
S. Pires
C. Saxton
T. Schrabback
G. Seidel
J. Walsh


















MPE Garching and USM Muenchen, Germany
IAP UPMC, France
ROE Edinburgh, UK
INAF OA Bologna, Italy
Oxford Univ., UK
Univ. Genève, Switzerland
CEA/IRFU Saclay, France
CEA/IRFU Saclay, France
UCL-MSSL London, UK
SLAC CA, USA and USM Muenchen, Germany
MPIA Heidelberg, Germany
ESO Garching, Germany





Contributions from the Euclid Science Working Group (not in previous lists):
N. Aghanim (IAS, Orsay), L. Amendola (MPIA, Heidelberg), J. Bartlett (APC, Paris), C. Baccigalupi (SISSA Trieste), J. -P.
Beaulieu (IAP, Paris), K. Benabed (IAP, Paris), J. -G. Cuby (LAM, Marseille), D. Elbaz (CEA, Saclay), P. Fosalba (IEEC,
Barcelona), G. Gavazzi (U. Milano Bicocca), A. Helmi (U. Groningen), I. Hook (U. Oxford, and OA Roma), M. Irwin (U.
Cambridge), J.-P. Kneib (LAM, Marseille), M. Kunz (U. Genève), F. Mannucci (OA Arcetri), L. Moscardini (U. Bologna), C. Tao
(CPPM, Marseille), R. Teyssier (CEA Saclay and U. Zurich), J. Weller (USM München, MPE Garching), G. Zamorani (OA
Bologna), M.R. Zapatero Osorio (IAC, Spain)
Contributions from the Euclid VIS instrument (not in previous lists):
O. Boulade (CEA Saclay), J. J. Foumond (IAS, Orsay), A. Di Giorgio (IFSI Roma), P. Guttridge (UCL-MSSL, London), A. James
(UCL-MSSL, London), M. Kemp (UCL-MSSL, London), J. Martignac (CEA, Saclay), A. Spencer (UCL-MSSL London), D. Walton
(UCL-MSSL London)
Contributions from the Euclid NISP instrument (not in previous lists):
T. Blümchen (MPIA Heidelberg), C. Bonoli (OA Padova), F. Bortoletto (OA Padova), C. Cerna (CPPM, Marseille), L. Corcione
(OA Torino), C. Fabron (LAM, Marseille), K. Jahnke (MPIA Heidelberg), S. Ligori (OA Torino), F. Madrid (IEEC Barcelona), L.
Martin (LAM Marseille), G. Morgante (IASFBO Bologna), T. Pamplona (LAM, Marseille), E. Prieto (LAM Marseille), M. Riva
(OA Brera), R. Toledo (UPCT, Cartagena), M. Trifoglio (IASFBO Bologna), F. Zerbi (OA Brera),
Contributions from the Euclid Science Ground Segment (not in previous lists):
F. Abdalla (UCL London), M. Douspis (IAS, Orsay), C. Grenet (IAP, Paris), S. Borgani (U. Trieste), R. Bouwens (U. Leiden), F.
Courbin (EPFL, Lausanne), J.-M. Delouis (IAP, Paris), P. Dubath (U. Genève), A. Fontana (OA Roma), M. Frailis (OA Trieste), A.
Grazian (OA Roma), J. Koppenhöfer (MPE Garching), O. Mansutti (OA Trieste), M. Melchior (FHNW Windisch), M. Mignoli (OA
Bologna), J. Mohr (USM München), C. Neissner (PIC, Barcelona), K. Noddle (ROE Edinburgh), M. Poncet (CNES, Toulouse), M.
Scodeggio (IASFMI, Milan), S. Serrano (IEEC Barcelona), N. Shane (UCL-MSSL London), J.-L. Starck (CEA Saclay), C. Surace
(LAM, Marseille), A. Taylor (ROE Edinburgh), G. Verdoes-Kleijn (U. Groningen), C. Vuerli (OA Trieste), O. R. Williams (U.
