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It is another warm and breezy day. I have been very lucky with the weather. Hardly needing to cover the huge pile of timber overnight.
I woke early and was muddling over a design for the observatory roof in my head as I waited for a suitable time to get out of bed.
A double pitched [Mansard] roof is as close as it is possible to get to an arc using only two straight lines. Rotating those lines into an octagon provides a polygon which is close enough to a circle. The result is a simplified hemispherical "dome." One which uses only flat surfaces and straight edges.
This double pitched roof is not as material efficient as a perfect sphere. It must be slightly taller and of larger radius to provide the same internal clear radius. This step is achieved by choosing the minimum acceptable clear radius [for swinging long telescopes inside] and then simply drawing the two tangents.
There is even room for a handy joining strut to reinforce the bent joint without obstructing the arc of the internal radius. Note how the upper and lower tangents may be slid around the arc to change the shape provided the edges still meet in the middle.
The next trick is to avoid having heavy timber braces inside.These would greatly reduce the internal swept volume. In other words, the internal radius of minimum arc would shrink considerably.
So I'm now looking at constructing an octagonal, double-pitched framework from aluminium angle profile. Logic suggests eight, long, isosceles triangles should run from the top down to the edge of the roof. A tall, thin triangle which is bent at half way.An important pint is that the lower inside edges must clear the building's own octagon. So one octagon rotates relative to the other. The projecting "points" between the two octagons must not interfere with each during roof rotation. This is obviously not as geometrically efficient as two circles rotating around each other.
Consideration of stiffness suggests a top triangle and a lower trapezium. This automatically provides right angle, edge connecting surfaces between the eight upper, triangular panels. The joint between the upper triangles and the lower trapeziums will enjoy a similar connection but would need to be angled. An obtuse angle is required to set the change in slope midway down the roof.
Critics will point to the slight error in the standard right angle profiles where they meet. This problem could be overcome by a slight bending of the edges of the cladding aluminium. Or a fairly stiff, but compressible, sealing material could provide a wedge between the angle profiles where they meet. A weather strip over the vertical joints is normal. While a slight overhang of the upper triangle cladding, over the lower trapezium, will ensure rain runs off the upper panel and onto the lower one. What could possibly go wrong? 😏
The rotating roof must obviously be furnished with an open slit for the telescope to look through. Is it vital for the telescope to be able to see vertically overhead? [Zenith.] Most amateur astronomers would suggest it is. This would require an open hole at the apex of the roof triangles. Life is suddenly getting much more complicated. A strong, closed ring obviously isn't much use. So it must be an equally strong, open semicircle leading downward into the open slit. The slit is a local break in the strength of the roof. The closed panels are sheathed in aluminium for a stressed skin effect. Fresh air has no value when it comes to strength of an open panel.
What works aesthetically in the way of a shutter, or shutters, on a double pitched octagonal roof? Can one of the octagonal panels be simply hinged aside? That might work with the upper triangle. Opening the entire lower trapezium would be much too wide! Think of the slightest wind dragging on that huge panel! Eek! Most dome, observing slits are parallel-sided regardless of the design of the shutters. Bi-parting shutters probably make most sense with an octagonal roof. What about the join between the upper and lower panels? The edge of the slit is going to need serious reinforcing. Not only to retain the geometric strength of the roof construction but also to support the shutters.
The rotating roof must obviously be furnished with an open slit for the telescope to look through. Is it vital for the telescope to be able to see vertically overhead? [Zenith.] Most amateur astronomers would suggest it is. This would require an open hole at the apex of the roof triangles. Life is suddenly getting much more complicated. A strong, closed ring obviously isn't much use. So it must be an equally strong, open semicircle leading downward into the open slit. The slit is a local break in the strength of the roof. The closed panels are sheathed in aluminium for a stressed skin effect. Fresh air has no value when it comes to strength of an open panel.
What works aesthetically in the way of a shutter, or shutters, on a double pitched octagonal roof? Can one of the octagonal panels be simply hinged aside? That might work with the upper triangle. Opening the entire lower trapezium would be much too wide! Think of the slightest wind dragging on that huge panel! Eek! Most dome, observing slits are parallel-sided regardless of the design of the shutters. Bi-parting shutters probably make most sense with an octagonal roof. What about the join between the upper and lower panels? The edge of the slit is going to need serious reinforcing. Not only to retain the geometric strength of the roof construction but also to support the shutters.
Click on any image for an enlargement.
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