Aluminising the primary mirror

This was originally perceived to be the largest obstacle we had to get over. The first thing was to establish if someone in Britain could do it. The next was the price. The only place that I could find that could do our mirror was Vacuum coatings in London. They originally quoted £180 back in January 1996. When we finished the mirror, and after I moved house, I gave them a call. They said that if we gave them a weeks notice and brought it down on a Tuesday or Wednesday that it could be done within the day! We arranged to bring it down on the 7th April 1998 and to arrive before 10am. We left the Wirral at 4:20 am BST and set off down the M56, M6, M1, M25 and M11 to 66 Barrett road E17. (0181 520 5353). They have a number of chambers the largest of which is a 36-inch chamber capable of taking a 34-inch mirror. Warrived early before anybody else and waited. We had a bit of a struggle getting it through the door but managed ok in the end.

Image right shows myself and Geoff getting the mirror in its protective transport box out of the car.

The mirror was cleaned using windowlean to remove the worst of any contaminates and then was thoroughly cleaned with swabs of alcohol to finally clean. The breath test was used to determine if the surface was clean enough to be aluminised.

Image right shows the mirror being cleaned prior to being put in the chamber. Notice the Windowlean!

Four strips of pure aluminium about three inches long and the thickness of a shoe lace where used at four locations in the chamber for four electrodes. The mirror would also be given a silica overcoat for additional protection. The silica was about the size of a few black pepper grains and these would be heated in a small bowl after the aluminising was completed. The mirror was placed in a jig face down into the chamber. Vacing down took about one and half-hours.

During this period we decided to goto Greenwich Observatory (being only a few miles away). It was a good sunny warm day and it was our first visit to the observatory. Among many of the objects seen where John Harrisons famous time pieces, William Herchals 9 foot reflector, some nice transit instruments, the 28 inch refractor in the 'onion dome' and a good view from Flamsteeds house across London. From which many of the famous sights could be seen including the building of the millennium dome.

Dave Owen standing on the 0degree longitude line at Greenwich

We left Greenwich and arrived back at Vacuum coatings at about 3.00pm. We had been speculating all day if we would still have one mirror at the end of the day or many and the concern if the surface was good enough. We had a nice surprise when John lifted the protective paper to find that the mirror looked almost perfect. The aluminising had taken well and only a few scratches left over from polishing could be seen. A slight hazing was seen at some parts of the edge but these where only minor and encroached the edge by only 1/4 to 1/2 an inch at most. Cost was a bargain. We packaged the mirror up and headed back home. We arrived back at 9.15 PM and placed the mirror in the garage. Dave Owen took a Polaroid of the mirror prior to leaving. All in all a good day out and a major step to full completion.

Above image shows Geoff Regan, Dave Owen and Dave Thomson with the completed mirror at Vacuum Coatings.

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