Groningen), A. Zacchei (OA Trieste)
Contributions from ESA
B. Altieri (ESAC), I. Escudero Sanz (ESTEC), R. Kohley (ESAC), T. Oosterbroek (ESTEC)
Other contributors:
P. Astier (LPNHE, Paris), D. Bacon (ICG, Portsmouth), S. Bardelli (OA Bologna), C. Baugh (U. Durham), F. Bellagamba (U.
Bologna), C. Benoist (OCA, Nice), D. Bianchi (Univ. Milan), A. Biviano (OA Trieste), E. Branchini (U. Roma), C. Carbone (U.
Bologna), V. Cardone (OA Roma), D. Clements (Imperial, London), S. Colombi (IAP Paris), C. Conselice (U. Nottingham), G.
Cresci (OA Arcetri), N. Deacon (UH Honolulu), J. Dunlop (IFA Edinburgh), C. Fedeli (U. Florida), F. Fontanot (OA Trieste), P.
Franzetti (IASFMI Milan), C. Giocoli (U. Bologna), J. Garcia-Bellido (UAM Madrid), J. Gow (Open U.), A. Heavens (IFA
Edinburgh), P. Hewett (U. Cambridge), C. Heymans (IFA Edinburgh), A. Holland (Open U.), Z. Huang (OA Roma), O. Ilbert (LAM,
Marseille), B. Joachimi (ROE Edinburgh), E. Jennins (U. Durham), E. Kerins (U. Manchester), A. Kiessling (ROE, Edinburgh), D.
Kirk (UCL, London), R. Kotak (QUB Belfast), O. Krause (MPIA Heidelberg), O. Lahav (UCL), F. van Leeuwen (U. Cambridge), J.
Lesgourgues (CERN Geneva), M. Lombardi (U. Milano), M. Magliocchetti (IFSI), K. Maguire (U. Oxford), E. Majerotto (OA
Brera), R. Maoli (U. Roma), F. Marulli (U. Bologna), S. Maurogordato (OCA Nice), H. McCracken (IAP, Paris), R. McLure (IFA
Edinburgh), A. Melchiorri (U. Roma), A. Merson (U. Durham), M. Moresco (U. Bologna), M. Nonino (OA Trieste), P. Norberg
(ROE Edinburgh), J. Peacock (IfA Edinburgh), R. Pello (IRAP Toulouse), M. Penny (U. Manchester), V. Pettorino (SISSA, Trieste),
C. Di Porto (U. Roma), L. Pozzetti (OA Bologna), C. Quercellini (OA Roma), M. Radovich (OA Padova), A. Rassat (EPFL,
Lausanne), N. Roche (OA Bologna), S. Ronayette (CEA Saclay), E. Rossetti (U. Bologna), B. Sartoris (U. Trieste), P. Schneider (U.
Bonn), E. Semboloni (U. Leiden), S. Serjeant (Open U.), F. Simpson (ROE Edinburgh), C. Skordis (U. Nottingham), G. Smadja
(IPNL, Lyon), S. Smartt (QUB Belfast), P. Spano (OA Brera), S. Spiro (OA Roma), M. Sullivan (U. Oxford), A. Tilquin (CPPM
Marseille), R. Trotta (Imperial London), L. Verde (ICC Barcelona), Y. Wang (U. Oklahoma), G. Williger (U. Louisville), G. Zhao
(U. Portsmouth), J. Zoubian (LAM, Marseille), E. Zucca (OA Bologna)

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1. Executive Summary

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1 Executive Summary
Understanding the acceleration of the expansion of the Universe is one of the most compelling challenges of
cosmology and fundamental physics. The Euclid surveys will show how cosmic acceleration modifies the
expansion history and the 3-dimensional distribution of matter in the Universe. To achieve this, Euclid will
measure the shapes over a billion galaxies and accurate redshifts of tens of millions of galaxies for weak
gravitational lensing and galaxy clustering studies. The Phase A study of Euclid has led to a payload and a
mission design concept able to support the requirements imposed by the need of high visible image quality,
and of near-infrared imaging photometry and slitless spectroscopy over a very large sky area. With the ex-
pected mission performance and exquisite control of systematics, the Euclid space mission puts Europe in a
leading position to address a most fascinating question that may revolutionise physics.
