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. |
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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.
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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).
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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. | |
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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:
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