Project Status - Large Dome


Well I've possibly cracked the money problem - I discovered a comet! Along with comet discoveries comes some money from the Comet Wilson award which I think I'll put towards the dome project.

Now to decide on the design which can be built within the new budget.


The plans are drawn and will be submitted to council for approval to build in the next week or so.

It will be a 6-metre diameter, 2-story all-steel dome. The lower floor will serve as a workshop/storage area (and perhaps a spectrohelioscope once I've got time to make one) while the upper floor will be large enough for a nice big telescope (but there isn't enough money for that so my existing 31-cm will go in there for the moment). I'll post plans and pictures once construction actually starts in a few months. Stay tuned...

Finally submitted the plans to council (2000/03/16). The delay was due to getting the plans reviewed by an independent engineer. I gave them to him at the start of the year, but he took a lot longer than promised to return them with his report. So far the project has cost me $730 in fees (council, engineer and owner/builder license) and nothing actually done (other than peg out where it's going). Soon...

The plans were approved by council 2000/03/27 (although I didn't get them for a few days), so construction could begin in earnest. Yesterday I removed the sheet of black plastic I had covered the site with (to kill the grass) and started digging. Here's a photo of the area immediately before the first sod was turned. I then dug the trench for the piers. (It's a trench because the mounting that will go there (or at least probably will with the current plan) is an English (or cross-axis) mount and it requires two piers. Today I started bending the steel. Here is a picture of the machine I was using, with Brendan (whose machine it is) in the background. The piece in the machine is one of the sections for the base ring. This was only the second pass through the roller and so it only has a mild curve. By the end of (quite!) a few hours it, and two other pieces had achieved a 6-m diameter and there were definite signs that it might work...

Yesterday saw the remainder of the rolling. Here is a picture of more rolling, this time one of the 50mm sections for the overhead arches. Next is a picture of the truck packed at the end of the day with all the material - an observatory kit almost. Today saw Brendan welding the base ring and arches in place - I'll do the rest. First some minor adjustments were necessary before welding. Then the arches were installed. And now it looks like this after the top bar was put in place. You can see my existing roll-off roof observatory in the background and to the right the site for the new one. There will be a delay now while the ground is prepared for the concrete.


On 2000/06/13 the concrete for the piers was poured. I made a steel cage for reinforcing the concrete which can be seen in this picture.

2000/06/23 saw the start of preparing the rest of the site for the slab. It was leveled and a trench dug around the edge for additional strength. Here you can see the finished pier base surrounded by the leveled area and defining ring for the base edge. The slab was poured on 2000/06/29, delayed a little due to a busy time for the cement truck. It was almost delayed more due to the poor weather - it was actually raining some of the time but remained dry enough to do the job. My friend Peter Neilson (from Dubbo Observatory) and his girlfriend Cindy came up to help as did another friend, Bill Ward, on holiday in Australia from Scotland. Many thanks for their efforts. This shows only some of the fun we (Bill, me, Cindy and Peter) had that morning in the rain. 4·2 cubic metres of concrete were used (plus 1·6 for the base for the piers).

Now that the base was poured it was time to concentrate on the dome. The ribs, already rolled, were cut and welded between the base and arches. This part at least went quickly, although some problems were encountered in that not all the ribs had the right curve (despite looking like they did) and had to be manually re-bent to make sure that they were a better fit. This is my father welding the ribs in place while I supervise (which I did a lot!).

Then it was time to make the bi-parting shutters. This did not go well. The problem here is that as I designed them, they aren't stiff enough to hold their shape without flexing. 25mm RHS was used and to this was added another layer. The shutter was first clamped in place on the dome and then the second 25mm RHS was clamped on top of the first and then welded in place.

Finally it all had to be painted. Red!

Now it was time for the posts to go up for the tower. Here's the first post going up, assisted by my wonderful wife, Pam. These are 3·3 meter tall, 100mm by 4mm wall thickness RHS steel. They are not light. At last they are all up - it only took the two of us a day.

To link the tower together and provide a place to which to screw the wall, steel girts (25mm RHS) are welded to the posts. Here is my friend (and neighbour) Rob welding the final (and highest) girts in place.