Primary Science Objectives: Our understanding of cosmology is that of the Universe evolving from a
homogeneous state after the Big Bang to a hierarchical assembly of galaxies, clusters and superclusters at our
epoch. However this view relies on two untested assumptions, about the initial conditions of the Universe
and the nature of gravity itself, and the existence of two dominant components whose nature is entirely un-
known. Of these unknown components, 76% of the energy density is in the form of dark energy, which is
causing the Universe expansion to accelerate. Dark energy is in conflict with our knowledge of fundamental
physics, if it behaves as predicted from the cosmological constant (introduced by Einstein), then its value is
1060 times smaller than that predicted from theory; this is the largest discrepancy between theory and obser-
vation ever encountered in modern physics. Another 20% of the energy density of the Universe is in the form
of dark matter, which exerts a gravitational attraction as normal matter, but does not emit or absorb light.
While several candidates for dark matter exist in particle physics, its nature remains unknown. Another
possibility to explain one or both of these puzzles is that Einstein's General Relativity, and thus our under-
standing of gravity, needs to be revised. This diversity of theoretical ideas shows our current ignorance but
also defines the need for future observations. Based on our present-day knowledge, the existing plausible
models will only change observational signatures by tiny amounts that can only be decisively distinguished
by using high-precision astronomical surveys covering a major fraction of the sky.
Euclid is a survey mission designed to understand the origin of the Universe’s accelerating expansion. It will
use cosmological probes to investigate the nature of dark energy, dark matter and gravity by tracking their
observational signatures on the geometry of the universe and on the cosmic history of structure formation.
Euclid will map large-scale structure over a cosmic time covering the last 10 billion years, more than 75%
the age of the Universe. The mission is optimised for two independent primary cosmological probes: Weak
gravitational Lensing (WL) and Baryonic Acoustic Oscillations (BAO). WL is a technique to map dark
matter and measure dark energy by quantifying the apparent distortions of galaxy images, a change in a
galaxy’s observed ellipticity, caused by mass inhomogeneities along the line-of-sight. The lensing signal is
derived from the measurement of shape and distance of galaxies. BAO are wiggle patterns imprinted in the
clustering of galaxies that provide a standard ruler to measure the expansion of the Universe. The properties
of the wiggles are derived from accurate distance measurements of galaxies. Surveyed in the same cosmic
volume, these two probes provide necessary cross-checks on systematic errors. They also provide a measure-
ment of large scale structure via different physical fields (potential, density and velocity), which are required
for testing dark energy and gravity at all scales. In addition, the Euclid surveys yield data of several
important complementary cosmological probes such as galaxy clusters, redshift space distortions and the
integrated Sachs Wolfe effect.
WL requires a high image quality on sub-arcsec scales for the galaxy shape measurements, and photometry
at visible and infrared wavelengths to measure the photometric distances of each lensed galaxy out to z≥2.
BAO requires near-infrared spectroscopic capabilities to measure accurate redshifts of galaxies out to z≥0.7.
Both probes require a very high degree of system stability to minimise systematic effects, and the ability to
survey a major fraction of the extra-galactic sky. Such a combination of requirements cannot be met from the
ground, and demands a wide-field-of-view space mission. Euclid is designed for that purpose.
