Build a roll-off roof garden observatory

Part 3 of 3: Install a low-voltage electrical system and make your setup solar powered

This final part of our series covers how to take a roll-off roof (ROR) observatory ‘off grid’ with solar power, and make a power distribution box for the equipment inside. By using solar power we can keep everything to a nominal 12V, so there is no electric shock hazard, but there is still the potential for an electrical short circuit to cause overheating and a possible fire. Always have your circuitry checked by a qualified electrician.

To begin, check the power or current requirements of your mount, cameras, dew heaters, and any other accessories, and make a realistic estimation of how long they will be used; our estimation was for up to three (five-hour) sessions per week. Next, calculate the size of the solar panels. Begin by assuming that the panels will supply a maximum of 80 per cent of the stated power, and take into account the effect of their orientation (visit bit.ly/30A2heP for details).

We mounted our solar panels nearly flat to reduce their exposure to the wind (windage); this reduces their output to 84 per cent of what it would have been at the optimal orientation of 30–40° to the horizontal, facing due south. You must also allow for your longest nights, when the daytime – solar charging time – and the Sun’s strength, are at their least. The daily intake of the Sun’s energy (insolation) at 53°N ranges from 7kWh/m ² in midsummer to just 10 per cent of this in midwinter (visit bit.ly/30DXeJX for more details). We worked on the assumption that if we had three observing nights per week, we could expect three sunny days as well. Now decide how many ‘extra’ sessions you need your battery to store, and work out the battery size. You’ll need a deep-cycle SLA (sealed lead-acid), Li-ion or LiFePO4 battery.

We built our power distribution board into a weatherproof junction box. Each circuit can be independently switched on and off and is fused via a six-way fuse box. It has the following outputs: a 12V nominal unregulated output for the dew heaters, and two 12V regulated circuits for the mount, cameras and other equipment. The battery voltage can sometimes exceed 14V, which could damage 12V circuitry, hence the regulated output. We used GX-12 condensationproof sockets for these outputs. There are also two USB sockets for peripherals, plus red lights (car LED marker lights) and a white (number plate) light.

The observatory equipment is controlled by a Raspberry Pi single-board computer running StellarMate software (www.stellarmate.com), which communicates over Wi-Fi with a desktop computer. Along with KStars astronomy freeware (edu.kde.org/kstars) and its integrated Ekos astrophotography suite, this setup enables you to remote control your scope and cameras; a separate project to cover later.

Once your roll-off roof observatory is complete, you have a functional, independently powered observing location to enjoy the clear night skies, be it for viewing or imaging pleasure.

What you’ll need

Solar panels; a solar panel mounting kit; MC4 connectors and branch connectors; an MC4 crimping tool; a 12V deep-cycle battery; an MPPT (multi power point tracking) charge controller; insulated cable, circuit breakers and line fuses; a six-way fuse box and a Mini Busbar.

Sundry connectors, plugs, sockets and switches; an earth spike and cable; red LED marker lights and an LED number plate light; a dual USB car socket.

A weatherproof junction box; bituminous tape, cable entry housing; sundry screws, nuts, washers and bolts; cable clips.

Step by step

Step 1

Attach the mountings to the solar panel, put it in place, mark the positions of the mounting holes, and drill through the EPDM (ethylene propylene diene terpolymer) rubber membrane into the joists. Add patches of bituminous tape between the mountings and the EPDM and secure the mountings with lag bolts.

Step 2

Next, you should put a circuit breaker and fuse between the solar panels and the charge controller, as well as between the charge controller and the battery. Look at the charge controller documentation from the supplier, as this will tell you what size fuse and cable you should use in each part of the circuit.

Step 3

Wire in the panels using the weatherproof MC-4 connectors, ensuring that they are properly locked. It is advised to have them underneath a solar panel and suspended off the roof surface. The solar panel frames should be grounded to an earth spike.

Step 4

Follow the manufacturer’s instructions and, after ensuring that the circuit breakers are open, connect the charge controller to the battery and the solar panels. We used an MPPT controller, as it is more efficient and cheaper than the PWM variety.

Step 5

Check the circuitry with a meter, ensuring there is no short circuits. Close the battery circuit breaker and set the battery type on the controller. Then close the solar panel circuit breaker and wait for confirmation that your panel is charging the battery.

Step 6

Connect your distribution box, via a fuse and SAE connector, to the load output of the charge controller. Then switch the distribution box on and check the output from each of the circuits within the box. If all is well, you will be able to connect your telescope, cameras, dew heaters etc, and you are ready to go!

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ALL PICTURES: STEVE TONKIN