  Millimeter-wave continuum astronomy is today an indispensable tool for both
general Astrophysics studies and Cosmology. General purpose, large
field-of-view instruments are needed to map the sky at intermediate angular
scales not accessible by the high-resolution interferometers and by the coarse
angular resolution space-borne or ground-based surveys. These instruments have
to be installed at the focal plane of the largest single-dish telescopes. In
this context, we have constructed and deployed a multi-thousands pixels
dual-band (150 and 260 GHz, respectively 2mm and 1.15mm wavelengths) camera to
image an instantaneous field-of-view of 6.5arc-min and configurable to map the
linear polarization at 260GHz. We are providing a detailed description of this
instrument, named NIKA2 (New IRAM KID Arrays 2), in particular focusing on the
cryogenics, the optics, the focal plane arrays based on Kinetic Inductance
Detectors (KID) and the readout electronics. We are presenting the performance
measured on the sky during the commissioning runs that took place between
October 2015 and April 2017 at the 30-meter IRAM (Institut of Millimetric Radio
Astronomy) telescope at Pico Veleta. NIKA2 has been successfully deployed and
commissioned, performing in-line with the ambitious expectations. In
particular, NIKA2 exhibits FWHM angular resolutions of around 11 and 17.5
arc-seconds at respectively 260 and 150GHz. The NEFD (Noise Equivalent Flux
Densities) demonstrated on the maps are, at these two respective frequencies,
33 and 8 mJy*sqrt(s). A first successful science verification run has been
achieved in April 2017. The instrument is currently offered to the astronomical
community during the coming winter and will remain available for at least the
next ten years.
