CFHT operating manual - Homepage Usask

ESPaDOnS: description of critical items http://webast.ast.obs-mip.fr/magnetisme/espadons_new/critical.html
smaller than lambda/5 rms.
All surfaces were coated with Dentons FSS-99 high-reflection
silver coating, ensuring a reflectivity larger than 98% in most of
the spectral domain (from 400 to 1000nm), dropping progressively in the blue down to 85% at 370nm.
The twin prism cross disperser
The two cross-dispersing prisms are made of PBL25Y in GSPLA-2 quality (Ohara equivalent for Schott LF5 in PH3 quality),
with an apex angle of 34.5deg and a free aperture of 220mm. Grinding and polishing was performed by Optique Fichou
(France).
The entrance and output surfaces are polished to ensure that the wavefront distortion remains smaller than lambda/4, as
demonstrated by the interferogram provided by the constructor.
The prism apex angle was set to ensure that the minimum distance between orders is 0.4mm at ccd detector level. With the
present set up, this distance varies from about 0.4mm in the red up to about 1.2mm in the blue (see curve).
Both prisms are coated with a broad band antireflection coating optimised for an angle of incidence of 28deg, ensuring that the
average reflection per air/glass surface is less than 1% in average over the full spectral range, ie that only 4% of the photons are
reflected off the main beam (in average) while passing through the prism train.
The R2 diffraction echelle grating
The R2 echelle grating used for ESPaDOnS (blaze angle 63.4deg) has a ruled area of 204x408mm featuring 79 lines/mm, and
was manufactured by Richardson Lab (USA). It is used in quasi-Littrow configuration, with the output beam being tilted from
the input beam by 1.2deg perpendicularly to the dispersion.
The reflectivity as measured by the constructor shows that the average efficiency over the full spectral range is about 65%. The
wavefront distortion measured on the full aperture is everywhere smaller than the lambda/2 p-v specification.
© Jean-François Donati, last update June 09 2004
2 of 2 08/07/04 11:34 PM