  The Probe of Inflation and Cosmic Origins (PICO) is a probe-class mission
concept currently under study by NASA. PICO will probe the physics of the Big
Bang and the energy scale of inflation, constrain the sum of neutrino masses,
measure the growth of structures in the universe, and constrain its
reionization history by making full sky maps of the cosmic microwave background
with sensitivity 80 times higher than the Planck space mission. With bands at
21-799 GHz and arcmin resolution at the highest frequencies, PICO will make
polarization maps of Galactic synchrotron and dust emission to observe the role
of magnetic fields in Milky Way's evolution and star formation. We discuss
PICO's optical system, focal plane, and give current best case noise estimates.
The optical design is a two-reflector optimized open-Dragone design with a cold
aperture stop. It gives a diffraction limited field of view (DLFOV) with
throughput of 910 square cm sr at 21 GHz. The large 82 square degree DLFOV
hosts 12,996 transition edge sensor bolometers distributed in 21 frequency
bands and maintained at 0.1 K. We use focal plane technologies that are
currently implemented on operating CMB instruments including three-color
multi-chroic pixels and multiplexed readouts. To our knowledge, this is the
first use of an open-Dragone design for mm-wave astrophysical observations, and
the only monolithic CMB instrument to have such a broad frequency coverage.
With current best case estimate polarization depth of 0.65 microK(CMB}-arcmin
over the entire sky, PICO is the most sensitive CMB instrument designed to
date.
