|
Dome Reconstruction 2007 Please click on any picture
for an enlarged version The observatory opened in 1982 had started to show problems in the early
1990s.The dome is only supported on three wheels and the stresses and
weight of the skin had made the observing slot had start to close. By
the turn of the century the gap part way up was over 50mm narrower than
at both ends. This put pressure on the hardboard skin which started to
buckle.
There were several committee meetings to discuss the materials to be used and eventually Oil Tempered Hardboard was agreed as the new skin material. This had proved itself on the original dome and in places that had not been subjected to stress, was as good as new, even though it was over 25 years old. The telescope and old shutter were removed at the start and a
steel bar put in place between the two main side ribs to stabilise
the structure. This would leave a large gap where the shutter had
been but this would be covered with a blue tarpaulin after every
work party. Structural Changes
|
|||||||||||||||||||||||||||||||||||||||||||
 |
 |
 |
Replacing the Skin
The skin was then removed, section by section and the new ones fitted. These had been pre painted for extra protection before cutting to size. Some rot was discovered in places on the box ring (right), but the plywood frame was in very good condition considering its age.
 |
 |
 |
None of the 22 sections of skin were the same size To make a new
piece, each old one was used as a template and placed over the
prepainted 8 by 4 ft sheet and cut around. Extra wood was left
to be marked accurately using the dome as a template and each one
trimmed to fit.
 |
 |
Final fitting and cutting was done in situ. Each panel was then glued and nailed to the frame with 100 nails. The dome is 18 feet in diameter and so 8 feet sheets would not cover from the box ring to the top. A join was made at the 8 feet level. It was then just a case of continuing around the structure to replace all 22 petals.
 |
 |
To make the skin waterproof, each joint between the panels was sealed with glass fibre sheet
A second coat of aluminium primer was then painted over the outer
surface before a single top coat was applied
 |
 |
Priming the wood before fitting helped as it was not necessary
to cover the newly fitted petals between sessions..
Building the Shutter
The new shutter is a plywood cover over a welded galvanised steel
frame for strength. The structure moves on six wheels, three on
each side, over two steel tracks.
 |
 |
Two sheets of plywood are needed to cover the length whole of
the shutter. This is tied to the frame.
The sides of the shutter are made of two sheets of 6mm plywood, carefully cut to size.
 |
 |
These are then screwed and glued to the frame and outer cover.
 |
 |
The new shutter is finished and ready for painting.
Two coats of primer were used together with aluminium top coat to seal the plywood against the damp.
Shutter Running Tracks
The running track are made of steel welded together to form two tracks nearly 25 feet long. These were then fixed to the dome. The tracks are wider than the main ribs to form a lip on the inside of the slot. Wheels on the shutter engage this lip in high winds, stopping the shutter blowing off.
 |
 |
 |
Shutter moving Mechanism
The shutter is opened by a system of pulleys and steel rope. The pulleys are plastic and fitted between two plywood sheets with steel bolts.
 |
 |
 |
These are fixed to the two main ribs over the full length of the shutter slot. The ropes are then fed through these to form a continuous loop on each side. The opening mechanism is the worm and gear from a car trailer- suitably modified.
Here we see the steel ropes in place with the main shutter
closed.
 |
 |
Telescope Mount
The telescope had been removed before work began on the dome. Here
the original mounting srews are being removed and an extra 12 inches
of support added to raise the telecope. It had always been impossible
to see below Declination 14 degrees south due to the box ring.
This change would enable the centre of the galaxy to be observed.
A new concrete block was added to the top of the 18 feet high pillar.
This was then painted.
The telescope was refitted and realigned. During the dome reconstruction we took the opportunity to have the mirrors reluminised.
To use the new dome the top shutter is first opened by rotating a handle on the rear box section.
This tensions two continuous steel rope loops and lifts the top shutter. These
loops can be seen left and centre.
 |
 |
 |
The lower shutter is then untied and pushed out. Part of the reason for having
a seperate lower shutter was to reduce light pollution from the houses on the
street above the society. The dome can be used with this lower shutter still
closed.
 |
 |
At the end of the observing session, the lower shutter is closed first. The handle is rotated until the upper shutter covers the top of the lower shutter and locks it in place. The dome is then rotated to its storage position.
The project started in the spring of 2006, took until September 2007 to complete. The largest cost was the paint and not the skin, but it was completely within the £1000 budget that had been allocated.
Members who worked on the dome refurbishment
Colin Reeve
Dave Castledine
Dave Garbutt
Phil Norton
Bill Booth
Laura Lacey