To understand the nature of dark energy its equation of state needs to be determined. Euclid uses WL and
BAO to measure the constant and time varying terms of the dark energy equation of state to a 1-sigma
precision of 0.02 and 0.1, sufficient to make a decisive statement on the nature of dark energy. Euclid tests

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1. Executive Summary 8
the validity of General Relativity by measuring the rate of cosmic structure growth to a 1-sigma precision of
< 0.02, sufficient to distinguish General Relativity from a wide range of modified-gravity theories. As Euclid
maps the dark matter distribution with unprecedented accuracy, subtle features produced by neutrinos are
measured, providing constraints on the sum of the neutrino masses with a 1-sigma precision better than 0.03
eV. Likewise, the initial conditions of the seeds of cosmic structure growth are unveiled by determining the
power spectrum of density perturbations to one percent accuracy. Euclid and Planck together measure
deviations to a Gaussian distribution of initial perturbations with a precision one order of magnitude better
than current constraints, allowing Euclid to test a broad range of inflation models. Euclid is therefore poised
to uncover new physics by challenging all sectors of the cosmological model.
Legacy Science: The Euclid wide and a deep surveys yield a treasure-trove with unique legacy science in
various fields of astrophysics and a primary data base for next generation multi-wavelength surveys.
Euclid produces a legacy dataset with images and photometry of more than a billion galaxies and several
million spectra, out to high redshifts z>2. At low redshift, Euclid resolves the stellar population of all
galaxies within ~5 Mpc, providing a complete census of all morphological and spectral types of galaxies in
our neighborhood. It also delivers morphologies, masses, and star-formation rates out to z~2 with a 4 times
better resolution, and 3 NIR magnitudes deeper, than possible from ground. Euclid derives the mass function
of galaxy clusters (in combination with eROSITA, Planck and SZ telescopes), and finds over 105 strong
lensing systems. Gravitational lensing together with near infrared photometry of lensing sources explores the
relationship between light, baryons and dark matter between galaxy and super cluster scales as function of
look-back time and environment.
The Euclid deep survey will be the primary target for follow-up observations. Deep data contain thousands
of objects at z>6 and several tens of z>8 galaxy or quasar candidates that will be critical targets for JWST
and E-ELT. As the deep survey fields are visited repeatedly over a time span of several years they are a
unique baseline for the discovery of variable sources.
Euclid probes the formation and evolution of our Galaxy. Euclid augments the Gaia survey, taking it several
magnitudes deeper, and provides complementary information, adding infrared colours and spectra for every
Gaia stars it observes. This breaks the age-metallicity degeneracy, which is critical for the chemical
enrichment history of our Galaxy. Also, the wide sky coverage of Euclid will detect nearby extremely low
surface brightness tidal streams of stars.
Payload: The Euclid payload consists of a 1.2 m Korsch telescope designed to provide a large field of view.
The telescope directs the light to two instruments via a dichroic filter in the exit pupil. The reflected light is
led to the visual instrument (VIS) and the transmitted light from the dichroic feeds the near infrared
instrument (NISP) which contains a slitless spectrometer and a three bands photometer. Both instruments
cover a large common field-of-view of ~0.54 deg2.
VIS is equipped with 36 CCDs. It measures the shapes of galaxies with a resolution better than 0.2 arcsec
(PSF FWHM) with 0.1 arcsec pixels in one wide visible band (R+I+Z). The NISP photometer contains three
NIR bands (Y, J, H), employing 16 HgCdTe NIR detectors with 0.3 arcsec pixels. The spectroscopic channel
of NISP operates in the wavelength range 1.1-2.0 micron at a mean spectral resolution λ/Δλ ~ 250,
employing 0.3 arcsec pixels. While the VIS and NISP operate in parallel, the NISP performs the
spectroscopy and photometry measurements in sequence by selecting a grism wheel in case of spectroscopy
and a filter wheel in case of photometry.
Mission: Euclid will be launched in 2018 on a Soyuz ST-2.1B rocket, with an all-year round launch window.
A direct transfer of ~30 days is targeted to a large-amplitude free-insertion orbit at the 2nd Lagrange Point of
the Sun-Earth System. It takes 6 years to complete a wide survey with the deep survey interspersed.
Spacecraft commissioning, performance verification and initial calibration require an additional 3-6 months.
