A wide field-of-view low-resolution spectrometer at APEX: instrument
  design and science forecast
  Characterise the large-scale structure in the Universe from present times to
the high redshift epoch of reionisation is essential to constraining the
cosmology, the history of star formation and reionisation, measuring the gas
content of the Universe and obtaining a better understanding of the physical
process that drive galaxy formation and evolution. Using the integrated
emission from unresolved galaxies or gas clouds, line intensity mapping (LIM)
provides a new observational window to measure the larger properties of
structure. This very promising technique motivates the community to plan for
LIM experiments.
  We describe the development of a large field-of-view instrument, named
CONCERTO, operating in the range 130-310 GHz from the APEX 12-meters telescope.
CONCERTO is a low-resolution spectrometer based on the Lumped Element Kinetic
Inductance Detectors technology. Spectra are obtained using a fast Fourier
Transform Spectrometer (FTS), coupled to a dilution cryostat with base
temperature of 0.1K. Two 2 kilo-pixels arrays of LEKID are mounted inside the
cryostat that also contains the cold optics and the front-end electronics.
  We present in detail the technological choices leading to the instrumental
concept, together with the design and fabrication of the instrument and
preliminary laboratory tests on the detectors. We also give our best estimates
of CONCERTO sensitivity and give predictions for two of the main scientific
goals of CONCERTO, i.e. a [CII]-intensity mapping survey and observations of
galaxy clusters.
  We provide a detail description of the instrument design. Based on realistic
comparisons with existing instruments developed by our group (NIKA, NIKA2, and
KISS), and on laboratory detectors characterisation, we provide an estimate of
CONCERTO sensitivity on sky. Finally, we describe in detail two out of the main
science goals offered by CONCERTO at APEX.
