The MuSiCoS Polarimeter




The MuSiCoS échelle spectrograph (installed on the 2m Télescope Bernard Lyot at Pic du Midi) is now equipped with a dedicated polarimeter, available for the whole astronomical community. It was designed and constructed at Observatoire Midi-Pyrénées in 1996, with CNRS/INSU, MENESR and Région Midi Pyrénées funds.

This new instrument is one of the only spectropolarimetric facilities worldwide and should be extremely well adapted for studying stellar magnetic fields and in particular their topologies through rotational modulation of linear (Stokes Q and U) and circular polarisation (Stokes V) Zeeman signatures in line profiles. It is also a very interesting tool for investigating geometries of non-axisymmetric circumstellar environments through depolarisation structures of spectral lines formed within the scattering envelope.

This page presents the basic characteristics of this instrument, describes its operation and details the first results obtained to date during the first three observing runs at TBL (August 1996, February 1997) and CFH (November 1996). A more practical users' guide to the MuSiCoS polarimeter is also available for new observers.


Description of the instrument

The new MuSiCoS polarimeter is mounted at the TBL Cassegrain focus through an interface module including all usual spectroscopic calibration facilities (halogen and thorium/argon lamp). As shown below, stellar light is collected in a 2" (500 micron) entrance aperture. Linear/circular sheet polarisers can be inserted in the beam for all tuning and checkout procedures. One quarter-wave and one half-wave retarder can also be inserted in the beam and rotated to achieve, in conjunction with a beamsplitter, a circular or linear analysis of the stellar light respectively. A focal reducer then converts the beam aperture from f/25 to f/2.5. Finally, two 50 micron core optical fibres collect the two beams emerging from the Zeeman analyser (corresponding to both orthogonal states of the selected polarisation form) and convey them to the MuSiCoS spectrograph.

The two retarders were supposed to be Halle superachromatic Quartz-MgF2 wave plates, i.e. the only commercial crystalline retarders with close to nominal retardance and fast axis direction throughout the whole spectral domain of the MuSiCoS spectrograph (390 to 870 nm). However, we discovered that these wave plates introduce large ripples both in polarisation and intensity spectra (with respective peak-to-peak amplitudes of about 1% and 2% respectively), most likely due to interference within the multiple cement layers of the retarders. This is very likely a general problem of all Halle superachromatic retarders (rather than a specific defect of ours), since all similar plates that we know of (those of the AAT and WHT polarimetric modules) also generate such ripples. We therefore switched to a less achromatic Fichou quarter wave plate for which such ripples are at least 20 times weaker. The expense is of course that circular spectropolarimetry is now restricted to a wavelength range of 400 to 700 nm only (till the Halle waveplates are fixed and if they can be fixed) and requires a special "intermediate" setup of the MuSiCoS spectrograph between the two nominal "blue" and "red" configurations. For the same reason, we no longer use the Halle half-wave plate (also replaced by a Fichou half-wave plate) and rotate instead the polarimeter as a whole, making linear spectropolarimetry perfectly achromatic throughout the whole wavelength range of the MuSiCoS spectrograph but slightly less convenient to operate than circular spectropolarimetry (see below).

The beamsplitter is made of two crossed calcite blocks (Savart plate). The advantage of such a design is that both beams emerging from the analyser are shifted by the same amount with respect to the incident beam and are therefore more symmetric than with a conventional single calcite block.

The focal reducer was optimised for our purpose to minimise longitudinal chromatic aberrations.

The optical fibre we selected to collect the two beams and send them to the MuSiCoS spectrograph is the H-treated low-OH Ceram-Optec fibre with 50/60 micron core/cladding diameters, whose transmission is optimised in the 350-1100 nm wavelength band. Transmission measurements indicate that this fibre does not match our expectations in the infrared domain, but is still acceptable up to 700 nm.

The total throughput of the MuSiCoS spectropolarimeter (atmosphere, telescope and detector included) peaks at about 0.85%, i.e. the same as that of the MuSiCoS spectrograph.


