HERMES, A Fibre-Fed High-Resolution Spectrograph for the Mercator Telescope (HERMES, een optische hoge-resolutie spectrograaf voor de Mercator telescoop)

Spectroscopy is one of the most important tools for the astrophysicist and a wealth of knowledge about the celestial sources was and will be acquired through the use of optical spectrographs. Hence, the HERMES project - an acronym for High Efficiency and Resolution Mercator Echelle Spectrograph - was initiated to meet the spectroscopic needs of the Mercator telescope. The aim of this dissertation is the complete optical and mechanical design, the realisation and the performance analysis of HERMES, an optical high-resolution (HR) spectrograph, featuring outstanding efficiency and excellent instrumental stability.Spectrograph designA unique optical design has been developed for the HERMES spectrograph. It matches in a highly-optimised way the telescope parameters, the available echelle diffraction gratings, the prism cross-disperser characteristics and the detector geometry, to the stringent requirements defined by the astronomical community. After careful evaluation of alternative designs, a white-pupil configuration was adopted for the HERMES spectrograph.All optical components of the spectrograph, including a large dioptric camera, were custom designed, providing the instrument with outstanding image quality. A detailed tolerance analysis leaded to manufacturing and mounting specifications that completely avoided any degradation of the spectrograph performance. These specifications were used for the procurement of the optical components and served as input for the opto-mechanical design. For all large optical elements, (semi-) kinematic mounts were designed that limit mounting stress on the optical glass. The mounts of the optical parts that are most sensitive to temperature variations, resulting in image quality degradation or spectral drift, incorporate a passive thermal compensation system. To improve the environmental stability, the spectrograph is housed in a highly-isolated and temperature-controlled chamber. Finally, as the spectrograph covers a broad spectral range in a single exposure, it could be designed with a completely fixed configuration without the need for movable parts, further increasing the instrumental stability.As spectrograph detector, a large 2kx4.5k pixels thinned back-illuminated CCD was selected. A special feature of this detector is its novel anti-reflection (AR) coating technique. As the spectrograph has a completely fixed spectral format, it was possible to optimise locally the thickness of the AR coating for the wavelengths hitting that region of the detector. The associated quantum efficiency gain is smaller than expected, but because for each pixel the AR coating is now matched to the incident monochromatic wavelength, Fresnel reflections are strongly reduced. Hence, the fringing amplitude is up to 9 times smaller compared to a standard-coating detector.Spectrograph entranceHERMES is too large to be mounted directly at a focal station of the Mercator telescope. Therefore, it is installed in a dedicated room and the telescope beam is relayed through optical fibres to the spectrograph. This fibre link offers the advantage of scrambling the image at the fibre entrance, this way reducing the effect of seeing and telescope guiding fluctuations. This increases the illumination stability of the spectrograph. To compromise between efficiency, high spectral resolution and instrumental stability, HERMES is equipped with two science fibres. The high-resolution fibre (HRF) is optimised for the highest possible throughput thanks to a comfortably large sky aperture of 2.5 arcsec. A specially-designed two-slice image slicer cuts the fibre diameter in two semi-circles. This doubles the spectral resolution of this fibre. The low-resolution fibre (LRF) has a 2.15-arcsec aperture and is optimised for the highest instrumental stability at the cost of lower efficiency. A set of fibre-entrance and exit optics was designed and built, to inject the telescope beam in an efficient way into the optical fibres. At the other end, these optics convert the fibre beam to the correct focal ratio for the spectrograph. An original and very useful exposure meter was developed to capture part of the spectrograph beam that would otherwise be lost in the fibre link, sending it to a photo-multiplier tube for continuous real-time flux monitoring.The fibres are mounted in the telescope interface, which is installed at the telescope focus. Several other important elements were built for this interface: an atmospheric dispersion corrector to correct for the differential atmospheric refraction, a fibre viewer for centring the target on the optical fibre and for automatic telescope guiding, and a calibration light projector to inject the fibres with calibration light. For this purpose, a calibration unit was developed, offering a series of selectable calibration sources. The telescope interface and the calibration unit house the mechanisms that select one of the spectrograph observing modes.Spectrograph performanceThe performance analysis and the first two years of science observations, showed that HERMES was built successfully and in a robust way, fully compliant with the demanding requirements. The spectral resolution of 85000 (HRF) and 63000 (LRF), values that exceed the initial expectations, confirm the excellent image quality of the spectrograph. Moreover, HERMES spectra are virtually ghost-free in normal unsaturated conditions. Small ghosts are detected at relative intensities below 0.01%. Careful baffling resulted in very little straylight contamination. The instrumental stability is excellent and radial velocity accuracies of 2 m/s, corresponding with a spatial shift of less than 20 nm at the detector, are achieved. HERMES covers a large spectral range, starting at 377 nm and reaching to 900 nm.The most important asset of HERMES lies in its outstanding efficiency (28% at 500 nm in HRF mode), rivalling or outperforming most other world-class instruments. The addition of HERMES to the instrument suite of Mercator lifted the telescope to a higher level and greatly increased the scientific output of the telescope. For high-resolution spectroscopy, Mercator is now fully competitive with telescopes that are one class up in mirror size.status: publishe