Dome build: A new trapdoor design:

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Tuesday: I covered the dome with an old, white, lightweight tarpaulin so I could work with power tools if it rained. Notching the ribs was done with a cheap, Chinese router working freehand. The tarpaulin gives an idea of how the dome would appear in white. The neighbour's trees and hedges have grown considerably since I last worried about dome colour.

Wednesday: The tarpaulin is doing quite a good job except where it is too short across one diameter. At least the observatory floor stayed dry. I keep rotating the dome to match the wind direction to keep the rain out.

I have been looking at the trapdoor again and fiddling with the ladder height with blocks. I need the top tread level with the rest of the floor for safety. I also need a level surface beyond the top tread for my overhanging toes.

Deciding on the hinge line is the main problem. It obviously has to be well beyond the ladder top or the trapdoor will stop natural footfall as I reach the top tread. My latest idea is a plate of aluminium to lie flat on the top ladder tread. This will be hinged about 8" forwards of the ladder top. Since the trapdoor will be normally closed the plate will only lie in the gap between the ladder handrails. It will not be used as part of the normal floor except when looking directly east. I could get some adhesive grip tape to avoid any slipperiness when standing on the metal plate. It will on the route to the veranda via double doors.

Sturdy T-strap hinges are an alternative to separate hinges and a metal plate. The hatch will be supported on  metal dowels projecting from each of the floor joists for even support. I have to be careful that the dowels are domed to void injury while climbing the ladder.

The new trapdoor will probably be a plywood sandwich. Using 1/2" grooved boards top and bottom. This will help to simulate the rest of the larch flooring and to provide some grip. Unfortunately the larch boards proved to be massively heavy. I had made a larch trapdoor ages ago but could hardly lift it up as I reached the top of the ladder. So I have done without a trapdoor ever since.

Stepping repeatedly over the large hole in the observatory floor has taught me to respect the not inconsiderable risk. I looked at boot scraper style doormats as a ventilated hatch option but they were also far too heavy.

Counterbalancing a heavy trapdoor is not an easy task when the mechanism must not impede movement around the telescope in the dark. Cables most not cross walkways nor risk any chance of tripping the user during normal use. There must be no crossing levers which could remove digits in the dark. Fool-proofing under all circumstances is absolutely vital.

I have just realised that cable pulleys could be attached to the stepladder handrail extensions. This would keep the cables well out of the way of normal movement within the observatory.  See the image left where I have lifted the boards beyond the ladder top for measuring. The hatch opening is on the right.

The weight cables need only small holes to run through vertically below the far side of the pulleys. The weights would fall near the floor, inside the west wall of the octagon. Where they can do no harm if a cable joint gives way. This area is below the stepladder so is not a normal thoroughfare.

The leverage applied by the cables changes with the angle of lift. Lift is modest as the trapdoor starts to rise. Is at a maximum at the halfway point and then declines as the trapdoor moves between the handrails. This is an ideal situation during normal hatch lifting. The trapdoor stays safely in the horizontal position. Except when it needs to open.

While counterbalancing could aid in parking the trapdoor in the open position this must not resist easy closing. Now I need to find a way to effortlessly close the hatch once I have safely gained the observatory floor after climbing the stepladder. The answer to easy closing may be simply to make the weights too light to hold the hatch open once it is past the [vertical] tipping point. Once started on its descent it would drop automatically.

I have lots of barbel weights to finely adjust the counterbalancing. A knotted rope could hang down below the hatch when I want to pull it closed from below while descending the ladder.

Gas struts, as for car boots and hatches, are an alternative lift-lower device but unlikely to provide the long term, ease of movement of cables and weights.

Click on any image for an enlargement.

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