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Field of View
The Small Fibre module was successfully commissioned in July 2001, and has been in use since then. With the AF2 software control and Linux PC, a complete setup of fibres at the telescope usually takes around 20 minutes.
Fiducial bundlesThere are 10 fiducial bundles 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.
Figure 1 is of a fiducial bundle flat frame (twilight sky flats) that shows the defects in each bundle. The cleanest fibre seems to be #80 while #64 is particularly bad.
In principle, these defects do not affect the field acquisition, however there is some evidence that fibre #64 usually is not placed correctly by the robot due to a bad bundle centroid calculation.
The Small Fibres 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. The error budget estimated for the fibre positioning predicts errors up to 0.4 arcsec in the central 20 arcmin radius field of view (more details in Target acquisition). This gives no room for astrometrical errors, which should be as accurate as possible and account for the stars proper motions. As well the selection of fiducial stars is very important, their number and distribution being key for good acquisition and guiding (see User's recommendations). We caution observers to take care of all these issues, as bad fibre positioning may cancel all the gains that small fibres offer.
The relative throughput of the science fibres was measured in February 2013 with the WHTWFC detector. Information about fibre
relative throughput can be found here.
"Bad" fibresOver many years of use, several fibres have been broken and are disabled in the file small_fibres.dat, so that they are not taken into account when creating the configuration files with the af2_configure software. The broken/disabled fibres are:
#1, #17, #25, #37, #83, #99, #117, #136, #145, #156, #159
As well as the disabled fibres, some of the active science fibres have quality worse than average, due to defects in the fibre itself, or in the optical elements attached to it, e.g. possible damage to prisms, accumulated dirt and debris, increased opacity of optical cements, etc. This can affect either the fibre throughput or the image quality.
Although it is difficult to avoid them while preparing configuration fields, the worst fibres, which significantly degrade the resolution (i.e. have a "donut-like" projected image) are:
#8, #14, #15, #23, #28, #39, #79, #102 and #143.
The fibres with the lowest throughput, being < 60% of the average of the fibres are:
#40, #133, #154
Fibre positioning limitations
Due 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 running 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 characterize 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 field of view).
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