CFHT, Instruments, Spectroscopy, ESPaDOnS

ESPaDOnS: an Echelle SpectroPolarimetric Device
for the Observation of Stars at CFHT

Description of ESPaDOnS

ESPaDOnS project

ESPaDOnS was designed and constructed at Observatoire Midi-Pyrénées (OMP) in France, by a team of 15 scientists, engineers, technicians, and administrators. J-F Donati is the principal investigator and system scientist, and J-P Dupin was the project manager and system engineer. C. Catala (France), J. Landstreet (Canada), and B. Foing (Netherland) were mostly involved in finding the funds (755k euros), which were provided by France (CNRS/MEN), CFHT, Canada (NSERC), and ESA/ESTEC/RSSD.

CFHT scientists, engineers and technicians were also involved in the Final Design Reviews, integration of the instrument, tests, and in implementing the Graphical User Interface.

For more details on the people involved and the budget, please see the Project team and budget page from OMP.

ESPaDOnS general description

ESPaDOnS is a bench-mounted high-resolution echelle spectrograph/spectropolarimeter [picture] fiber-fed from a Cassegrain module [photo]. This module includes the calibration and guiding facilities, and an optional polarization analyzer. The spectrograph is located in the 3rd floor Coude room, and is housed in a thermal enclosure [photo] to minimize temperature and pressure fluctuations, which affect the spectrograph's stability.

ESPaDOnS was designed to obtain a complete optical spectrum (from 370 to 1,050 nm, with 3 very small gaps: 922.4-923.4 nm, 960.8-963.6 nm, 1002.6-1007.4 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).

The total peak throughput is between 15% and 20% (telescope and detector included). This high throughput was obtained using 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 fibers and image slicers).

ESPaDOnS gives continuum subtracted linear and circular polarization spectra of the stellar light (in polarimetric mode). The use of Fresnel rhombs instead of standard crystalline plates suppresses the usual problems of interference patterns in the collected spectra, with the additional advantage of being much more achromatic.

ESPaDOnS has a fiber agitator, a device that shakes or agitates the optical fiber just before it enters the spectrograph. This device is used to remove modal noise present in optical fibers.

The Cassegrain module

The calibration/guiding module [photo] includes an atmospheric dispersion corrector (made of 2 separate null-deviation prisms rotating independently from each other and cancelling out in real time the atmospheric refraction), a compact 1kx1k ccd camera [photo] looking at the instrument aperture (that can be used to autoguide on the star of interest or on any other star present in the 100" camera field of view), and a calibration wheel that can replace the stellar beam by various sorts of calibration light. The instrument's aperture is a 1.6 arcsec hole drilled in a tilted mirror [photo]. A bigger hole (also seen in the previous photo) is used to collect the light from the sky for one of the spectroscopic modes. The light reflected in the tilted surface goes to the guiding camera.

There are 2 flat field (tungsten) lamps. One is used at a low intensity with a red filter and the other is used at a higher intensity with a blue filter. This produces a flat field [image] with flux in both the red and the blue part of the spectrum. There are also Thorium comparison lamps [image]. Diffusers and polarizers with known orientations can also be inserted in the beam.

The polarimeter includes one quarter-wave and two half-wave Fresnel rhombs, and a Wollaston prism, providing a very achromatic polarization analysis of the stellar light without producing the usual spectral interference patterns [figure]. Two images of the main 1.6" instrument aperture are produced at polarimeter output, each image gathering the photons from the incoming beam associated with one of the two orthogonal vibration states of the selected polarization).

In non polarimetric mode, the Wollaston prism is removed and replaced with a wedge plate producing at polarimeter output a single image gathering all photons from the incoming beam (a second image is also produced in this mode, gathering photons from a second instrument aperture offset from the main one by about 8" and with which we estimate the spectral contribution from the sky background, if needed).

A bundle of 3 optical fibers collects photons at the bottom of the Cassegrain module. Only one fiber is used in the spectroscopic 'star only' mode, while 2 different pairs of fibers are used for the other 2 modes [figure].

The spectrograph

The light brought by the optical fibers get to the spectrograph's tunable Bowen-Walraven image slicer [photo]. The slicer slices the twin circular images of the fiber heads at a rate of 3 or 6 slices [photo] per fiber (depending on the selected instrument configuration) into a pair of narrow images at the spectrograph slit level. About 40% to 45% of the stellar photons that reached the telescope made get to the spectrograph.

The spectrograph is set up in dual pupil configuration and features a 190mm pupil, a double set of high-reflectance collimators [photo] (cut from a single 680mm parabolic parent with 1500mm focal length), a 79 gr/mm R2 200x400mm monolithic grating [photo], a fully dioptric f/2 camera with 388mm focal lens and a 210mm free diameter (7 lenses in 4 blocks, one of them being a 220mm quadruplet) [photo], a high dispersion prism cross disperser (made of a train of 2 identical PBL25Y prisms with 35deg apex and 220mm cross section) [photo] and a ccd detector with 2kx4.5k 0.0135mm square pixels [photo].

This design gives a full spectral coverage of the optical domain (from grating order #61 centered at 372nm to grating order #22 centered at 1029nm, with 3 very small gaps: 922.4-923.4 nm, 960.8-963.6 nm, 1002.6-1007.4 nm) in a single exposure with a resolution in excess of 65,000.

One CCD pixel is equivalent to 2.6 km/s, while the spectra are actually recovered with bin sizes of 1.8 km/s (in other words, the spectral sampling bin is 1.8 km/s or 0.6923 CCD pixel). See the " Spectral domain and resolution" web page [at OMP | CFHT's copy of the page]

The peak throughput of the spectrograph (with ccd detector) is about 40% to 45%, bringing the total instrument peak efficiency at a level of about 15% to 20%.

Many more pictures can be found on the OMP ESPaDOnS webpage [OMP page | CFHT's copy of it]

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This CFHT Web page is maintained by Nadine Manset (manset -=AT=-