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The HERMES Project: The Spectrograph

The spectrograph is housed in a clean, temperature controlled, room located inside the AAT coudé laboratory. Input light comes from a 400-fiber feed coming from 2dF. Each 140 μm fiber covers 2 arcseconds on the sky and their F/3.65 output is optically relayed at F/6.3 (γ=2) to the HERMES slit.

Hermes room
The HERMES area on the 4th floor of the AAT building.

Spectrograph Optical Design

As shown in the Figure, the HERMES optical design features the following elements:

Each camera feeds one 4096 (spectral direction) x 4096 (spatial direction), 15 μm pixel, Charge Coupled Device (CCD). Spectral resolution is 27,000-30,000 for the 4 pixel sampling of the 2 arcsecond slit width (the averaged projection over a circular fibre reduces the projected 5 pixel sampling to an effective 4 pixels). Spectral coverage is ~ λc/25 around the 4 central wavelengths λc set by the VPH gratings. A slit mask can be inserted to get the same wavelength coverage with a higher (~ 50,000) 2-pixel spectral resolution, at the cost of ~ 40% light loss.


Hermes Final Optical Design

The beam splitters, gratings and cameras coatings are optimized for their respective spectral ranges. Predicted image quality for the GA bands is superb and the end to end 10% efficiency requirement from the telescope to the detector signal is expected to be fulfilled. Baseline beam splitter and grating specifications have been optimized for the Galactic Archaeology Survey case. Alternatives might be purchased later in order to cover other science domains.

The most challenging components are the VPH gratings which will have the highest blaze angles ever made (68 degrees) and, which, due to their large size, will have to be done by a stitching technique. Prototypes are being tested as of Q2, 2010. Another significant challenge is to keep scattered light in the 2-D spectra below the 1% level.

Spectrograph Mechanical Design

One driving requirement is the 1/10th of a pixel (1.5 μm) mechanical and thermal stability during an observation. To that effect, all subsystems (slit assembly, main optics, cryostats & detectors, etc) are assembled on a 4.5 m x 3.4 m bolted aluminum space frame, housed in a thermally controlled room (± 0.5 °C) located in the AAT coudé laboratory and connected to the coudé floor through vibration isolators. Regular focusing of the main optical components (collimator & cameras) is planned. Detailed simulations show that the 1/10th of a pixel goal should then be achieved.

HERMES space

Man and Machine

The spectrograph slit assembly holds two interchangeable slit units. It provides accurate and stable interfaces for the two fiber feeds coming from 2dF, each containing 400 fibers regrouped in 40 Slitlets. Each of the 40 V-grooved channels in the slit bodies houses a lens relay that changes the F/3.16 focal ratio output of the fibers to feed the collimator at F/6.32. To optimize image quality, the slit is curved (convex and spherical) with a radius of curvature of 935.9 mm.

Slit assembly

When needed, a slit mask can be inserted manually on a kinematical mount. The slit assembly also holds a back illumination system, used to position precisely the fibers on the sky target positions: it is engaged by activating pneumatic actuators with guided rods.

The four 4k x 4k detectors are housed in a cryostat operating at -100 °C. The in-house AAO2 controller drives and reads the 4 detectors in parallel. It provides fast & slow reading, pixel binning, and nod & shuffle capabilities.

                  CCD detector
HERMES 4K by 4K E2V CCD detector
Cryostat with final camera lens and detector.
Proximity electronics box attached on the right.

The large spectral resolution and wide global spectral range of HERMES make wavelength calibration difficult. After thorough testing, the baseline solution is to use 4 Thorium-Xenon arc lamps instead of the traditional Thorium-Argon mixture. Flat fielding in the UV part of the blue channel is also hard. Again after testing, we selected 75 W UV Quartz halogen lamps as the baseline; a study of an alternative Xenon short-arc lamp is ongoing.

Ucles ThXe arc
A blue to near-infrared Thorium-Xenon spectrum, covering the four GA channels, made with the AAT echelle spectrograph UCLES (Blue up, near-Infrared down). This shows that this arc provides sufficient unsaturated lines over the whole HERMES GA range. (The curved features at the bottom of the image are reflections in UCLES from very strong lines in higher orders - these will not be seen in HERMES.)

Assembly, Integration & Testing

After fabrication and integration, the spectrograph and fiber feed will undergo an extensive testing phase at Epping in order to fully characterize the HERMES facility, followed by a thorough reintegration and commissioning phase at the AAT.