Images of the primary being aluminised in 2002

Images below show the aluminising of the TROK 30 inch primary at Vacuum coatings in London for the first time.

It has since been recoated in 2006. A report on this is given after these images.



Report on re-aluminisng the TROK primary in 2006

LAS report on re-aluminising the TROK primary (2006)

The 30 inch TROK telescope had not been recoated since it's first coat back in 2002.
We decided to use vacuum coatings again as I needed to complete the job in one day. Galv Optics needed one week's turnaround. Dave, Geoff and Lew Brown came to my house a few weeks before to help in the removing of the old coating. We removed the mirror through the upper aperture of the mirror box and set it up on a work mate on the dive into the garage. This is loose stone and would allow waste chemicals to drain safely away.We used paint stripped to carefully remove the black enamel on the bevel.

This proved quite slow, but cleaned it off quite well in the end. In future, the edge of the mirror will have a removable aperture ring and not be painted or marked with an indelible marker.

To remove the primary coating we used a diluted (10:1) solution of sodium hydroxide (caustic soda). Personnel protective equipment is required for this as it can be very dangerous. Long thick gloves, suites, goggles and masks are required. A ready supply of water is required for cleaning and in case of emergencies. The stand pipe was next to the working area.

The coating came off quite quickly when very gently wiped with pre soaked cotton wool. But it came off even quicker when poured directly onto the aluminium surface.

The mirror wash then washed with water and finally rinsed with deionised water, dried and boxed ready for coating. The intention was to finally clean with isopropyl alcohol. This in the end was not required by Vacuum coatings.

The mirror was taken down to Vacuum very early on Thursday 7th September 2006. We needed to get this done ready for when John Dobson arrived in a few weeks. As it happened, this was the only spare day the Vacuum coatings had during the next month.

We arrived early and unloaded the boxed mirror. The door into vacuum coatings is very narrow!! The mirror was unpacked and vacuum coatings got to work in finally cleaning the mirror to go in the 36 inch chamber.

While they prepared and pumped down the chamber, we decided to go off the Greenwich Observatory for a few hours. The weather was kind and we had some great views of London from the top of the hill at Greenwich. Highlights of the visit were seeing Harrisons clocks again, the camera obscura and the 28inch reflector within the onion dome. We had some lunch, fed the squirrels and then made our way back to Vacuum coatings.

We had arrived a little early and had to wait an hour before the mirror came out of the chamber. The mirror looked very shiny, but did appear to have some faint 'coloured rings' at the 70% zone at shallow angles. We had no time to argue the point and the mirror looked fine viewed direct on. The mirror was packed up and loaded back into the car ready for the long drive home again.

We left the mirror for nearly two weeks before it was put back into the telescope. The mirror already had some dust, but looked 1000% better than it had before being recoated. The old coating was tarnished and very uneven. We stimated 30 to 40% had been washed off, or been eaten off by the atmosphere over the years.

First views showed that the mirror was still ok and performing well. We now needed a dark sky to get the most of it. John Dobson, on his recent visit to the society, said that the mirror was good with a slight turned edge. This was encouraging as this is what he said last time he looked through it back
in 2002.




TROK mirror cleaning February 2011

The bad winter of 2010 had taken its toll on the mirror coating. An annulus of contamination (haze) had appeared around the edge of the mirror. we decided to clean the mirror and prolong the caotings life.

Dave Owen's images:-