2000/07/16 saw the 200mm I-beams put in place. Brendan brought out his crane and easily lifted them into place. The first one is down, helped by (L-R) Colin, Brendan (hiding behind the ladder), my father, Rob and me up the ladder actually caught doing something other than supervising. Here's Rob and I guiding the second beam into place.

Then there's a lot more painting to be done. Here's me doing the door frame. (I did the dome with a spray gun, but the rest was done by hand.) Once the lower section was painted (mostly by Colin) we could start putting up the wall, which consists of sheets of "colorbond" (a protected steel) screwed to the girts. Simple but effective.

But then my holidays ran out and I had to go back to work. My father had already left and Colin went home the next day. So progress is now much slower without my helpers. As of yesterday, all the painting is done and Pam and I started skinning the dome - not much but enough to think that it might work - and the wheels atop the tower are in place. More pictures soon...


The dome skinning went slowly. Here's the the first few panels (done at the start of August) to see if were possible. It was more than a month later that this shot was taken. I managed to convince my friend Colin to come back for another week's "holiday" and so things progressed very quickly then. More progress, then on to the shutters. As can be seen, it can get a little precarious at times. The skin is 0·55mm thick galvanised steel riveted in place, with a bead of silicone between each sheet to help ensure a waterproof seam. I used a "lazy tong" riveter which can be seen in action here (that's Colin's hand from the inside of the dome waving to the audience). Once all the rivets were in place, Colin then went round and put a blob of silicone over each one just to make sure there was no place for water to get in. I should point out that the weather was clear and hot - much more so than is usual for this time of year. With the shiny steel we were cooked from both sides by the sun. It was finally finished. Here's the front view showing the shutters with weather seal in place.

Now it was time to put the dome on the tower. Brendan came out with his crane and we hooked it up. The shutters were tied open and the crane jib put in through the opening to the centre. Chains were attached around the base ring and then it was up and away! Down to the tower and put it on. As simple as that! Here's the view from where the dome was by the house, and then when the crane was out of the way .

The dome has been tied down so that it can't blow away and then the task of putting on the remaining colorbond was started. As the time of writing (2000/9/21) all the colorbond is in place and there is a lock on the door. From the outside it looks complete - but the inside now needs a floor and stairs. Wait for the next installment...

2000/09/22 - I've just borrowed a digital camera for this wide angle shot showing both the old observatory and the new, and this close-up.

2002/10/27 - Final update. The dome is finished and I've been too busy using it to update this page. So here's a summary of what happened and some pictures to go with it.

The next job was inside - to put in the stairs and the observing floor. The stairs went in first, but are a little hard to photograph due to their size. They are made in 3 straight sections wrapping around the outer edge of the tower wall. Here is a picture of me standing on the finished stairs, starting to put up the floor joists. (You'll note that already the under-stair area is being used as storage space - and the stairs aren't even painted!) This picture taken a while later (the floor was finished by this stage) shows a better view of the stairs (and the junk under them). Note the yellow blobs of stuff protruding up from the floor. This is expanding foam (comes in a pressure pack can) used to fill the space between the round concrete floor and the wriggly Colorbond wall. This effectively seals the inside from the outside (about a million spiders had quickly decided that my dome made excellent living quarters for them!) and was simple to install.

Once some of the floor joists were in place, Rob and I stacked all the floor sheets upstairs. Once the remaining joists were in place, it took me another day to cut the sheets to size and screw them down. (I had decided on the layout of the floor boards from a scale drawing, trying to minimise cutting and wastage.) Here's what it looks like from underneath. The joists are 150×50 mm treated pine across the 200mm I-beams (carefully aligned so as to miss the piers when they go in), while the floor is made from "standard" 22mm thick floor sheets, 900mm wide by 3600mm long. The stairs are made from the same material (and are 900mm wide - the width of a sheet).

I then spent ages painting the floor - or at least it felt like ages (I hate painting!). The hole in the floor at the top of the stairs and the beginnings of the safety rail is being inspected here by my son Alexander.

It should be noted that the dome isn't waterproof yet. There are still 2 small holes at the top of the dome, plus the skirt isn't in place. I was relying on the lovely dry conditions we usually have to save me from ruin. In practice, it rained a couple of times but the contents of the dome were saved from ruin by some large plastic sheets. I couldn't fix the holes at the top of the dome until the floor was in place as I needed a support for the ladder in order to reach them. It's a long way up there but I finally finished it.

The final part of waterproofing the dome was to put the skirt on. The dome diameter is about the same as the tower and so the skirt has to be shaped accordingly. U-shaped brackets were welded to the dome base ring and then sections of 5mm thick, 25mm wide steel strap were welded all the way around the dome. This provided a support for the galvanised steel sheets (the same material as the dome skin) to be riveted, as shown here. The wider view showing the overlap of the skirt over the tower is shown here. The idea of such a large skirt is to provide a gap for (passive) ventilation. The top of the shutters has a large space to allow the hot air to vent. The waterproof seal is made by applying lots of silicon sealant around the junctions. It seems to work well enough.

Once the dome was waterproof it was time to move on to the details. One major effort was the dome drive. The whole point of having the wheels mounted on the tower and not on the dome is that this becomes much easier. One of the wheels has a shaft attached which a motor drives via several pulley-wheel reducers. The motor was salvaged from an old washing machine some 20 years ago. I thought at the time that it would make a good dome drive and I've been carting it about with me ever since. The motor has a specially-machined 1·5-inch diameter pulley wheel driving a 12-inch wheel (yes, these are imperial wheels) which in turn has a 2-inch wheel driving another 12-inch wheel. This gives me the right reduction (via a 150mm diameter dome wheel) to rotate the dome in just under 60 seconds - which is about the right speed, I think. This view shows the drive and electronic control box and handset. One of the 4 lateral alignment wheels can also be seen, but this one is offset because of the drive. (The whole drive mechanism is covered while in use to stop fingers getting caught.) The control box houses 2 relays to drive the dome. One (a zero-crossing type to minimise power-on surges) switches power to the 240V AC motor, while the other is used to switch windings in the motor to enable it to be reversed. The handset is on a 6-m lead so that the dome can be controlled from anywhere in the dome, and has just a power button and direction switch (and LED to indicate power is on).

There are 4 wheels which support the dome. Three look like this, while the other is sprung. The wheels are 150mm diameter, 75mm wide neoprene-coated (so no noise) capable of supporting 350Kg each (the dome weighs approximately 900Kg). Four 12mm bolts allow for individual height and angle adjustments (the wheel surfaces were machined at a small angle to encourage movment in a circle rather than a straight line). Lateral adjustment wheels made from nylon stop the dome from wandering too far out of line. There are covers over all of the wheels now.

The other necessity was a desk, which is built above the dead space of the stair well. (I should have put a drawer in there too, but didn't think of it at the time.) You can also see that it is carpeted. I found enought carpet to do both upstairs and downstairs, a luxury worth having I feel.

To help the shutters open and close easily I attached some pulleys and wire rope to the lower and upper parts of the shutter opening. This allows the shutters to open evenly without twisting.

Eventually I had time to make a pier. The mount I intend putting in the dome is an English (cross-axis) style, requiring 2 piers. Making that mount work is another project in itself and so I don't need 2 piers yet. I bought the blocks for 2 piers but only cemented one in place, the other is just stacked. This shot is a panorama stitched together to give a wide angle view so that the piers can be seen in context. My 31-cm Dobsonian now sits atop one pier and shows that it should be stable enough for serious observing.

The final essential aspect was power and permanent lighting. Twin fluorescent lights give adequate bright lighting, while red lights are from old darkroom fittings. The stairs have separate red lights from the rest of the area. There are no major lights in the dome (just a desk lamp), but there's enough light to see during the day by from the light which filters in from the wide air gap around the shutters and dome base. Power points around the wall downstairs and in several places in the dome complete the fittings. The power points, dome power and lighting are separately protected.

There is more I intend to do, but the dome is as finished as it needs to be for observing. I've already cabled up the network to the roll-off roof observatory and to the house so I can monitor the CCD while doing some visual observing. Insulation and air conditioning are priorities for the future, as is the 25-inch telescope.

In use, the dome is excellent. The wide aperture provides good views of the sky, but you can still attain better dark adaption than in the open. Dome rotation is smooth and quiet and adequately fast. The pier is stable, and there's enough room for lots of friends. I'm in observing heaven.

Page last updated 2003/04/02
Steven Lee