description of critical items

The most critical items of ESPaDOnS are mainly the large optical components used in the spectrograph: 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 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 picture on the rights shows two prism train mounted in their mechanical support. One handle of the transfer collimator cage is observed through the prism train, visualising the effect of ESPaDOnS crossdisperser.

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 image on the right shows the grating in its mechanical mount (with its holding base removed). Sun light going through a regular hole pattern in a nearby shutter is dispersed by the grating, producing the bright roughly horizontal line pattern seen in the image.

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.

The Bowen-Walraven image slicer

To transform the circular image of each fibre into a narrow elongated strip at spectrograph entrance, we use a Bowen-Walraven image slicer high enough to process the images of all three fibres in a row. Rotating slightly the slicer about a vertical axis enables to change the number of obtainable slices per fibre, making this device very flexible.

The image on the right presents the image slicer built for us by Optique Fichou (France). The slicer is made of 3 individual (optically contacted) components, a small prism on the front (entrance prism), a parallel place inclined at 45deg in the middle and a large prism on the rear (output prism). The image to be sliced comes in by the small front prism, is reflected an even number of times within the thin parallel plate in the middle and goes out through the rear prism once slicing is complete.

The total size of this device is rather small, being about 15mm high in its largest section.

© Jean-François Donati, last update July 21 2004