Having installed the pier and leveled the top plate (as described in the previous article), I made sure that everything was rock solid and that I could easily polar align the telescope before beginning construction of the observatory itself.

First Ideas

At the time I was considering building the observatory I checked out a variety of plans and approaches that were available on the internet ranging from simple roll-off roof designs to increasingly more sophisticated hinged roof section designs and on to backyard observatories implementing geodesic domes. The Doghouse roll-off roof in the June 2000 issue of Sky & Telescope was of particular interest.

My skills being very meager meant that a dome design was just too sophisticated and well beyond my abilities. So I decided on constructing a roll-off roof design. The various plans I'd investigated gave me some ideas but because some parts used in those (principally American) designs were unavailable, I ended up designing the observatory from scratch rather than adhering to someone else's plans.

How Big?

The first thing to take into consideration was the overall size of the observatory. My back garden is large, so I wasn't limited by the space available. What was important was measuring the full length of the telescope with attached CCD camera and other accessories and making an allowance for my own girth at the back of these optical combinations. The overall width of the observatory was worked out to be six feet (outer wall to outer wall). [A real mixture of imperial and metric measurements would be utilised in the final construction!]

Picture 1: The floor frame covered in plastic for damp prooftng. Four 3"x3"fence posts are bolted to each side of the floor leaving 3"x3"gaps at the corners

Since I also needed room for keeping various accessories and a work table, the length was chosen to be 8 feet rather than making the observatory square.

Another consideration was how high to make the observatory. Since I live beside a lane-way, I didn't want the observatory to be easily seen by passing opportunists. This limited the overall height, including roof, to 6.5 feet. The observatory walls also had to be high enough to protect the telescope from being buffeted by the wind and from low lying stray light but not tall enough to restrict horizon views. Due to surrounding trees, houses and walls, the minimum elevation I can reliably see from the site is about 30 degrees above the horizon. With all this in mind, a wall height of 5 feet was chosen.

What Orientation?

The garden is roughly oriented with the cardinal compass directions, so the issue of which direction the roof would roll off was based on which area of the sky was worst affected by light pollution, surrounding trees and intrusive street lights. I also didn't want to have the rails and roof intruding further into the main garden area and spoiling it.

Picture 2: First attempt at laying the floor, flush with the edges of the floor frame

I eventually decided to align the building's long axis in the east-west direction and have the roof roll to the east. That part of the sky is partly blocked by nearby trees anyway and suffers light pollution from neighbouring buildings.

Construction Begins

Looking around the local DIY suppliers, I ended up buying treated 2"x3" beams for the construction of the various frames in the observatory. The wood was cut to size using a hand saw and nailed together using 150mm nails.

I started out with the floor frame. The struts were placed 20" apart (10" apart either side of the pier) with a small 10" square frame being centred in the floor to surround the pier. The pier had to be passed through this hole so the floor frame could be positioned correctly. The frame was then roughly lined up parallel with the wall. The corner points were marked, the frame rotated out of position and 1 foot deep holes dug at each marked point. These would later take fence posts which would be cemented into position.

The floor was then rotated back into position and four 3"x3" treated fence posts were bolted to each of the four sides of the floor frame (see Picture 1). The floor was then leveled by placing wooden shims under it where necessary. Thick plastic (a mixture of damp-proofing plastic and hardwood underlay plastic left over from a re-flooring job in the house) was then stapled on top of the frame to prevent damp rising from the ground and into the observatory. It would also prevent the floor from prematurely rotting.

The floor itself was cut from two 8'4 pieces of plywood with a 10" diameter semicircle being cut in each piece which would fit around the pier. The two pieces were then screwed down onto the floor frame. 3" square cut outs were taken from the corners where the vertical fence posts would eventually fit (see Picture 2).

Walls

Picture 3: Erection of the wall frames for the left and back of the observatory

Three wall frames were then constructed - one each for the short sides of the observatory and one for the long side at the back of the observatory. These were screwed into position, through the floor and into the fence posts previously bolted to the floor frame, using 150mm coach screws (see Picture 3). These screws (which have hexagonal heads for use with ratchet spanners) can be hard to find. Home base had a selection whose entire stock I eventually purchased! But even that wasn't enough. Luckily, the local old-style hardware store had a stock of them as well ...at half the price Home base were charging!

Since I also used up Homebase's supply of 2"x3"s, I contacted a timber merchant about supplying the rest of the timber and plywood for the project. No surprises for guessing that the prices were again about half that charged by Home base! If only I'd thought of it sooner!

Design Mistake

With three wall frames in position (see Picture 3), heavy clouds rolled in and a tumultuous downpour began. Sometime later, when the Sun reappeared, I inspected the water damage. As expected the wall frames were fine (being treated wood) but the floor (even though it was marine grade plywood) was saturated. Since the wall frames were flush over the edges of the plywood floor, all water that had run off the frames was now seeping into the floor through its exposed edges.