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M. F. Blanken (ING), G. Talbot (ING), M. Aderin (UCL)
Over
the last year (2002–2003) ING was subcontracted by University College London
(UCL) to produce a module of 18 science fibres for the bHROS instrument,
one of the instruments on the Gemini South telescope.
bHROS is a high-resolution (ℜ=150,000) prism cross dispersed echelle
spectrograph, situated in the pier of Gemini-South. It is fed by optical
fibres mounted on the GMOS instrument located at the Cassegrain focus of
the telescope. bHROS will have the highest spectral resolution among the
optical spectrographs currently being designed and built for 8–10m class
telescopes.
The optical fibre connection between GMOS and bHROS consists of 18 fibres;
9 fibres with a 120µm core diameter and 9 fibres with a 160µm
core diameter. Both types of fibre had to be ground and polished at both
ends. The GMOS end also had to be mounted and aligned in an optical assembly
(a body plate). 10 fibres of each type were delivered by UCL to the ING
out of which 9 of each were to be used for science. The 10th fibre of both
types was manufactured in case of any breakage.
.
Figure 1 (left). Fibre grounding
and polishing [ JPEG | TIFF
]. Figure 2 (top right). Fibres in body plate gluing [ JPEG | TIFF ]. Figure 3
(bottom right). Metal tube gluing [ JPEG | TIFF ].
The first stage after receiving the fibres was cutting them to the desired
length. After this metal tubes were glued over the fibre ends to make the
grounding, polishing and handling easier. The metal tubes were also connected
to the outer PTFE sleeve of the fibre, using heat shrinks, to give extra
strength and reduce the risk of breaking. Both ends of the fibres were ground
and polished to a flatness of <¼ of a wavelength (632nm).
The body plate for the GMOS end consists of 18 sapphire ball lenses of
two sizes (3mm and 4mm diameter) and 18 silica optical windows (3mm diameter,
300µm thick and 4mm diameter, 400µm thick). The balls and the
windows were glued in the body plate using UV-optical curing glue. Before
the fibres were aligned in the body plate a throughput test was done to
check the relative transmission of the 20 fibres. The best 18 fibres were
aligned on top of the silica windows and the sapphire ball lenses in the
body plate. The alignment was done using a target that simulates the Gemini
telescope pupil (fibre positioning tolerances were 0.02mm). After the alignment,
the fibres were glued in the body plate by using the UV-optical curing glue
and super glue.
After the polishing, aligning and gluing the fibres were sent to the
UK for installation of the optics for the bHROS end. The fibres are now
complete and are waiting to be installed between GMOS and the bHROS instruments
in Chile. bHROS will be fully integrated with the telescope in 2004.
ING is experienced in fibre work after making several successful fibre
projects. One project in particular, “Small fibres” consisted of 160 fibres
with a core diameter of 90µm for the Autofib2 (robotic positioner)/
WYFFOS (optical spectrograph) commissioned in July 2001 (see alsoING Newsl., 4, 26 and ING Newsl., 5, 19 for more information
and first light report). The procedures and experience of the “Small fibres”
project were used in the GMOS/bHROS project. ING is actively looking for
more fibre work from external institutes for the future.
For more information on the GMOS/ bHROS project please visit the following
sites:
