The Hector Instrument: optical design of the new higher-resolution spectrograph

The Hector instrument is the new multi-object facility at the Anglo-Australian Telescope. It consists of new-format hexabundle IFU’s, complex robotic positioner with magnetic system, unique sky-fibre system, guiding system, optical cable and two spectrographs. Light is captured at the telescope prime focus by optical fiber imaging bundles (hexabundles) at f/3.25 and delivered to the spectrograph slit via ~50 m long fiber cable. At the spectrograph end, the fibers are reformatted into a curved slit relaying unconverted telescope input. The spectrograph optics includes fast collimators and cameras reimaging the slit onto 4k x 4k E2V detectors at f/1.3 with magnification 1/2.5. The challenge of good image quality with the large pupil size (180 mm) and the field of view (±12° at detector) was met by introduction of several aspheric surfaces in the all-refractive design. The blue and red arms, 372-591 nm and 571-778 nm, respectively, are implemented with the help of a dichroic beam splitter in the diverging beam followed by a collimating doublet lens for each arm. An upgrade is possible to the infra-red arm with the help of an additional dichroic beam splitter. The dispersers are asymmetric VPH gratings with slanted fringes optimised for the passband of each arm. Optical performance of the dichroic beamsplitter and gratings has been confirmed and complemented by in-house metrology. The spectrograph throughput is predicted based on transmittance of materials and coatings.