The sky mapping mode is step and stare. Image stability is maintained by scanning the sky along circles of
constant solar aspect angle. Possible variations in the solar aspect angle between fields are kept to a
maximum of 5 deg to minimise overheads for thermal stabilisation. At least one ground station is available to
receive the science data from the spacecraft at a rate of at most 850 Gbit over a daily pass time of 4 hours.
Surveys: The wide survey covers 15,000 deg2 of the extragalactic sky and is complemented by two 20 deg2
deep fields observed on a monthly basis. For WL, Euclid measures the shapes of 30 resolved galaxies per

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1. Executive Summary

9
arcmin2 in one broad visible R+I+Z band (550-920 nm) down to AB mag 24.5 (10 ). The photometric
redshifts for these galaxies reaches a precision of σz/(1+z) < 0.05. They are derived from three additional
Euclid NIR bands (Y, J, H in the range 0.92-2.0 micron) reaching AB mag 24 (5 ) in each, complemented
by ground based photometry in visible bands derived from public data or through engaged collaborations
with projects such as DES, KiDS, and Pan-STARRS. To measure the shear from galaxy ellipticities,
requirements are imposed on the PSF such that it can be reconstructed with an error ≤ 2x10-4 in ellipticity and
its dimension varies by less than 2x10-3 across the FoV. The BAO are determined from a spectroscopic
survey with a redshift accuracy σz/(1+z) ≤0.001. The slitless spectrometer, with λ/Δλ~250, predominantly
detects Hα emission line galaxies. The limiting line flux is 3x10−16 erg s−1cm−2 (1 arcsec extended source, 3.5
sigma at 1.6 micron), yielding over 50 million galaxy redshifts with a completeness higher than 45%.
The deep survey is two magnitudes deeper than the wide survey. This is needed for calibration of the slitless
spectroscopy and is also unique as a self-standing survey. The deep survey monitors the stability of the
spacecraft and payload through repeated visits of the same regions.
Expected performance: The end-to-end simulations carried out to assess the performance of VIS demon-
strate that the PSF of the VIS channel can be known to a high level of accuracy. The spatio-temporal
variations of the PSF which are most critical for WL are sampled and modeled with sufficient accuracy by
using the ≥1,800 stars suitable for calibration spread over each Euclid field. The PSF smearing produced by
the CCD charge transfer inefficiency is corrected using the standard method routinely applied to HST images,
provided the readout noise of the Euclid detectors is lower than 4.5 electrons. Overall, the residuals in the
knowledge of the size and the ellipticity of the PSF will be lower than the upper limit requirements during
the whole life of the Euclid mission. Furthermore, the throughput of the VIS channel guarantees that the
galaxy number density of ~30 gal./arcmin2 down to AB 24.5 mag can be achieved in less than 4000 s.
Similar simulations carried out for the NISP imaging mode show detection limits of YAB, JAB and HAB = 24
mag (5σ) in less than 6 minutes per filter, while preserving the required encircled energy in the three near-
infrared bands. The NISP imaging mode provides Y, J and H photometric data for 95% of VIS sources
suitable for WL analysis. The limited number of near-infrared detectors results in an under-sampled design,
with a pixel scale of 0.3 arcsec, which will be compensated by multiple dither observing sequences.
End to end simulations of NISP in spectroscopic mode show that Euclid can measure 3,000 redshifts/deg2
with the required S/N, completeness (fraction of spectra measured above a given line flux limit) and purity
(fraction of spectra for which the measured redshift is correct) over the whole range of redshifts explored by
the Euclid BAO sample and down to the expected H-alpha flux limit. The redshift completeness for BAO is
predicted to be significantly higher than the required 35%.
Mission management: For the space segment, ESA provides the spacecraft and telescope through a selected
industrial contractor, as well as the CCD and NIR detectors. The Euclid Mission Consortium (EMC), funded
by national agencies, has been selected to provide the VIS and NISP instruments, and elements of the science
ground segment (SGS) related to the scientific pipelines generating the data products and the instrument in-
orbit maintenance and operations.
