4 Conclusion

We have presented in this paper the technical characteristics of a new polarimeter dedicated to the MuSiCoS échelle spectrograph that can collect high resolution circularly and linearly polarised spectra of stellar objects.

This polarimeter was initially designed to be very achromatic in the 390 to 870 nm range, by using in particular the Halle superachromatic crystalline retarders. However, we discovered that such wave plates generate large amplitude ($1-2\%$ peak-to-peak) intensity and polarisation ripples and turn out to be unusable for our purpose. We replaced the quarter-wave retarder with a less achromatic Fichou plate (for which such ripples are at least 20 times smaller), at the expense of restricting circular spectropolarimetric experiments to a 400 to 700 nm interval. We avoid using a half wave plate by simply rotating our instrument at specific azimuths.

We find that this instrument is very good at measuring polarisation or depolarisation structures in line profiles, with a relative accuracy of down to at least 0.002%. It is therefore one of the only facilities worldwide for studying magnetic topologies of active and chemically peculiar stars (through rotational modulation of linearly and circularly polarised Zeeman signatures in line profiles). It should also be a very interesting tool for investigating geometries of non-axisymmetric circumstellar environments (through depolarisation of spectral lines formed within the scattering envelope). This instrument could in principle also be used to estimate linear and circular continuum polarisation of stellar objects, but our fibre setup limits the accuracy of such measurements to a level of about 1%.

Additional up to date information on the MuSiCoS spectropolarimeter (in its TBL configuration) and on its operation is available on the world-wide-web at URL http://www.obs-mip.fr/omp/umr5572/magnetisme/polarmus.html.

Acknowledgements

We thank Cyril Delaigue for writing the dedicated microcontroller code, Laurent Parès for a few additional ray tracing computations, Gerardo Avila and Jacques Baudrand for measuring the transmission of our optical fibre, Torsten Böhm and John Landstreet for their involvement in data collection, and Meir Semel, François Ménard, Jean-Louis Leroy and John Landstreet for discussions on polarisation and polarimetric devices. We are also grateful to the referee, Ian Howarth, for his careful reading of the manuscript.