What is "planetpol"

PlanetPol was a ground based, high sensitivity polarimeter based at the William Herschel Telescope that has now been decommissioned. It was the most sensitive astronomical visual polarimeter ever built in fractional polarisation, a mantle that since its decommissioning now belongs to HIPPI. Although the device could be used for a wide range of astronomy, its primary use was the detection of extrasolar planets.