The EMC is organised to support the instrument development, assessment of scientific requirements and
performance, and the SGS. Together with ESA, the consortium has worked out the SGS operations concept,
which has led to an agreed set of science implementation requirements for the SGS. The implementation
encompasses the definition of tasks and interfaces of the science data centres (SDCs) and the architecture of
the SGS that includes operation of the mission and the legacy archives.
Data release: Scientifically validated data are released via the Euclid Legacy Archive on an annual basis.
The scientific products are categorised in three data levels. The first data level consists of the raw decom-
pressed telemetry frames, the second level consists of calibrated data with instrument signatures removed,
and the third level consists of extracted scientific information such as catalogues. In addition, Euclid pro-
vides quick-release data, Level Q, representing transient products suitable for most purposes in astronomy,
except for the core cosmology objectives. The earliest public data release takes place 14 months after the
start of the routine operations, and contains Level Q products of the first year of routine operations. The
other associated data levels are released 12 months later, together with the Level Q products of the second
year, and so on.

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Table of Contents 10
Table of Contents
Euclid Mission Summary..................................................................................................................................3
Foreword ...........................................................................................................................................................4
Authorship and Acknowledgements..................................................................................................................5
1
Executive Summary...................................................................................................................................7
Table of Contents ............................................................................................................................................10
2
Euclid Science Objectives.......................................................................................................................13
2.1
Cosmology today ............................................................................................................................13
2.1.1
A dark Universe......................................................................................................................... 13
2.1.2
The key questions...................................................................................................................... 14
2.2
Mapping the luminous and dark Universe......................................................................................18
2.2.1
Mapping the Expansion History of the Universe....................................................................... 19
2.2.2
Mapping the growth of large scale structure in the Universe.................................................... 20
2.2.3
Combining the primary probes.................................................................................................. 20
2.2.4
Additional cosmological probes ................................................................................................ 20
2.3
Precise and accurate cosmology with Euclid..................................................................................22
2.3.1
Cosmological performance........................................................................................................ 23
2.3.2
Unprecedented cosmology constraints ...................................................................................... 24
2.4
Legacy science................................................................................................................................27
2.4.1
Euclid legacy in numbers........................................................................................................... 28
2.4.2
The high redshift Universe ........................................................................................................ 29
2.4.3
The cosmic co-evolution of galaxies and active galactic nuclei................................................ 30
2.4.4
The relationship between dark and baryonic matter.................................................................. 32
2.4.5
Near-field cosmology and astrophysics..................................................................................... 33
2.4.6
Euclid synergies with multi-wavelength surveys ...................................................................... 34
2.4.7
Prospects for additional surveys................................................................................................ 35
3
Scientific Requirements...........................................................................................................................37
3.1
Choice of techniques.......................................................................................................................37
3.2
Designing an accurate dark energy experiment ..............................................................................37
3.2.1
Survey area ................................................................................................................................ 37
3.2.2
Galaxy density from visible imaging......................................................................................... 38
3.2.3
Photometric data requirements .................................................................................................. 39
3.2.4
Ground based imaging data....................................................................................................... 40
3.2.5
Ground based spectroscopic data............................................................................................... 41
3.2.6
Galaxy density from NIR spectroscopy..................................................................................... 42
3.3
Control of systematics.....................................................................................................................44
3.3.1
Image quality............................................................................................................................. 44
3.3.2
Redshift measurement and contaminants .................................................................................. 46
3.4
The need for a space mission..........................................................................................................49
3.5
Legacy science requirements for the deep survey...........................................................................50
3.6
Hardware and mission requirements...............................................................................................51
3.6.1
Technological readiness............................................................................................................. 51
3.6.2
Telescope aperture..................................................................................................................... 52
3.6.3
Mission duration........................................................................................................................ 52
3.6.4
