The science fibres and fiducial bundles are terminated at their input ends with SF5 microprisms whose input faces are wide-band coated. Their 92-degree angle provides median compensation for non-telecentricity, which amounts to four degrees at the edge of the field. The residual maximum tilt of two degrees is still enough to produce a field-dependent degradation of focal ratio from f/2.8 to f/2.3 in addition to the intrinsic focal-ratio degradation (FRD) of the fibre. This is one reason why an FRD-insensitive design was chosen for the spectrograph.
Each fibre is equipped with a magnetic button which holds it onto the (flat) field plate. The circular plate and its entourage of radially-disposed fibres are supported in a box from which trail the fibre cables to the spectrograph; the complete assembly constitutes a `fibre module', which is easily removed from the robot to be interchanged with another. Coded proximity switches tell the Autofib control system which fibre module is in place.
The first fibre module contains 126 high OH
fibres in 14
individually-cabled bundles of nine fibres, but it will be possible
to image only about 120 of these onto the spectrograph detector at any
one time. The fibres subtend 2.7 arcsec on the sky, and will be used
principally for galaxy observations. Each of the 14 bundles has a
nine-way fibre connector some 8.5 metres from the Autofib end to allow
the lower part of the feed to remain permanently attached to the
telescope.
At their output ends, the fibre bundles are brought together in a slit
unit, where each group of nine is disposed in a skewed 3
3
array. The 14 arrays are then lined up in the collimator focus. The
spectrograph thus sees three staggered fibre `slits', an arrangement
needed to accommodate the 2-mm diameter sapphire balls that terminate
each fibre. These are used in a reverse-Fabry configuration, so that the
fibre is imaged accurately onto the collimator while a pupil formed
0.9 mm from the surface of the microlens acts as the entrance slit.
The pupil has a nominal diameter of 0.5 mm, but varies with the focal
ratio of the beam emerging from the fibre; thus a change in focal
ratio alters resolution rather than throughput.
Because the large fibres admit a high level of sky background, and
because point images are not well scrambled at fast focal
ratios (Watson & Terry, Gemini, No. 42 (1993), 32),
a smaller second set will be provided (probably late in
1995) for observations of point sources. They will capitalise on the
throughput-stability of the spectrograph for accurate sky-subtraction,
and will make the best use of the site's sky conditions. For best
possible performance, the connectors will be dispensed with and a
wide-band hydrogenated low OH
fibre-type will be considered as
an alternative to the current one. It may also be possible to
replace the sapphire balls with graded-index lenses.