Onehouse Observatory

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Constructing the Onehouse observatory

The basic structure

The basis of the building is a timber shed, of ship-lap construction and apex roof design, measuring 2.13 x 2.13m (Fig 1). The side walls are 1.88m high and the apex height is 2.54m. The frame timbers are 50 x 50mm. The shed was custom-built without a roof or windows and with a circular hole (0.54m in diameter) in the middle of the floor. The latter was designed to accommodate the pillar, as described below. The manufacturers, on their own initiative, added a square box-frame underneath the floor around the hole to provide extra support. They erected the shed on a pre-existing concrete base. The apex is oriented WNW-ESE.

Fig 1  The shed, as supplied

Constructing the roof

The observatory roof consists of two panels, each made from a standard-sized sheet of shuttering quality plywood measuring 2440 x 1220 x 12mm. These are screwed on to rectangular timber frames built with 50 x 50mm planed timber. Each frame has a central cross-member across its short axis to prevent sagging in the middle of the panel.

The ends of the frames rest on top of the gable walls of the shed. The lower edge of each frame is attached to the top-rail at the side of the shed with three steel hinges. During construction it was found that the hinges raised the lower edge of each panel and prevented its frame from lying flush on top of the gable walls. The solution was to attach thin strips of wood, about 20mm thick, to the tops of the gable walls (Fig 2).

Fig 2  The roof panels in place

Once the roof panels were in place, the ends of the plywood sheets were trimmed so that they did not project beyond the lines of the gable walls. In order to seal the gap of about 20mm between the panels at the apex of the roof, a V-shaped ridge piece was made from two 2.13m lengths of 19 x 150mm planed timber. These were joined with right-angled metal brackets that were then bent to the required angle (Fig 3).

Fig 3  The ridge piece being tried for size

The roof panels and ridge piece were individually covered with shed-quality roofing felt, and the latter was fixed with 10mm bolts to the roof panel on the south side of the building (Fig 4). To increase the strength of this joint the bolts pass through both the plywood sheet and the underlying 50 x 50mm frame timber. The bolt-heads were sealed with silicone sealant. Barge-boards were fitted to the ends of the roof panels, at front and rear. Sturdy hooks and eyes were used inside the building to lock the roof panels together when closed.

Fig 4  The roof, after felting but lacking barge-boards

It soon became apparent that rainwater tended to drain off the roof and through the gap between the side walls and the lower, hinged edge of the roof panels. A normal garden shed has projecting eaves to prevent this. Some rubber car-mats were nailed to the lower edges of the roof panels to cover the gaps – in retrospect it would have been better to attach the mats before the roofing felt was put on. To avoid problems of damp caused by rainwater running off the rubber mats and down the outside of the walls, wooden battens were added to hold the mats away from the walls, the mats being held in place with half-round beading (Fig 5).

Finally, to prevent wind-blown rain from entering the building at the gable ends of the roof, varnished wooden disks (in fact, hardwood chopping boards) were mounted on metal brackets in front of the gap between the barge boards at the apex of the roof (Fig 5).

The roof mechanism

The resulting structure was watertight and secure, and it was time to decide how the heavy roof panels (each weighing 70 pounds or more) were to be opened. Also, there needed to be some means of supporting the open panels at the required angle. The latter was solved easily by running chains from eyebolts in the top corners of the roof panels to another eyebolt fitted near the apex of each gable wall. Arriving at a method for opening the roof was more problematic. Experiment showed that by standing on a step ladder it was relatively easy for one person to lift each hinged panel to a vertical position, but once the panel went beyond that point it could not be controlled, even with the help of a retaining rope. Clearly, a system of counterweights was required.

The solution was to use plastic, 5 gallon water containers, obtained from a camping shop. A length of nylon rope (8mm diameter) runs from the eyebolt in the top corner of each roof panel (that to which the chains are attached), through another eyebolt in the top corner of the opposite wall. Each rope is fastened to the handle of a water container standing on the floor in the corner of the building. When the roof is closed, these ropes are slack. Once a roof panel is opened to a vertical position the slack on its two ropes is taken up. As the panel is pushed open a little further the two water containers on the opposite side of the building are lifted off the floor (Fig 6). From this point on, the weight of water in the containers is just enough to balance the weight of the roof panel and allow it to be opened to its full extent in a controlled manner.


