Because spectropolarimetry is the most sensitive of the available
field discovery methods, astronomers have been using ISIS on the William Herschel Telescope (WHT), FORS on
the Very Large Telescope (VLT), and Espadons on the Canada-France-Hawaii Telescope (CFHT). Each of these instruments has
specific strengths.
Both ISIS and FORS are particularly well suited to
detecting very weak fields in relatively faint (V > 14) white
dwarfs. Remarkably, because ISIS can do spectropolarimetry
at an optimal resolving power around the Halpha line in the red, it is possible to obtain the most sensitive field measurements, even though the telescope area is only one-quarter of
that of the VLT.
The ongoing ISIS survey to find more weak-field white dwarfs has the potential to
substantially improve the knowledge of the actual distribution of
magnetic field strengths among white dwarfs, to provide more bright examples of
weak-field stars for detailed modelling and analysis, and to assist us
in understanding whether magnetic fields decay during white dwarf
cooling or whether some process(es) generate new magnetic flux.
More information:
J. D. Landstreet, S. Bagnulo, S., G. Valyavin, A. F. Valeev, 2018, "Monitoring and modelling of white dwarfs with extremely weak magnetic fields. WD 2047+372 and WD 2359-434", A&A, 607, A92 [ADS].
S. Bagnulo and J. D. Landstreet, 2018, "Searching for the weakest detectable magnetic fields in white dwarfs. Highly-sensitive measurements from first VLT and WHT surveys", A&A, 618, A113 [ADS]