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It is blowing a gale and the rain has poured down. I have just had to go up and lash down the lightweight tarpaulins which had broken loose overnight. This exercise gave me a firm reminder of the power of the wind. Bi-parting shutters might easily become an embarrassment in our normally gusty conditions.
A typically curved, "up and over" shutter would not suit the faceted dome. It would need to be evenly curved to avoid complication. Flat, concertina panels, the height of the facets would be impossible without very tall channels to contain the ends. The hinges might leak like a sieve!
In the dim distant past some observatories had "cloth" shutters. Canvas or oilcloth would be the norm back then. A long strip of material, with cross sticks for maintaining smooth surfaces, would be hauled up and down via ropes. This would open and close off the observation slit. The "cross sticks" could be aluminium poles for lightness and stiffness.
Two ropes would be needed to maintain parallelism. A pulley each side, at the top, with eyelets inside the slit for the free ends of the rope. This would help to keep things tidy. A closed horizontal channel, each side of the slit, would keep the rain out. They would also hold the shutter close to the dome's angular surface in a gale. The whole thing would collapse into a concertina heap at the base of the slit when the ropes were let out. Distance between the cross sticks would only be one panel high. Or about 60cm or 2' in ye olde money.
Cosmetically it could be the most attractive option for my tiered and paneled dome. I'd probably choose a medium weight, PVC tarpaulin for long life and a suitable degree of stiffness. Though I'm not really sure how well it would cope with really cold conditions. It might become too stiff to self-fold easily. Though it would always be within reach to help it on its way down to the bottom. Presumably it would eventually take a set in its folds to ease the final descent. A box could be provided for the concertina to avoid it looking too untidy.
There should be considerable weight saving over any form of plywood, shutter construction. Less weight means it is easier to rotate the dome.
The image shows a mock-up of how a drop-down shutter might look.
Tuesday: I spent time sealing the edges of the covering panels. I found an easier way to cover the dome. I tie two large tarpaulins together by the eyelets and then drag the whole lot up to the top of the dome with a rope. It is then fairly easy to spread them out working through the top of the slit from the stepladders.
I have also started making heavier panels for the top of the dome using 15mm birch plywood. I want to get some strength into this space and struts weren't doing it. I have made three panels so far to go at the rear of the dome using a cardboard template.
For the corners and side panels I will need more material and think 12mm ply might be enough and certainly lighter. I brought out one of my longer Record wood planes to straighten the edges of the panels after using a jig saw to cut out the triangles.
Expert advice has put me completely off having a roll/fold down PVC shutter. I have decided that I'd go with traditional bi-parting doors. My original choice was to make them faceted like the dome. However, the sides of the shutters would need enough depth to be strong enough.
"Faceting" them would seriously cut into the depth of the side ribs. So it's a conventional rounded shutter look. I can always revisit the faceted idea if I find a way to do it well. For the moment I just need a usable observatory. Preferably one which doesn't need to be covered every time it rains!
I had better get some materials for the shutters while I'm collecting more ply for the top panels. Unnecessary journeys with my trailer, the ten miles to the builders merchant and then back again, is best avoided. Not least because of the time involved. 4'x8' sheets of ply would be better than 5'x5'. Cutting curved ribs is horribly wasteful of expensive materials. Though I can usually find another use for all the off-cuts. Precise cutting saves a lot of waste too.
Wednesday: Drove to the builders' merchants for 3 sheets of 12mm birch ply but only available in 5'x5'. Went on making top tier panels. They are all the same base width but with different lengths and angles at the narrow ends.
Thursday: Very wet all day! Tidied the shed so I can have the miter saw ready and out of the rain. I fitted some large, heavy duty, shelf brackets to get stuff up and out of the way. The saw now sits happily underneath.
I realised the other day that my dome could be duplicated with much less effort in aluminium. Using square section for the ribs they need only to be V-notched precisely and bent to the closing angle. A dihedral plate will then be fixed over the inner side of the joint for the horizontal members to be attached.
