Yesterday Mike drafted a plan to stake out the locations of the foundation posts. It involves identifying the north-south line through the center of the telescope pier, then measuring from the pier to the post locations. Here is the plan drawing. Today's measurements are shown in red.
We strung a line between two lengths of steel conduit left over from our straw bale garden deer fence, and centered it directly over the north-south bolts embedded in the top of the concrete pier.
Then we marked the center and end post locations on a 10' 2x6, and laid this on the ground beneath the string. Louise dropped a plumb bob from the string, and Mike adjusted the 2x6 so the center reference mark was directly under the plumb bob.
Next we measured from the pier's north centerline, and rotated the 2x6 until the end marks were the same (and correct) distance from the pier. We were gratified that these measurements came out exactly right. We sprayed marking paint at each post's location. This photo shows the general setup.
We repeated this process on the south end, then measured between opposite corners and made minor adjustments to get the diagonals equal. One minor nuisance was the tape measure had only feet-and-inch markings, but Mike's plan showed all dimensions in inches. Thankfully, Mike has a construction calculator app on his cell phone, and this quickly converted inches to feet-and-inches.
The final step was to measure between same-side corners to locate the intermediate posts. In all, we accurately marked all 12 foundation posts. Tomorrow we'll bore holes for them.
We used the power earth auger to bore holes for the 12 foundation posts marked yesterday. The job went very well, and we finished in less than an hour. The auger encountered only two rocks – unlike much of our land, where rocks are everywhere.
A depth gauge made quick work of onfirming the holes are at least 21" deep. Tomorrow we'll use a second gauge to check the depth is 18" after shoveling 3" of crushed stone into each hole. Then we can set the posts.
We began to set the 12 foundation posts and attach the cross beams between them. Even though we began in early morning, the full sun quickly became too much for us, so we quit at 10:30, and came back at 6pm, when the sun was behind some trees.
We made better progress in the cooler temperature. We first cut the two south corner posts to their respective lengths, then attached a 10' 2x6 between them. We poured 3" of crushed stone into each hole, and set the post-beam assembly into their two holes. We plumbed the posts, then measured from the pier's south centerline to the center of each post, and adjusted the posts until the distances were equal. When everything was right, we poured concrete into the two holes.
With light fading, we worked quickly to repeat this process for the north corner posts. We cut them to length, added the 10' 2x6 cross beam, set the posts in their holes, and plumbed them. We made minor adjustments until the posts were equidistant from the pier, then we secured them until tomorrow with large wedges made from old 2x4s.
Finally, we measured diagonally between opposite corner posts, and were estatic that both distances were equal to within a tiny fraction of an inch.
This photo shows today's progress at 8:44pm. The south posts and beam are in the foreground. The north posts and beam in the distance still must be adjusted to the same height as the reference post on the southwest corner (closest to the camera, near the black bucket). Then they too will be fixed in place with concrete.
We use an electronic water level to adjust each post to the correct height. It's accurate to 1/16".
We got an early start this morning, but still set just two posts and their cross beam before it became too hot to work. Here is the morning's result. Only one pair of posts remain to complete the foundation perimeter.
We returned about 7:30pm and managed to set the final pair of posts and their cross beam. It was 9:15pm and almost completely dark as we collected our tools and headed for the house.
But oops! We didn't notice these posts and beam were offset 1" toward the west, and ½" too high. Mike had to fix these problems days later
Mike shot this northwest view of tonight's work the next morning, but we didn't work that day. The final post-beam assembly is second from the left. The remaining four posts to be set along the observatory's centerline are leaning on the other three beams.
We got another early start, and set the final four posts along the observatory's centerline. The foundation is complete. Next up – frame the deck.