Important
Note

The ESPaDOnS web site, prepared by J.F. Donati, gives a wealth of information on the instrument. The reader will find below a few essential facts, which are mostly excerpts from the original web site.



ESPaDOnS?
The acronym ESPaDOnS stands for an Echelle SpectroPolarimetric Device for the Observation of Stars. The instrument has been built at Observatoire Midi-Pyrenees under the leadership of J.F. Donati.
 
ESPaDOnS has been funded through an international partnership :
- France (CNRS/INSU, Ministère de la Recherche, LATT, Observatoire Midi-Pyrénées, Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, Observatoire de Paris-Meudon),
- Canada (NSERC), 
- the European Space Agency (ESTEC/RSSD),
- the Canada-France-Hawaii Telescope Corporation.

CFHT  is operated by the National Research Council of Canada, the Institut National des Science de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.


Polarimetry Radiation from a light source can be polarised. When the electric vector of the electromagnetic radiation vibrates in a fixed plane, the radiation is called linearly polarised. When the electric vector describes an helix about the direction of propagation, the radiation is said to be circularly polarised. A combination of both usually yields elliptical polarisation. Polarimetry aims at measuring the degree to which a radiation from a light source is polarised, as well the polarisation state of the corresponding light.

A polarimeter usually involves retardation components (cristalline plates or Fresnel rhombs, retarding one component of the electric vibration with respect to the other by a fixed amount) in association with a birefringent cristal (which splits the two orthogonal states of linear polarisation of the incoming beam into two separate beams). To perform a circular polarisation light analysis for instance, one normally uses at least a quarter-wave plate (changing the incoming circular polarisation into linear polarisation) and a birefringent cristal (to search for potential linear polarisation emerging from the quarter-wave plate).
 
Polarimetric observations of stellar sources can inform us on the geometry and chemical composition of circumstellar environments (winds, discs, dust envelopes) through the continuum polarisation scattering processes produce.

It can also tell us about stellar surface magnetic fields through the line profile polarisation that the Zeeman effect generates in photospheric spectral lines.


Spectropolarimetry Although spectroscopy of unpolarised light and broadband photopolarimetry are quite common tools for modern astronomers, the combination of both, called spectropolarimetry, is much more unusual. The coupling of both instruments is indeed not totally trivial. While the polarisation analysis is usually performed at the primary or Cassegrain focus of a telescope (to minimise instrumental polarisation produced by oblique reflections in the telescope), high-resolution spectroscopy is often done at Coudé focus (for better spectrograph stability). A double-fibre feed (one fibre for each orthogonal polarisation state) must therefore be used to convey the light from the Cassegrain focus down to the Coudé room and couple both foci with no compromise either on the accuracy of the polarisation analysis or on the spectrograph stability.


ESPaDOnS
Main Features
ESPaDOnS is a bench-mounted high-resolution echelle spectrograph/spectropolarimeter fibre-fed from a Cassegrain module including calibration and guiding facilities, as well as an optional polarisation analyser. It can deliver:
    - a complete optical spectrum (from 370 to 1,050 nm) in a single exposure with a resolving power of about 68,000 (in spectropolarimetric and 'object+sky' spectroscopic mode) and up to 81,000 (in 'object only' spectroscopic mode); with a 79 gr/mm grating and a 2kx4.5k ccd detector, the full spectrum spans 40 grating orders (from order #61 in the blue to order #22 in the red);

    - 15% to 20% peak throughput (telescope and detector included); this performance is obtained thanks to the very efficient dual pupil design of Baranne (along which many modern spectrographs such as uves, feros and harps were designed) as well as to the most recent advances in glass and coating technologies (allowing to produce large dioptric optics with low reflectance and absorption as well as high efficiency optical fibres and image slicers);

    - continuum subtracted linear and circular polarisation spectra of the stellar light (in polarimetric mode); using Fresnel rhombs instead of standard cristalline plates suppresses the usual problems of interference patterns in the collected spectra, with the additional advantage of being much more achromatic.
 
Polarimetric observations of stellar sources can inform us on the geometry and chemical composition of circumstellar environments (winds, discs, dust envelopes) through the continuum polarisation scattering processes produce.

It can also tell us about stellar surface magnetic fields through the line profile polarisation that the Zeeman effect generates in photospheric spectral lines.