The dome could then be covered in thin aluminium sheet. Hopefully a local engineer would guillotine the shapes to order from drawings or patterns. Each tier of panels would be identical to each other. Pop riveting and/or gluing the covering to the skeleton will make the dome incredibly strong yet light. The panels could even be cut as a long tapering strip if there is enough guillotine capacity. They only need a gentle bend to be applied where they meet the joints.
Friday: Quite a pleasant day peaking at 65F. I trimmed and fitted the first three 15mm birch top panels. They are so pale the dome now looks like a snow capped mountain. I haven't routed the underneath edges to lower their visible profile. They form the backbone of the dome where it joins the slit frame. I'll life with them for now to see if I'm feeling obsessive about it. I have a sheet of 12mm birch ply marked out for a lot more panels but need to confirm their lengths first.
There should be considerable weight saving over any form of plywood, shutter construction. Less weight means it is easier to rotate the dome.The image shows a mock-up of how a drop-down shutter might look.
Tuesday: I spent time sealing the edges of the covering panels. I found an easier way to cover the dome. I tie two large tarpaulins together by the eyelets and then drag the whole lot up to the top of the dome with a rope. It is then fairly easy to spread them out working through the top of the slit from the stepladders.
I have also started making heavier panels for the top of the dome using 15mm birch plywood. I want to get some strength into this space and struts weren't doing it. I have made three panels so far to go at the rear of the dome using a cardboard template.
For the corners and side panels I will need more material and think 12mm ply might be enough and certainly lighter. I brought out one of my longer Record wood planes to straighten the edges of the panels after using a jig saw to cut out the triangles.
Expert advice has put me completely off having a roll/fold down PVC shutter. I have decided that I'd go with traditional bi-parting doors. My original choice was to make them faceted like the dome. However, the sides of the shutters would need enough depth to be strong enough.
"Faceting" them would seriously cut into the depth of the side ribs. So it's a conventional rounded shutter look. I can always revisit the faceted idea if I find a way to do it well. For the moment I just need a usable observatory. Preferably one which doesn't need to be covered every time it rains!
I had better get some materials for the shutters while I'm collecting more ply for the top panels. Unnecessary journeys with my trailer, the ten miles to the builders merchant and then back again, is best avoided. Not least because of the time involved. 4'x8' sheets of ply would be better than 5'x5'. Cutting curved ribs is horribly wasteful of expensive materials. Though I can usually find another use for all the off-cuts. Precise cutting saves a lot of waste too.
Wednesday: Drove to the builders' merchants for 3 sheets of 12mm birch ply but only available in 5'x5'. Went on making top tier panels. They are all the same base width but with different lengths and angles at the narrow ends.
Thursday: Very wet all day! Tidied the shed so I can have the miter saw ready and out of the rain. I fitted some large, heavy duty, shelf brackets to get stuff up and out of the way. The saw now sits happily underneath.
I realised the other day that my dome could be duplicated with much less effort in aluminium. Using square section for the ribs they need only to be V-notched precisely and bent to the closing angle. A dihedral plate will then be fixed over the inner side of the joint for the horizontal members to be attached.
The dome could then be covered in thin aluminium sheet. Hopefully a local engineer would guillotine the shapes to order from drawings or patterns. Each tier of panels would be identical to each other. Pop riveting and/or gluing the covering to the skeleton will make the dome incredibly strong yet light. The panels could even be cut as a long tapering strip if there is enough guillotine capacity. They only need a gentle bend to be applied where they meet the joints.Friday: Quite a pleasant day peaking at 65F. I trimmed and fitted the first three 15mm birch top panels. They are so pale the dome now looks like a snow capped mountain. I haven't routed the underneath edges to lower their visible profile. They form the backbone of the dome where it joins the slit frame. I'll life with them for now to see if I'm feeling obsessive about it. I have a sheet of 12mm birch ply marked out for a lot more panels but need to confirm their lengths first.
Click on any image for an enlargement.
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