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FibresField of View
Fibres can be positioned over a field of 1 degree diameter; though it should be noted that there is some vignetting starting at a radius of 20 arcminutes, and that the image quality outside the 25 arcmin radius is quite poor, the PSF showing a large elongation that causes a lot of light to be lost for targets in this outer area. We thus recommend users to avoid putting science fibres in the annulus outside the 25 arcmin radius, and consider there there will be some moderate light loss due to the optics also at radii between 20 and 25 arcmin from the centre. Fibre Characteristics
The Small Fibre module was successfully commissioned on July 2001, and is offered since then. With the AF2 software control and Linux PC, a complete setup of fibres at the telescope usually takes around 20 minutes. Science fibresThe Small Fibre module contains 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. 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 small fibres offer. "Bad" fibresSome of the science fibres have quality worse than the average, due to some defects in the fibre itself, or in the optical elements attached to it. Although it is difficult to avoid them while preparaing configuration fields, the worst fibres, where most of the light is concentrated in an external annulus (i.e. have a "donut-like" projected image, which significantly degrades resolution) are: #25, #28, #79, #83, 99, and #143.Fiducial bundles10 fiducial bundles are available for field acquisition and autoguiding. Each fiducial bundle (450 micron diameter) contains some 10000 fibres transmitting coherent light and therefore providing a rough imaging capability over a 8 arcsec round field of view.
Fibre positioning limitationsDue to the size of the fibre buttons in AF2, fibres can not be placed closer than approximately 25 arcsec from each other. More important, the fibre rods run radially out from a positioned fibre block a much larger area. It should also be noted that fibres cannot cross with AF2/WYFFOS. Simulations using af2_configure have been done in order to bring forward these limitations. An `ideal' case where no fiducial and sky fibres are needed, and where the target sources are randomly spread over the investigated area was considered. For real setups the number of placeable fibres will be less. The table below gives the number of fibres placed, as a function of number density [number of objects / square degree] of the target sources and the radius [arcmin] of the area on the sky over which the targets are distributed. The table gives the number of placed fibres and the number of sources within the given radius. The 20 arcmin radius field corresponds to the unvignetted field of view of AF2. The 30 arcmin radius field corresponds to the full field of view of AF2 (but see above the limitations of the outer regions of the f.o.v.).
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