The Polarimetric and Helioseismic Imager (PHI) is a solar polarimeter and magnetograph designed to measure the vector magnetic field and line-of-sight velocity in the solar photosphere. The science objectives for PHI are to: investigate the links between the solar surface, corona, and inner heliosphere; explore, at all latitudes, the energetics, dynamics and fine-scale structure of the Sun’s magnetized atmosphere; and probe the solar dynamo by observing the Sun’s high-latitude field, flows and seismic waves.

PHI is a diffraction limited, wavelength tunable, quasi-monochromatic, polarization sensitive imager with two telescopes; a High Resolution Telescope (HRT) and a Full-Disc Telescope (FDT) which alternatively feed a common filtergraph and a 2048 x 2048 pixel focal plane array. The working wavelength is 617.3 nm (Fe spectral line). The instrument is mounted behind the sunshield, with heat-rejecting entrance window apertures for the telescopes. It is made of an aluminum-beryllium alloy and low-expansion carbon-fiber reinforced plastic. The HRT is a decentered Ritchey-Chretien design with a 140mm diameter pupil. It has a field-of-view of 0.28 x 0.28 degrees and a spatial resolution of about 200 km at closest perihelion. The FDT has an entrance pupil diameter of 17.5 mm and an effective focal length of 579 mm. The field-of-view is 2 degrees, resolution at 0.28 AU is 725 km/pixel.

The filtergraph consists of a lithium niobide (LiNbO3) solid-state etalon which can change refractive index as a function of voltage and select passbands of order 100 milliangstroms wide. A Polarization Modulation Package performs polarimetric analysis for each telescope. It comprises two nematic liquid crystal retarders and a linear polarizer analyzer.

For more detailed information on PHI, see:
https://arxiv.org/pdf/1502.03368.pdf