Daily telemetry rate ................................................................................................................... 52
3.6.5
Orbit........................................................................................................................................... 52
3.6.6
Instruments ................................................................................................................................ 52
4
Payload....................................................................................................................................................53
4.1
Optical design and telescope description........................................................................................53
4.2
Visible instrument (VIS).................................................................................................................55
4.2.1
Visible Channel description....................................................................................................... 55
4.2.2
Thermal architecture.................................................................................................................. 58
4.2.3
Electronics architecture ............................................................................................................. 59

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11
4.2.4
4.2.5
4.3
4.3.1
4.3.2
4.3.3
4.3.4
4.3.6
4.3.7
4.4
Mission Design........................................................................................................................................70
5.1
Mission analysis..............................................................................................................................70
5.2
Mission operations concept.............................................................................................................71
5.2.1
Survey Field Observation.......................................................................................................... 71
5.2.2
Reference survey strategy.......................................................................................................... 72
5.2.3
Euclid Reference Survey ........................................................................................................... 73
5.4
Spacecraft........................................................................................................................................75
5.4.1
Attitude and orbit control system............................................................................................... 75
5.4.2
Propulsion and actuators............................................................................................................ 76
5.4.3
Communications........................................................................................................................ 76
5.4.4
Data handling............................................................................................................................. 76
5.4.5
Thermal...................................................................................................................................... 76
5.4.6
Power......................................................................................................................................... 77
5.4.7
Overall configuration and budgets............................................................................................. 77
5.5
Payload interfaces...........................................................................................................................77
5.6
AIV and Development Issues..........................................................................................................77
5.6.1
System Level ............................................................................................................................. 78
5.6.2
Payload Level ............................................................................................................................ 79
6.
Performance.............................................................................................................................................80
6.1
Instrument simulation and performance..........................................................................................80
6.1.1
Visible imaging.......................................................................................................................... 80
6.1.2
Near-Infrared photometry.......................................................................................................... 84
6.1.3
Near-infrared Spectroscopy....................................................................................................... 85
6.2
Wide survey implementation ..........................................................................................................88
6.2.1
Survey simulation...................................................................................................................... 88
6.2.2
Expected performance............................................................................................................... 89
6.3
Scientific performance....................................................................................................................89
6.3.1
Weak Lensing Performance....................................................................................................... 89
6.3.2
Galaxy Clustering Performance................................................................................................. 93
7
Ground Segment and Data Handling.......................................................................................................97
7.1
Operations Ground Segment and Science Ground Segment...........................................................97
7.2
Science Operations..........................................................................................................................97
7.3
SGS Organisation............................................................................................................................98
7.4.1
Data processing levels and data production overview............................................................... 99
7.3.2
Quality Control.......................................................................................................................... 99
7.3.3
A data-centric approach........................................................................................................... 100
7.4
Data Processing.............................................................................................................................100
7.4.1
Data processing functions........................................................................................................ 100
7.4.2
Data processing architecture and organisation ........................................................................ 102
7.4.3
Data processing responsibilities and expertise ........................................................................ 104
8
Management..........................................................................................................................................106
8.1
Introduction...................................................................................................................................106
8.2
