New Heraeus STU optical fibres will be provided for multiple-object observing with AAOmega. Currently 2dF uses 8m Polymicro High OH fibres with a core size of 140 microns corresponding to ~2.1" on the sky. The size of the 2dF fibres was determined after extensive modelling of the expected S/N for various fibre diameters, seeing, spectral resolution, object brightness, and object extent (extended or point source). The conclusion was that a fibre diameter of ~2.1" was the best compromise between point and extended sources, even in good seeing (~1"). We have carried out a similar analysis for AAOmega, and decided that a fibre size of 140 microns/2.1" is also the best compromise for AAOmega. The AAOmega fibres will be approximately 35m long to reach from the AAT top end to the new bench-mounted spectrograph in the AAT Coude Room.
Fig. 1 below shows the throughput for 27m lengths (conclusions do not
change for 35m lengths) of various Silica
fibre types (High OH, Low OH, and new Heraeus STU fibres, and the present
8m High OH 2dF fibres). It can be seen that the STU fibre is the
best choice, as it has better performance in the red and is only slightly
worse than High OH Silica in the UV. Unfortunately, the necessity
of longer STU fibres with AAOmega will
result in a decrease in blue transmission, but note that the fibre throughput
is still ~50% at 3500Å.
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| Fig. 1: Plot of transmission of an 8m high OH optical fibre, as used in 2dF, compared with 27m of high OH, low OH, and STU fibre. AAOmega will need 35m fibre lengths to transport light from the AAT's prime focus top-end to the bench mounted spectrograph. |
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