Backyard Pier Construction
The Fall of 2011 we started rebuilding our back yard after tearing down an old wooden deck, so it was a good time to make a telescope pier. Our back yard is far from ideal as a telescope site. It's in a sea of strong light pollution, and there is no horizon to speak of. But I can do narrow-band work whenever the sky is clear, I can see Polaris over the roof of our house, and I can see enough of the southern sky to observe all of Orion when it rises high enough. There is enough in the sky directly overhead to keep my 11 inch busy for years.
I've never worked with concrete before, so I planned every step carefully. It went pretty well, but I would tweak a few things if I ever did it again.
Step 1. Dig a Hole
My usual place to set up a tripod was over the center of a 2’x2’ paving stone – a good size for a pier footing, and the other stones helped to stabilize the edges and make a good work platform. The ground is clay, with a layer of fine gravel at the surface. I put some coarser gravel in the bottom, and then some stone dust to fill up the pore space. The footing thickness was to be about 1 foot.
Step 2. Place Rebar
The vertical rebar only needed to extend a short distance, because the ¾” bolts for the mounting hardware were 3 feet long and overlapped the rebar, giving additional support to the column. I drove eight 1 foot long rebar rods into the ground; hopefully they will stabilize the footing against sideways flow. The ground is mercifully flat here and soil creep should not be an issue.
Step 3. Position the Form Tube
I passed up the flimsy 12 inch diameter tubes available at home improvement stores for a firm, 14 inch diameter tube from a contract supplier. The bottom was held by 3 planks, and the top by four 2x2s, all firmly attached to the tube with carriage bolts. The tube was of course levelled, and the wood supports firmly braced with heavy paving stones. The bottom of the tube was positioned at what was to be the upper surface of the footing.
Step 4. Pour #1
The hole was filled up to the bottom edge of the tube, and a small amount of cement went into the tube to seal it. This was given 2 hours to partially set, so that when the tube was filled the cement wouldn’t just run out the bottom. The brace for the vertical rebar, used to ensure proper spacing from the sides of the tube, was removed.
Step 5. Pours #2 and #3
The tube was filled to a depth of 1 foot above the end of the ¾” bolts, suspended from the steel plate which will hold the mount. For this cement, a latex additive was used to improve adherence to the footing. The plate was immediately positioned, taking care to give it the correct compass position. After another 2 hour set, the plate was removed and the tube filled to the predetermined correct level, having covered the ends of the bolts with plastic bags to prevent concrete getting into the threads. The plate went back on immediately. Here a slight problem occurred – the bolts were pushed outward slightly by the weight of the concrete, and had to be convinced to return to the correct positions so the plate could slide over them.
Step 6. Rest. Eat. Sleep. Peel off the tube after 24 hours.
Peeling off the form tube revealed my inexperience with concrete – I should have taken more care to pack it by poking and stirring it inside the tube after each bag was poured (=6 inches depth). I’m sure there is less porosity inside the column than we see on the outer surface, and I used a crack-resistant concrete with special fibers to increase its strength. So I’m not worried that it isn’t strong enough to support my scopes for years to come. It looks a little better now that I've parged it (below, right), and in the spring I may improve it further with tiles of polished granite.
Phase 2: Enhancements
I don't mind snoozing in a chair beside the pier in summer, but in winter it's very unpleasant and the House Steering Committee dislikes me tromping in and out through the back door. I found that good-quality USB extension cables will successfully carry signals between the mount, camera, etc. and a computer in the house. So I buried this plastic pipe before laying the patio, and ran the cables through the window, where I can comfortably sit or doze on the couch while things progress. I put downward-pointing bends over each end to ensure water wouldn't run in, and plugged them with plastic to keep unwelcome life forms from taking up residence in the tube. Ends are color-coded and the best cable is reserved for the imaging camera.
Finally, I'm trying to turn the pier into an object of beauty in the yard, by covering the ugly grey concrete with a nice polished granite. The next step is to put some sort of colored grout in the gap between the tiles, and make a top for it. It's a rapakivi granite, sold as "Baltic Brown Granite" but the boxes were labeled "Made in China". So the name may mean no more than "forest green", and I have no idea what country the granite is actually from.