CFHT operating manual - Homepage Usask

ESPaDOnS: instrument details and configurations http://webast.ast.obs-mip.fr/magnetisme/espadons_new/configs.html
Cassegrain unit by the fibre link and image slicer module.
The specific role of these four modules is described below:
the calibration/guiding module includes an atmospheric dispersion corrector (made of 2 separate null-deviation
prisms rotating independantly from each other and cancelling out in
real time the atmospheric refraction), a compact 1kx1k ccd camera
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 (composite featureless spectra from
tungsten lamps for flat fielding purposes, thorium spectra used as a
wavelength reference, fully polarised light with known directions of
vibration);
the polarimeter, including one quarter-wave and two half-wave
Fresnel rhombs coupled to a Wollaston prism, provides a very
achromatic polarisation analysis of the stellar light without producing
the usual spectral interference patterns; 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 polarisation); the
optical design on the right shows the beam passing through the
instrument aperture (top right of image), through the three rhombs
and Wollaston prism (performing the polarisation analysis and
duplicating the input beam) and through the two reimaging triplets
(working at infinite conjugate ratio and bracketing the polarisation
optics), before being refocussed on the optical fibres (bottom left of
image, not shown on picture);
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);
the multiple fibre link collects photons at polarimeter output (one fibre per image) and conveys them to a tunable
Bowen-Walraven image slicer device (with attendant optics) at the entrance of the spectrograph; this device slices the
twin circular images of the fiber heads at a rate of 3 or 6 slices per fibre (depending on the selected instrument
configuration) into a pair of narrow images at the spectrograph slit level; a peak fraction of about 40% to 45% of the
stellar photons that reached the telescope made their way through the previous instrument modules and are injected
into the spectrograph;
the spectrograph, set up in dual pupil configuration, features a 190mm pupil, a double set of high-reflectance
collimators (cut from a single 680mm parabolic
parent with 1500mm focal length), a 79 gr/mm
R2 200x400mm monolithic grating, 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), a high
dispersion prism crossdisperser (made of a
train of 2 identical PBL25Y prisms with 35deg
apex and 220mm cross section) and a ccd
detector with 2kx4.5k 0.0135mm square pixels;
the optical design on the right shows the beam
entering the spectrograph (in dark blue, just
below the grating in the top centre of image),
bouncing successively off the main collimator,
grating, main collimator, flat mirror and
transfer collimator (all shown as light green
surfaces in the image) before passing trough
the double prism cross disperser, the 4-block
fully dipotric camera and the ccd dewar
window (all shown as light blue volumes);
this configuration yields full spectral coverage
of the optical domain (from grating order #61
centred at 372nm to grating order #22 centred
at 1029nm) in a single exposure with a
resolution in excess of 65,000; 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%.
2 of 3 08/07/04 11:28 PM