Small fibres

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    AutoFib2+WYFFOS (AF2) is the multi-object, wide-field, fibre spectrograph working at the Prime focus of the 4.2m William Herschel Telescope (WHT). It contains 150 science fibres each of 1.6 arcsec diameter, and 10 fiducial bundles for acquisition and guiding. At the Prime focus, the fibres are placed onto a field plate by the robot positioner Autofib2 at user-defined sky coordinates. Object light is transmitted along fibres 26 metres in length to the Wide Field Fibre Optical Spectrograph (WYFFOS). The path from Prime focus to the spectrograph consists of a prism, fibre button, 26 metres of fibre, finger, microlens and the facet block.

    In 1998 AUTOFIB underwent major modifications to the chassis and fibre-griper. The new system shows much improved positional accuracy of the fibres.

    The small fibre (1.6 arcsec diameter) module was successfully commissioned on July 2001. The Small Fibre module contain 150 fibres with 1.6 arcsec diameter (90 micron), which run without connectors from AF2 to WYFFOS. The fibres are high-content OH fused silica made by Polymicro. Compared to the Large Fibre module, small fibres have the following main two advantages: 1) no light loss is due to fibre connectors, providing a more homogeneous distribution of fibre relative throughput; 2) the sky/background contribution in observations with the small fibres is 0.35 times smaller than with the large fibres. This means that, with the small fibres, the noise level in sky-limited observations is down by a factor of 0.6.

    The Small Fibres are imaged onto less than 2 pixels (FWHM) on the TEK6 detector in the spatial and spectral directions. The full spatial image of the fibres is therefore sampled by less than 3 pixels. There may be a slight gain in S/N of the extracted spectrum with respect to the Large Fibre case, as less pixels have to be extracted when sampling the wings of the spatial profile. For Small Fibres, the fibre distance in the WYFFOS entrance slit is 1mm, which transforms onto a peak-to-peak aperture distance of 6-7 pixels on the detector.

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