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The Linnhe Observatory web site

These pages feature extensive construction details for my observatory completed in the summer of 2001 for a Meade LX200 10" f/10 Schmidt-Cassegrain telescope

page 4


The pad

As mentioned earlier, I needed to make sure that the top of the concrete around the pier was 0.5" lower than the surface of the pad concrete. So I started on the pad, laying it out and creating a form for the concrete before pouring the pier concrete. Because the dome I was expecting was circular I designed an octagonal-shaped pad. This turned out to be quite fortuitous later when I had to construct a frame for the observatory I eventually built.

The pad mapped out with the turf removed image

The pad mapped out with the turf removed

I left the turf around the pier hole for some time to prevent its collapse at the top. The concrete for the pad is 3.5" deep. The ground proved to be uneven and on a gentle slope on the north-south axis. Hence, in the image below, the boards around the north side closest to the camera are below the grass surface whilst those at the north side are well above. It is very important to have a level pad if having a dome for the smooth rotation of the dome on its walls. It is possibly slightly less so for other types of observatory. I didn't bother with a hard core base, the sand was tramped down and very firm. It wasn't going to move.

The pier and pier hole construction image

The pier and pier hole construction

The concrete was poured into the pier hole with the pier held in place with some heavy pieces of wood and bricks. I had two builder's levels to ensure that the pier plate remained level at all times. I wasn't concerned about the pier being vertical, this doesn't matter. What does is the surface onto which the scope will sit. I also roughly aligned it north-south. However, you'll remember that there was provision for adjustment later with the second top plate.

The form for the pad complete image

The form for the pad complete

When the pier plate of the pier was welded on I had two holes made a quarter of the way up from the bottom (half way in the section in the hole) opposite each other. This allowed the concrete to pour into the pier and be one with the concrete on the outside of the pier. I also placed some steel rods through this hole. Hopefully these two measures will ensure the vertical stability of the pier. I also stood the pier on a couple of small bricks to allow concrete under the pier rather than let it sit on the sandy base of the hole. Concrete was poured into the access hole at the top filling the pier to just underneath this hole.  Over the top probably, from what I've read some of you Americans may not think so :-) but this 'bird table' isn't going anywhere in my lifetime - or the next!

The concrete laid and signing my name image

The concrete laid and signing my name

Electrical cable was run from the house inside a plastic conduit and buried under the lawn and comes out of the pad. You can see it here sticking up above the concrete and wrapped up to prevent the conduit being filled with concrete. At this stage I had not planned where I would take the electricity wires (to the pier or observatory walls) so this seemed a fair compromise. You may want to consider more than one electricity supply and another conduit containing serial, network or telephone cables if you intend to do some remote viewing using CCD or video cameras with your computer in the house. I always planned to use a laptop in the observatory so this wasn't necessary for me.

A membrane was placed over the base, stones having been removed first. Thin boards were placed over the pier concrete and the concrete poured. A single layer of chicken wire mesh was placed in the concrete about half way down to strengthen it.

The pad finished and set months later image

The pad finished and set months later

When the concrete had cured, the form was removed. The form around the pier concrete left a 0.5" gap between the pier concrete and pad concrete. As you can see here, the pier concrete is lower than the pad concrete so that when flooring is installed it doesn't touch the pier concrete. A large flower pot covers the electrical source, I had placed a metal clad RCD double socket here as I was using this pad for two years waiting for the dome that never arrived.

Before constructing a concrete pad you may want to look at HomeDome's site which provides an excellent document entitled "At Home in a Dome" which you can download. In it, there are extensive details on how to construct a concrete pad and pier construction designs.

So that was 1999. As I say, the dome didn't arrive, so I started to design something myself. I looked at lots of sites on the web - the best place to start then was Bill Arnett's "List of DIY Observatories" (now down). It quickly became evident to me that with my limited DIY and construction skills what I required was a ready-made shed of some sort I could adapt. It also seemed the cheapest and quickest way to proceed. But most sheds were of the wooden garden variety, thinly clad, no appreciable frame and with roofs that were not easily adapted to a roll-off design. Then I came across a site that changed everything.

 

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