Industrial organisation...................................................................................................................106
8.3
Payload Procurement ....................................................................................................................106
8.4
Euclid Schedule.............................................................................................................................107
Mass and power budget............................................................................................................. 61
VIS Critical items...................................................................................................................... 61
Near IR spectrometer and imaging photometer (NISP)..................................................................62
Opto-mechanical design ............................................................................................................ 63
Detector system ......................................................................................................................... 65
Thermal Architecture................................................................................................................. 65
Electronics Architecture ............................................................................................................ 66
Mass and power budget............................................................................................................. 68
NISP Critical Items.................................................................................................................... 68
Field of View gap-filling evaluation...............................................................................................69
5

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8.5
8.6
Science Management ....................................................................................................................107
Instrument procurement and EMC organisation...........................................................................108
8.6.1
Hardware activities by the EMC.............................................................................................. 108
8.6.2
Consortium organisation.......................................................................................................... 109
References.....................................................................................................................................................112
Acronyms ......................................................................................................................................................113

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2. Scientific Objectives

13
2 Euclid Science Objectives
2.1 Cosmology today
Our view of the Universe has changed dramatically over the past century: less than a hundred years ago it
was believed to consist of only our own Galaxy. The discovery of the expansion of the Universe and the sub-
sequent realisation that the Universe is very old, but had a beginning, are major triumphs of astronomy that
have changed our view of humanity’s place in the Universe. At the turn of the Millennium, further crucial
observational progress led to the emergence of the concordance cosmological model. This provides a
remarkably accurate description of a wide range of independent observations through a fully self-consistent
theoretical framework with a small number of parameters. However, it relies on two untested assumptions
about the initial conditions of the Universe and the nature of gravity itself, as well as the existence of two do-
minant components whose nature is entirely unknown. Of these unknown components, about 76% of the
overall mass-energy density is in the form of dark energy, which is causing the Universe to accelerate its
expansion at the current epoch. Another 20% is in the form of non-baryonic dark matter, which exerts a gra-
vitational attraction as normal matter, but cannot emit or absorb light. The biggest of all these puzzles, one
that raises a potential crisis in fundamental physics, is the nature of dark energy. There are a plethora of ideas,
ranging from including a “cosmological constant” in the Einstein field equations of General Relativity (GR),
to the inclusion of additional fields, or even a revision of our theory of gravity. More than an order of
magnitude improvement in the quality and quantity of observational data is needed to address this problem.
Euclid has been conceived to make this step forward by measuring the expansion history and growth of
large-scale structure with a precision that will allow us to distinguish time-evolving dark energy models from
a cosmological constant, and to test the theory of gravity on cosmological scales. The same measurements
also constrain the initial conditions in the very early Universe, by determining the statistical distribution of
the primordial density fluctuations with high precision, on scales that cannot be probed using observations of
the cosmic microwave background (CMB). As such, Euclid will not only provide insights into the ultimate
fate of the Universe, but also into how it began. Euclid will achieve this by taking the concept of galaxy sur-
veys into a new regime in terms of size and control of systematics. Surveys such as the Sloan Digital Sky
Survey (SDSS) have provided one of the primary pillars upon which the concordance model has been built.
Euclid will take this to a new level by surveying 15,000 deg2 of the extra-galactic sky. It will directly map
the dark matter distribution in the Universe through weak gravitational lensing by imaging 1.5 billion
galaxies with HST-like resolution and providing near infrared (NIR) photometry. At the same time, it will
carry out a spectroscopic redshift survey of 50 million galaxies over a volume 500 times larger than the
SDSS, observing galaxies over 75% of the lifetime of the Universe.
The scientific impact of Euclid, however, is not limited to cosmology and in fact spans most of extra-galactic
astronomy. The unique combination of high-resolution optical imaging, multi-band NIR imaging and
spectroscopy up to z~2 over most of the extra-galactic sky, will result in a vast range of non-cosmology
science. It is only possible to provide a selection of scientific highlights in this document. The legacy of
Euclid will last for many decades and will have an impact across all of physics and astronomy. Finally, when
we make the leap forward that Euclid enables us to do, important serendipitous discoveries will be almost a
certainty.
2.1.1 A dark Universe
Of the two disturbing ingredients of the concordance model, dark matter is the most familiar. Evidence for its
existence goes back to the 1930s, when Fritz Zwicky realised that the dynamical mass of the Coma cluster
exceeded that expected from the luminosities of its member galaxies, suggesting a dominant non-luminous
(dark) component. In the 1970s further support came from the measurements of flat rotation curves in spiral
galaxies. Finally, a significant dark matter contribution was found to be necessary to reconcile the low level
of anisotropy in the CMB with the very existence of galaxies and structures today. This, together with early
observations of large-scale structures by the first extensive galaxy redshift surveys during the 1980s, brought
forth the emergence of the Cold Dark Matter (CDM) paradigm. The “standard” CDM scenario was based on