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ESPaDOnS: description of critical items http://webast.ast.obs-mip.fr/magnetisme/espadons_new/critical.html
ESPaDOnS
description of critical items
The most critical items of ESPaDOnS are the large optical components used in the spectrograph, and in particular:
the f/2 dioptric camera;
the two large highly reflecting parabolic collimators;
the twin prism cross disperser;
the R2 diffraction grating.
Additional information about these components is given below.
The f/2 fully dioptric camera
The f/2 fully dioptric camera was built for ESPaDOnS by
EADS/Sodern (France). The image on the right shows the
camera while being qualified on the optical bench of
EADS/Sodern. With a focal length of 388mm, it includes 7
large lenses in 4 blocks, the first one being a massive
quadruplet with a free aperture diameter of 220mm (as can be
seen on the close-up view of the camera optical design).
This camera is designed to yield a spherical focal surface whose
curvature compensates that induced by the parabolic
collimators over the whole field of view (whose diagonal
reaches 9deg). The associated image quality is very good
throughout the whole wavelength domain with a spot diagram
featuring a full width at half maximum smaller than 0.010mm
except for the most distant field where it reaches about
0.013mm. The corresponding wavefront distortion is better
than lambda/5 rms except in the most distant field where it is
of order lambda/3 rms.
High transmission broadband antireflection coating was used on all air/glass surfaces to obtain the highest possible throughput.
The achieved throughput ensures that 85% of the photons reaching the camera are redirected to the ccd detector throughout
most of the wavelength domain.
The two parabolic collimators
The two Zerodur parabolic collimators of ESPaDOnS, cut
from a single 680mm parent with a focal length of 1500mm,
were polished and coated for ESPaDOnS by Optical Surfaces
(UK). The image on the right shows the main collimator seen
from behind, with its 6 invar holding pins (black circles) just
glued on the rear, top and side surfaces, and before being
mounted in its metallic cage.
The mirrors were polished at a surface accuracy of better than
lambda/30 rms (as demonstrated from the interferograms of
the main collimator and transfer collimator) to ensure that the
wavefront distortion resulting for a double pass on the main
collimator and a single pass on the transfer collimator remains
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