  Fig 5  The completed roof, open fully

The roof panels are closed by pulling on a rope attached to an eyebolt in the top centre of the panel. Once the panel starts to rise, the counterweights assist the lift. The only drawback is that once the panel has reached its vertical position and the counterweights are once again resting on the floor, the panel has to be supported manually and eased down into its closed position.

Some trial and error was required to get the weight of water and length of rope correct. It was surprising (given that the roof panels weigh in excess of 70 pounds) how little water was required in each container: 28 pounds in those balancing the southern panel (with the ridge piece attached) and 23 pounds in the others. This is because of friction at the point where the ropes runs through the eyebolts in the top corners of the walls. Originally this friction caused the ropes to fray slightly, but this is prevented by the occasional application of some vaseline.

Fig 6  View of the interior, showing one of the counterweights and the pillar

The pillar

The lower part of the pillar is a plastic, 168 litre rainwater butt, which sits inside the hole in the timber floor of the observatory. It is free standing on the concrete base. Inside the water butt there is a plastic, 1600mm diameter (4mm thick) sewer pipe (Fig 6). The pipe and the water butt are filled with concrete and rubble, and are immovable.

Three 500mm lengths of 8mm threaded rod are embedded in the concrete in the top of the pipe and anchored at their base by a metal plate. The rods protrude slightly above the top of the pipe. A 15mm thick hardwood disk (another chopping board) slots over the rods, and is retained by nuts. This board is the base plate for the equatorial mount, which is attached to it by a central bolt – this is a longer version of the bolt that would normally attach the equatorial mount to the head of a tripod.

A circular, plastic tray with a central hole slots over the pipe and rests on top of the water butt, where it serves as a rotating accessory tray (Fig 7).

Fig 7  Detail showing the base of the pillar and the accessory tray

Finishing touches

The power supply to the observatory comes via an extension cable from a nearby garage. Plug sockets and light fittings are mounted on a timber rail that runs around the walls (Fig 6). The floor of the observatory is carpeted, to provide an extra layer of insulation and to reduce the risk of damage to dropped eyepieces and accessories. A hinged wooden tray is attached to the top-rail in one corner, to hold books and charts. There is a wall-mounted heater, used mainly to deal with occasional damp or condensation. A computer and monitor stand on a rack in the south-eastern corner.

Pros and cons

The advantages of having a permanently mounted telescope, housed in a secure building that is provided with a power supply, are obvious. There are several other advantages to this particular design.

A hinged-roof observatory takes up less space than one with the more common roll-off roof, and looks as much like a garden shed as it possibly could. The roof panels can be opened or closed in a matter of minutes, and it is very useful to be able to open one panel only, as the closed panel provides some protection from neighbours’ lights and the wind.

The size of the observatory is just sufficient to house my 150mm refractor (focal length 1200mm), even when observing targets at a low altitude. At 2.13m long, it was possible to build the roof panels from single sheets of standard-sized plywood.

Having a raised wooden floor and carpet helps the observer to keep warm and comfortable. As anyone with a long focal length refractor will know, it is often necessary to kneel on the ground when observing objects near the zenith.

At the time of writing (January 2005) the observatory has been in use for two years. The level of maintenance has been minimal. The exterior is treated with timber preservative once a year and the roofing felt should be good for several years to come. The nylon ropes (despite some slight fraying in the first weeks of use) show no signs of needing to be replaced. Even the water in the counterweights is still clear, thanks to the addition of a handful of salt.

The main disadvantage to the design is in the weight of the roof panels. Although the procedure for opening and closing the panels is straightforward, it is not something that can be undertaken in a casual manner, particularly after a cold evening’s observing. It might have been possible to reduce the weight of the panels by using thinner plywood and frame timbers, or even corrugated plastic sheets, but this would have resulted in a more flimsy structure that might have flexed.

During the winter there is sometimes quite a lot of condensation on the pillar. Also the undersides of the roof panels get damp and are prone to mould, especially on the north side of the building, which receives less sunlight. Occasional use of the heater is enough to deal with this problem.

Time and cost

The roof was constructed in one day. Another 2-3 days was required to install the counterweight system, construct the pillar and complete the fitting out.

The shed cost £400. The timber, roofing felt, water butt, sewer pipe, rope, chain and other hardware came in at about £200.


I would like to thank Andy Hasluem, who introduced me to many of the concepts used in building the observatory, and assisted with the construction.

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