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ING Newsletter No. 6, October 2002

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ING Stick to the Task – Work for the NSST

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.

NSST mirror [ JPEG | TIFF ]
NSST pad [ JPEG | TIFF ]
SST [ JPEG | TIFF ] 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.

Email contact: Gordon Talbot (rgt@ing.iac.es

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Last modified: 13 December 2010