G. Talbot, A. Chopping, K. Dee, D. Gray, P. Jolley (ING)
he profile of the William Herschel Telescope (WHT) has changed since the beginning
of this year, with the addition of a new facility at one of the telescope’s
Nasmyth platforms. For many years the WHT has had the GHRIL building on the
Nasmyth1 platform — now the ING has added GRACE to the opposite side of the
telescope. GRACE (GRound based Adaptive optics Controlled Environment) is
a dedicated structure designed to facilitate the routine use of adaptive optics
(AO) at the WHT, using ING’s AO instrument suite. The design of GRACE allows
for the future use of laser guide stars.
The AO suite consists of the AO system NAOMI, the coronograph OSCA, near
IR imager INGRID and the optical integral field spectrograph OASIS. The bench-mounted
NAOMI system achieved first light in 2000 with INGRID as its science camera
in GHRIL, but until the advent of GRACE, the NAOMI optical bench complete
with delicate (and expensive!) optical and electronics components has had
to be craned in and out of GHRIL every time there has been an instrument change.
Quite apart from the risk involved, there has been a massive overhead for
ING staff in disconnecting and removing everything, with an even more massive
overhead in putting everything back, aligning it and getting it working —three
weeks was allowed for this!
After the first NAOMI instrument change, it was realised that for routine
operation moving NAOMI about was not sustainable. This was re-inforced with
the signing of the agreement with the Centre de Recherche Astronomique de
Lyon (CRAL) to bring OASIS to the WHT, which added a further large complex
instrument to the equation.
|Top left: NAOMI suspended from the
crane (by the yellow straps) above GRACE, just before being lowered through
the access hatch [ JPEG | TIFF ]. Middle top: The Eagle has landed! The NAOMI
bench complete with deformable mirror being lowered onto its kinematic mounts
inside GRACE [ JPEG | TIFF
]. Right top: Dr Ronald Stark, Dr Annejet Meijler and Dr René Rutten
standing up in front of WHT. GRACE can be seen on the Nasmyth platform on
the left, opposite to GHRIL [ JPEG | TIFF ]. Middle left: Inside GRACE [ JPEG | TIFF ]. Bottom
left: Inaugurating committee leaving GRACE [ JPEG
| TIFF ]. Bottom middle: Dr Annejet Meijler uncovering
the inauguration plate [ JPEG | TIFF ]. Bottom right: Dedication to ING staff [
JPEG | TIFF ].
Accordingly ING made the decision to create a dedicated facility on the
Nasmyth2 platform then used by the Utrecht Echelle Spectrograph (UES).
A suitable building was designed taking into account the requirements of
the AO suite. To provide sufficient space an extension to the Nasmyth platform
was needed and this was also designed at this time. A local La Palma company,
Grolei Servicios S. L., was successful in winning the contact to construct
the building. Their proximity to ING allowed progress to be monitored, problems
resolved and modifications to be made quickly resulting in an excellent building,
fulfilling ING’s requirements.
GRACE is not just a building, it offers all of the systems associated with
providing the controlled environment necessary, especially cooling and filtering
of the air, together with all of the services needed including electrical
supplies, lighting and a computer network. While the building was being built,
we continued to design and specify these systems.
Crucial to success is environmental control. The GRACE electronics room
is maintained at a constant temperature by an air-handling unit mounted on
the roof. For the optics room, the air is again maintained at a constant
temperature for the stability of the instruments, additionally being finely
filtered and introduced at low velocity through laminar flow units to minimise
air currents. The heat from GRACE is removed (not dumped in the dome) through
a water glycol circuit. The opportunity was taken to upgrade the cooling capacity
to the whole of the WHT, in order to meet future needs including a laser
guide star, by buying and installing a new external plant.
The first step was the removal of UES and the extension and modification
of the platform ready for the building.
Installation of the completed building, which was dismantled for transportation,
began late in 2002. A significant moment was reached on 18 March 2003 when
the NAOMI adaptive optics system on its bench was lifted into its new and
permanent home. This was followed quickly by its first light in GRACE using
INGRID on 13 April at the beginning of a nine-night run. The second ‘first
light’ event for GRACE followed soon after when, on the 11 July, OASIS went
on sky with NAOMI on its first commissioning night.
A key concern was that GRACE would not change the telescope performance
when replacing UES. During the construction phase several blocks (weighing
a tonne each) were used to ensure that the removal of UES did not unbalance
the telescope. These were progressively removed as GRACE was assembled. The
opportunity was taken to brace the new platform extension to the telescope
structure. Tests after GRACE installation was complete show this was effective
in raising the natural frequency considerably, away from the telescope’s
azimuth locked rotor frequency, which was the required result.
During the time of restructuring for ING, the creation of GRACE is a major
achievement, especially when set against the background of other project work
—not least adding the Universal Science Port to NAOMI to feed OASIS. Many,
if not most ING staff have contributed to GRACE and in getting the AO suite
installed and working. In the end —over the last months, then weeks— it’s
still hard to believe how much was done and how hard everyone worked.
Dr Annejet Meijler, the Director of the Council of Physical Sciences of
the NWO, formally inaugurated GRACE on May 2nd, 2003. ING staff take pride
in having a world class environment for AO which was, in the words of the
plaque unveiled, ‘conceived, designed and built’ by them.¤
Email contact: Gordon