The biggest telescopes require sophisticated control
to keep them in focus, using high performance load cells.
The Gemini telescopes have
1.8 metre mirrors,
weighing 23 tonnes.
Although there has been much publicity surrounding the success
of the Hubble Space telescope. there are many advantages in
building and operating large ground-based optical and infrared
telescopes.
Large telescopes are essential for spectroscopy of faint stars and
distinct galaxies in order to derive physical properties such as mass,
age and chemical composition but it is difficult to put a telescope
much larger than 4m into orbit.
Furthermore, when equipment fails in space it is very expensive to
replace. Although ground based telescopes can suffer from variable
weather and atmospheric effects, they can be built for about 3 per cent
of the cost of the Hubble telescope and can be easily repaired or
upgraded with the latest technology. However, building large ground
telescopes presents problems of its own, especially those of a
mechanical nature.
Cooperation
The Gemini project, which is an international cooperation between
Argentina, Australia, Brazil, Canada, Chile, the UK and the US has
involved the building of two massive 8.lm telescopes. These have
now seen 'first light' and already scientists are amazed at the quality
of the images being received.
In order to provide the best year round atmospheric conditions and
the widest possible view of the sky, one telescope is located in the
Northern hemisphere and one in the Southern. The northern telescope
is located at 4,200m on Mauna Kea on the island of Hawaii
(40 per cent through the atmosphere) while the southern telescope
is located at the peak of the 2,800m Cerro Pachon mountain in Chile.
When combined, these two sites provide an unrestrained view of the
whole sky, so that key astronomical objects, such as the centre of
our Milky Way, small satellite galaxies, the Magellanic clouds and
Andromeda galaxy are fully accessible to Gemini cameras and
spectrographs.
The Gemini telescopes are a magnificent achievement of modem
technology and provide a powerful optical - infrared astronomical
facility. In order to achieve good image quality, the shape of the primary
mirror must be maintained over a range of telescope orientations
and environmental conditions. The previous generation of large
telescopes did this by using primary mirrors that were very thick
and hence resisted distortion.
This simple 'brute force' method for maintaining rigidity is not feasible
for an 8 metre mirror. The mass increases approximately as the fourth
power of the diameter and if the Gemini mirrors had been built using
this approach they would weigh more than 230tonnes and retain heat
for many days, affecting image quality. Not only would the logistics
of transporting such mirrors be totally impractical (especially given
the remote location) but any support and drive mechanism would be
unmanageable.
Distortion
To overcome this problem, the Gemini telescopes use thin meniscus
mirrors mounted on an ingenious support system which can deform the
mirror to compensate for gravitational and thermal distortion. With a
mass of only 23 tonnes the primary mirrors are by comparison ultra-light.
The mirrors were assembled, like a giant jigsaw puzzle, from a large
number of hexagonal blocks of special ultra-low expansion titanium
silicate glass less than 200mm thick These blocks, or boules, were
fused together in a special furnace at the Corning Glass Company in
Canton, New York and then cooled to form a solid flat disc. To obtain
the necessary curvature to focus the light, the flat discs were reheated
and 'slumped' over a mould, shaped to define the final curvature.
From here the mirrors were shipped to REISC near Paris for final
polishing. The precision of this operation is such that the maximum peak
to valley deviation is only 141 billionths of a metre (141 nanometres).
Incredibly this means that if the mirrors were the diameter of the earth,
the largest bump would be less than 300mm high!
Each mirror can collect more light than 2million human eyes and when
combined with its adaptive optics, the telescopes can produce infrared
images that are sharp enough to resolve the separation between a pair
of car headlights at a distance of 3,000 miles, exceeding the resolution
of the Hubble telescope.
Support
Despite the advantages offered scientists by modern computer
techniques and electronics, much of the success of the project has
depended on the design of the mechanism supporting the mirrors.
The mirrors are made from hexagonal glass
blocks, above, fused together and held in
shape during use by a system of load cells
and hydraulic and pneumatic actuators.
Surprisingly, at the heart of the structure are hundreds of strain gauge
load cells - a technology first used 60 years ago to determine weight
and balance data for military aircraft.
The Royal Greenwich Observatory, based in Cambridge, was primarily
responsible for the design and building of the support mechanisms.
Each mirror is supported on 120 special nodes, equally spaced beneath
its surface, in conjunction with a large inflatable rubber cushion like a
giant air bag.
The load cells have been designed and built by transducer specialists
Applied Measurements Ltd., which has extensive experience in
providing bespoke force measuring solutions for a diverse range
of applications.
Each node consists of two load cells, mounted one on top of each other.
At the top, a 500N load cell is used for the fine control of the mirror, while
immediately below is a 2,000N load cell. Although the two load cells are
mounted in series, an ingenious force transfer mechanism ensures that
the top load cells do not carry the full load of the mirror if the main support
fails, the load being transferred to the lower cells.
All the support points are used in conjunction with pneumatic and
hydraulic actuators which can pull and push parts of the mirror to maintain
optimal shape and to compensate for distortions due to gravity and
wind buffeting. To keep the mirror circular as the orientation of the
telescope changes, 100 special 5,000N load cells are equally spaced
around the outside of the rim of the mirror (and in the same plane).
Traverse
The complete mechanism is part of a closed loop which is part of a
closed CAN bus computer system and allows the mirror to traverse
through an arc of ±9odegrees without loss of image quality. The overall
precise optical figure of the telescope is controlled and maintained by
an 'active optics system' that senses distortions in the mirror surface
using light from reference stars.
Given the precision of the whole system, the design demands placed
on the load cells were very exacting. It is vitally important that the load
cells give the same 'output per unit load', for both positive (push) and
negative (pull) loading to an accuracy of ± 0.02 per cent FS.
Each load cell has a custom designed electronics module incorporating
a microprocessor and amplifier which gives a fully balanced ± 5V output.
This enables calibration adjustments to be carried out via a dedicated
PC system. A further advantage of the approach is the ability to
automatically locate and identify any problems with individual load cells.
Peter Lewis, managing director of Applied Measurements, is
understandably proud of the part played by his company in this
prestigious international project. Although we have designed and
built many special load cells over the past ten years, the Gemini
units are for the most intriguing and complex application. Now that
we can see the finished telescopes and the quality of the first pictures
from outer space, we can only marvel at the ingenuity of the overall
design and construction."
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For more information, please contact :-
Applied Measurements Ltd.
Tel: +44(0) 118 981 7339 Fax: +44(0) 118 981 9121
Email: sales@appmeas.co.uk
Web: www.appmeas.co.uk
January 2001
