Small fibres Commissioning

 The small fibre (1.6 arcsec diameter) module was successfully commissioned on July 2001, and is offered starting from Semester 2001B.  Consequently, the Large Fibre (2.7 arcsec diameter) module is not offered any longer.

 Science fibres

 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:

•   No light loss is due to fibre connectors, providing a more homogeneous distribution of fibre relative throughput.
•   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.  Although the nominal spectral resolving power has increased as the ratio of Large to Small fibre diameters, the actual resolution is limited by the CCD pixels, because of the undersampling along the spectral direction. We expect the highest resolution to be around R~7500 in echelle mode.

The 1.6 arcsec fibres were chosen as a compromise between minimum sky contribution and maximal source contribution. As the positioning and (automated) guiding errors may add to 0.5 arcsec, there will be no room anymore for astrometrical errors. Field setups that suffer from inaccurate astrometry or an insufficient number of fiducial stars may suffer light losses of more than 50% at the fibre entrance. We caution observers for this effect, as bad astrometry may cancel all the gains that the new Small Fibre Module is offering.

 Fiducial bundles

With the Small Fibre module, 10 new fiducial bundles are available for field acquisition and (auto)guiding. Each fiducial bundle (450 micron diameter) contains 10000 coherent fibres providing a rough imaging capability over a 8 arcsec round field. Note that one fiducial bundle (#160) will NOT be available for the observing runs on October 2001.