The Polarimetric and Helioseismic Imager on Solar Orbiter
  This paper describes the Polarimetric and Helioseismic Imager on the Solar
Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument
to observe the Sun from outside the Sun-Earth line. It is the key instrument
meant to address the top-level science question: How does the solar dynamo work
and drive connections between the Sun and the heliosphere? SO/PHI will also
play an important role in answering the other top-level science questions of
Solar Orbiter, as well as hosting the potential of a rich return in further
science.
  SO/PHI measures the Zeeman effect and the Doppler shift in the FeI 617.3nm
spectral line. To this end, the instrument carries out narrow-band imaging
spectro-polarimetry using a tunable LiNbO_3 Fabry-Perot etalon, while the
polarisation modulation is done with liquid crystal variable retarders (LCVRs).
The line and the nearby continuum are sampled at six wavelength points and the
data are recorded by a 2kx2k CMOS detector. To save valuable telemetry, the raw
data are reduced on board, including being inverted under the assumption of a
Milne-Eddington atmosphere, although simpler reduction methods are also
available on board. SO/PHI is composed of two telescopes; one, the Full Disc
Telescope (FDT), covers the full solar disc at all phases of the orbit, while
the other, the High Resolution Telescope (HRT), can resolve structures as small
as 200km on the Sun at closest perihelion. The high heat load generated through
proximity to the Sun is greatly reduced by the multilayer-coated entrance
windows to the two telescopes that allow less than 4% of the total sunlight to
enter the instrument, most of it in a narrow wavelength band around the chosen
spectral line.
