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Construction of the Thanet Observatory

Here are some details of the renovation of the mirror for the 12″ Newtonian telescope in the main observatory, and a detailed account of the construction of the foundation for the telescope and observatory building. Did you think it would just be a case of digging a hole and filling it with concrete? Not quite!

telescope

The newly renovated and modernised telescope on show to the public for the first time in August 1999.

When we acquired our 12″ Newtonian reflector it was in a very sorry state, having been built in the 1950’s and for 10 years subject to all weathers in the back of a garden.

As a charity, we had to ensure that the instrument was worth spending the money that would be needed to modernise the telescope. With that in mind, the primary mirror was sent for testing by Alan Buckman, B.Sc, FRAS of AWR Technology.

Alan removed the mirror from its old three-point welded steel (9kg) mirror cell. The new cell is a much lighter nine point cell. The rear of the blank contained a message from Mr G. Hole of Brighton:

Refigured by G. Hole and Son Ltd. Brighton.

Astronomical parabola of 68″ F.L.

When properly mounted its performance cannot be excelled.

(signed) G Hole (no date)

Telescope foundations

The observatory foundations are begun with volunteer help

A very brave claim, to be sure, and not one that a manufacturer would dare make today. However, Mr. Hole’s confidence was largely justified. When the testing was carried out we were very pleased to find that the mirror is capable of resolving a tenth of a light wave – an extremely respectable figure.

Having assured ourselves that the mirror was worth utilising we made plans for modification of the mount, adding a motorised drive with full computer control, and ordering a skeleton tube, etc. Work then began on the observatory, which had to be suitable for public access and also aesthetically pleasing.

A seventeen foot diameter trench, three feet deep was dug, together with a six feet long by three feet wide by six feet deep telescope pier cavity. The spoil was used to infill old lorry (truck) ruts leading past the site.

Telescope foundations

The foundations nearly ready

The large volume of the pier below ground was thought necessary to support the telescope mount, the base of which had to be raised about three feet in order that at 7° the telescope view was not obstructed. One obvious advantage in going as deep as this was that this pier cavity extended well into the rock substrate, thus aiding stability.

The central pier support excavation still had to be taken down another three feet. This had to be done through chalk rock, which, though soft, offers a surprising resistance to the pickaxe. We resorted to breaking up the rock with an iron bar and sledge hammer and then shovelling out the resulting chalk rubble.

Four holes were then dug around the central pier (not shown in photograph) to provide supports for the suspended floor, which must not be allowed to come into contact with the telescope pier. This is to ensure that vibration is not transmitted to the telescope. Note that the work was carried out without much disturbance to the surroundings. It had been anticipated that construction could be completed without noticable detrimental effects to the flora and fauna of the Reserve.

telecope foundation

Shuttering completed in readiness for concrete pour

A long section of 12″ plastic water pipe was lowered into the pier cavity, and height adjusted  to bring the top of the pier to the required level.

This was then levelled and held in place with 2″ by 4″ timbers. Very careful measurements were  made to find the level for the main dome wall foundations and sections of iron pipe were  driven in at intervals to show the depth of concrete needed. Wooden shuttering was added  around the trench to complete the job.

The plastic pipe was added as an un-rustable re-inforcing. At this time, we had a site meeting  with the manufacturer of the dome to ensure that the components would mate up with the  dome base.

Telescope concrete foundation

The concrete goes in with lots of volunteer help from SEKAS (South East Kent Astronomical Society) and Thanet Countryside Trust members

The most stressful part of this job was the estimate of the amount of concrete required. We calculated the volume several times and in several different ways. It would have been bad enough to over-order but the nightmare was getting three-quarters of the way through filling the pier cavity and running out! As it was, our calculations were just about right, and we had only a little left over.

Another important decision was over the type of concrete used. We left this in the hands of the technical advisor of Brett Concrete Ltd. The mix used was similar to that used in bridge piers with a small alteration in aggregate size to allow easy flow into our pier support. Some extra water was added into the chute to aid us in moving the concrete around the trench. Note the lengths of iron piping indicating the required level for the main dome wall foundations.

telescope footing finished

Concreting finished and shuttering removed

Great care was needed when we filled the central pier and especially the 12″ diameter pipe.  First we had to fill the first foot of the pipe with concrete, making sure there were no air  spaces formed, then the telescope mount bolt assembly was lowered into place. This was  then carefully ‘polar aligned’, and the whole tube filled in.

The site was then left to overwinter until the last frosts had gone and dome wall construction  could begin.

During this period planning was going ahead concerning the construction of the 16′  diameter dome, and details were finalised with Beacon Hill Telescopes regarding the  construction of the skeletal tube.