Euclid Definition Study Report
  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.
