New Ultra-Fast Polarimeter Sees the First Light with High Precision and Accuracy

Polarimetry is a sensitive technique for detecting magnetic fields, atmospheres, surfaces, atoms, molecules and dust grains, in astronomical objects such as the Sun, other distant stars, solar and exo-planets, asteroids, nebulae, and even black holes. The new, high-precision astronomical imaging polarimeter DIPOL-UF (Double Image Polarimeter – Ultra Fast) has been built in cooperation between KIS and a research group from the University of Turku (UTU), Finland; see the UTU press release here: https://www.utu.fi/en/news/news/new-astro-polarimeter-sees-first-light-at-the-nordic-optical-telescope.

1) The DIPOL-UF polarimeter mounted at the 2.5m Nordic Optical Telescope, La Palma, Canary Islands, Spain. 2) A normal image of sky with stars of interest in red circles. A total stellar flux in different colors can be measured from such images. 3) The same area of sky as seen by the DIPOL-UF polarimeter: A special optical element, a plane-parallel calcite plate, inserted into the optical path splits each stellar image into two orthogonally polarised ones. These are marked as “o” (ordinary, which is not shifted) and “e” (extraordinary, which is shifted). The o- and e- images of the stars from the above image are marked by blue and yellow circles, respectively. The polarisation of stellar light is the difference between the e- and o- fluxes normalized to their sum (total flux).

The DIPOL-UF is equipped with three high-speed ANDOR EM-CCD cameras which are capable of recording images of the sky with the impressive rate of 56 frames per second. The instrument also employs three industrial grade mini-PCs and special optical layout to record images in polarised light simultaneously in three colours (blue, green and red). The first-light observations have been carried out in July 2019 at the Nordic Optical Telescope (NOT) on La Palma, Canary Islands. During two nights DIPOL-UF measured polarisation from a number of stars with an accuracy and precision better than ten parts per million. Such a polarimetric sensitivity is hard to achieve even for the Sun. Studying polarization of other stars and exoplanets allow us to understand the Sun and the Solar system in the context of our Galaxy and the Universe.

The process of observations with DIPOL-UF is highly automated and all observations can be done remotely from a few thousand kilometres away. In October 2019, the University of Turku becomes a co-owner of the NOT. The cooperation agreement between KIS and UTU will allow joint research programmes with the DIPOL-UF and the NOT.