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The weight of an observatory roof or dome depends on its shape, area and the choice of covering material. It is possible to make some rough calculations by using the information and online calculators without leaving your computer chair.
Duratool aluminium weights:
337967.qxd - 359285.pdf
The plywood calculator above suggests that 4mm Birch plywood weighs 2kg or 5lbs per m².
Some allowance must be made for variation in wood density, glue weight and moisture level.
According to the online calculator above a cone of 1.6m radius and 2m height has a slant or lateral surface area of 13m². You can't have the same height of cone as a hemisphere because it would cut into the hemisphere. The hemisphere of 1.5m radius is the minimum swept volume to clear the telescope. A straight-sided cone to slip over that dome and reach the equator would need to be larger in diameter and /or taller.
So 13m² x 2kg = 24kg or ~60lbs. This is for the plywood covering alone. A real observatory is expected to survive long term, outdoors in storms, snow load, etc. so badly needs its supporting timber framework. This framework could easily dwarf the weight of the covering depending on materials, number of braces, etc..
Beware of including the Base area in your cone area calculations unless you plan to cover the base too. Some online calculators offer the Base area, Lateral/Slant area and Total area. Keep this in mind.
Let's try using 1.5mm aluminium instead of 4mm plywood: 1.5mm sheet aluminium weighs 4kg m².
That's a doubling in weight over 4mm plywood! Though aluminium might be stronger, longer lasting and far more waterproof than plywood. Aluminium might be easier to support using smaller aluminium cross sections. Plywood usually has to be coated for long term weather protection. Paint and GRP can add considerable weight.
Historically, many observatory roofs or domes were sheet copper covered over wood planking.
1.5mm Copper sheet weighs 13.5kg or 30lbs per m². That's 3.5 times that of aluminium sheet of the same thickness. Now imagine the weight of all that supporting planking! It's no wonder they don't build aircraft this way! But then, they might have used thinner copper cladding for this purpose.
A 1.5m radius hemisphere has a surface area of 14m². Perhaps our original cone was rather undersized to fit over a hemisphere? But both would still be of a similar weight to each other.
The right, circular cone to fit over a hemisphere of 10' diameter is 15' wide and 7' tall.
That's 4.6m wide x 2.13m high. Running those figures through the cone calculator increases our slant surface area to 22m². That's quite an increase on 13m² ! Now we are up to 44kg or 90lbs!
Reducing the bottom diameter of the straight cone to 12' means a height increase to 9'. This is for a cone which touches the hemisphere. So the cone would need to be larger, both ways, to make room for supporting rafters and braces. The only sensible answer, is to bend the sloping walls of the octagonal "cone" to wrap it more tightly around the matching hemisphere.
A forum member has kindly suggested suitable dimensions for a 'bent' conical roof. I will now have to build a simple, full sized template to check clearance for the telescope in all positions.
Yesterday [Saturday] I fixed guttering along the shed roof adjacent to the observatory. This will prevent rain soaking the ground between them and stop splashing up against the walls of both buildings. Without access to sun and wind there was little chance for this area to dry out between showers.
The right, circular cone to fit over a hemisphere of 10' diameter is 15' wide and 7' tall.
That's 4.6m wide x 2.13m high. Running those figures through the cone calculator increases our slant surface area to 22m². That's quite an increase on 13m² ! Now we are up to 44kg or 90lbs!
Reducing the bottom diameter of the straight cone to 12' means a height increase to 9'. This is for a cone which touches the hemisphere. So the cone would need to be larger, both ways, to make room for supporting rafters and braces. The only sensible answer, is to bend the sloping walls of the octagonal "cone" to wrap it more tightly around the matching hemisphere.
A forum member has kindly suggested suitable dimensions for a 'bent' conical roof. I will now have to build a simple, full sized template to check clearance for the telescope in all positions.
Yesterday [Saturday] I fixed guttering along the shed roof adjacent to the observatory. This will prevent rain soaking the ground between them and stop splashing up against the walls of both buildings. Without access to sun and wind there was little chance for this area to dry out between showers.
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