Instrument control and data reduction

The polarimeter is controlled from a terminal in the TBL control room through a serial link, a microcontroller card and attendant electronics. Rotating the polarimeter wheel, the waveplate wheel, and the waveplate holders is achieved with three stepper motors and three proximity detectors. Four microswitches are used to check that the instrument has safely reached the requested positions. Thirteen polarimeter configurations are available altogether, each associated to a two-character command. A list of these commands (and error messages) is given here.

For each polarisation exposure, one needs to obtain a sequence of four individual subexposures at alternating waveplate or instrument azimuths:

The last operation consists in feeding the dedicated reduction package ESpRIT (installed on the DecAlpha workstation of the TBL control room) with the sequence of four subexposures (plus flat fields, arc and bias frames) to get the corresponding wavelength-calibrated optimally-extracted polarisation spectrum (within typically 5 min).

A more extensive users' guide to this instrument (description of set-up, operation and data reduction) is also available for potential observers.


First results

We find that the MuSiCoS spectropolarimeter performs very well for detecting polarisation/depolarisation structures in line profiles.




Our polarimeter can in principle also be used to estimate continuum polarisation in stellar spectra. However, we observe that our fibre setup introduces spurious continuum polarisation through small motions of the double image at fibre level (due to wave plate rotations, instrument flexures with telescope movement or simply small random fluctuations in light injection) coupled to slight differential mispositioning of the two images onto the two fibres (due to small magnification errors in the focal reducer, to slight azimuthal misalignment of the analyser with respect to the fibre or to the chromatism of the Zeeman analyser). The accuracy to which continuum polarisation can be estimated with our instrument is typically 0.8% rms for circular polarisation, and about twice as much for linear polarisation. For such programs, our instrument is thus poorly competitive with photopolarimeters like Sterenn or Cassegrain low-resolution spectropolarimeters, usually an order of magnitude more accurate at measuring continuum polarisations of stellar objects. Note that this problem does not affect measurements of line profile polarisation signatures as we usually remove a posteriori any true or spurious continuum polarisation from the observations in this case.


Any questions/comments/suggestions about either this page or the MuSiCoS polarimeter should be sent to jean-francois.donatiATnospam.ast.obs-mip.fr



Related publications

Shorlin S.L.S, Wade G.A., Donati J.-F., Landstreet J.D., Petit P., Sigut T.A.A., Strasser S., ``A sensitive search for magnetic fields in B, A and F stars'' (2002) A&A 392, 637

Donati J.-F., Wade G.A., Babel J., Henrichs H.F., de Jong J.A., Harries T.J., ``The magnetic field and wind confinement of beta Cephei: new clues for interpreting the Be phenomenon?'' (2001) MNRAS 326, 1265

Bagnulo S., Wade G.A., Donati J.-F., Landstreet J.D., Leone F., Monin D.N., Stift M.J., ``A study of polarised spectra of magnetic CP stars: predicted vs. observed Stokes IQUV profiles for beta CrB and 53 Cam'' (2001) A&A 369, 889

Wade G.A., Donati J.-F., Landstreet J.D., ``An atlas of Zeeman polarisation in the Stokes IQUV spectrum of Beta Coraonae Borealis'' (2001) New Astronomy 5, 455

Wade G.A., Donati J.-F., Landstreet J.D., Shorlin S.L.S., ``Spectropolarimetric measurements of magnetic Ap and Bp stars in all four Stokes parameters'' (1999) MNRAS 313, 823

Wade G.A., Donati J.-F., Landstreet J.D., Shorlin S.L.S., ``High precision magnetic field measurements of Ap and Bp stars'' (1999) MNRAS 313, 851

Donati J.-F., Wade G.A., ``On the magnetic field and circumstellar environment of the young O7 star theta1 Orionis C'' (1999) A&A 341, 216

Donati J.-F., Catala C., Wade G.A., Gallou G., Delaigue G., Rabou P., ``A dedicated polarimeter for the MuSiCoS échelle spectrograph'' (1999) A&AS 134, 149

Donati J.-F., Semel M., Carter B.D., Rees D.E., Cameron A.C., ``Spectropolarimetric observations of active stars'' (1997) MNRAS 291, 658

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© Jean-François Donati, last update on 2003 Jun. 16