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Gordon Talbot, Maarten Blanken, Alan Chopping, Paul Jolley and Juerg Rey
(ING)
The ING recently demonstrated the range
of services they can offer to other telescopes by completing a contract for
the Royal Academy of Sciences, Sweden for mirror handling, pad gluing and
aluminising a pair of 1.4 metre diameter mirrors to be installed in the New
Swedish Solar Telescope (NSST) on La Palma.
Mirror handling involved the need to invert the mirrors after unpacking,
then placing them face down so that a set of eighteen Invar axial pads could
be glued to the rear of each one. This involved rehearsing the procedure with
the dummy mirror supplied which matched the size and weight of the actual
mirrors (1400 mm diameter, 150 mm thick and 600 Kg). Rehearsal of these operations
is vital to avoid unforeseen occurrences that could damage optical components;
the more so when they are this size and weight. ING’s policy for all of this
type of work is that we look after the mirrors as if they were our own.
Once the mirrors were in position the template that defined the position
of the axial pads on the back of each mirror was aligned. The mirror surface
was prepared by carefully cleaning then grinding the Zerodur in the area of
each pad before applying a special primer. The surfaces of the axial pads
were roughened. The pads were then positioned and aligned using the jigs supplied,
before being attached by the specified two-pack flexible epoxy adhesive.
The jigs controlled the thickness of the epoxy under each axial pad to 75
microns. One mirror was completed before the template and jigs were transferred
to the second mirror.
The work was carried out in the WHT aluminising area. To cure the adhesive,
they had to be kept at an elevated temperature of at least 24 degrees C for
seven days. To achieve this a ‘tent’ was created for each mirror using polythene
sheeting and scaffolding, which was then warmed with space heaters to reach
the required temperature while minimising the transfer of additional heat
to the dome.
While the axial pads were being glued to the mirrors a total of three ‘test
pieces’ (one for each day of gluing) were made using some of the actual epoxy
mixes used. The test pieces were axial pads glued to steel plates, which had
had exactly the same surface treatment as the actual pads and mirrors. These
were left to cure alongside the mirrors. After the curing time each test
piece was then subjected to a pull-off test. Each test piece was sandwiched
between thick steel plates to avoid local distortions. A weighing device was
slung from the lifting crane. To this was attached a sling with the test piece
shackled underneath, and test weights were progressively added. All three
demonstrated they would withstand the specified 100 Kg load.
The mirrors complete with firmly attached axial pads were then turned right
side up, before being prepared then coated together in the WHT aluminising
plant.
After coating, but while still in the aluminising area, the mirrors were
then installed in their cells by NSST staff, then with assistance from ING
transported to the telescope for installation.
From top to bottom: one of the NSST mirror blanks
in the aluminising area at the WHT, pad gluing in the rear of one of the blanks
and the two mirrors assembled on top of the NSST tower.
A large number of ING staff contributed a wide range of skills to this work,
principally members of the Mechanical, Site Services and Telescope Operators
Groups. Some stages of the work required relatively large numbers of people
present, for instance the mirror handling required at least five to co-ordinate
all the actions of lifting and rotating the mirror.
The NSST representatives also assisted and of course made the whole thing
possible by first of all entrusting their mirrors to ING and with their detail
work in the design of the pads, templates and jigs.
Due to the current economic climate ING will be driven in the future to
seek to carry out more repayment services in areas where we have special
expertise or facilities, with the aim of retaining a wide skills base, while
maintaining both our proficiency and facilities by exercising them. These
areas extend beyond aluminising which has been the main service in the past,
to other areas, (which will include optical fibres) where ING have built
up experience from our operations and projects.
