New modifications to the observatory are on hold for a bit while I am on modified activity levels awaiting back surgery. Given that we think the construction of the pier was the cause of the back problems I am experiencing, I decided to make the pier construction the topic of this post. I have included a series of photos in the photos page that show the evolution of the pier construction.
It took awhile to decide where on the property I wanted to place the pier and ultimately the observatory. A lot of my property is wooded and that made it a bit of a challenge to find a place with appropriate horizon views. In the end, I chose a location nearer to the road than I wanted, but it gave me the best horizon available on my property. My plan was to address the light problem of passing vehicles in the near future. This would eventually be managed by the construction of a “light blind” which wil be the topic of a future post.
Before I began any actual work at the site, there was some prep work to due ahead of time. My pier construction basically followed the plans posted on the Tucson Amateur Astronomy Association and anyone looking to build a pier should consider reviewing that sites documentation, it is quite helpful.
Unlike the TAAA plans, I used an 8″ sono-tube rather than a 12″. In addition, I made a slight modification to the rebar method used. Like the TAAA, placed 2′ long rebar segments in the concrete in a criss-cross fashion. To manage the vertical rebar, rather than using stand alone rods as seen in the TAAA documents, I built a re-bar cage, 4 vertical rods 4′ in length with 6″ rods as the cross-braces at the top and bottom.
To attache the mount head to the pier I used a Meade tri-pod adapter. I wanted the adapter to be dead center on the pier, or at least as close as possible. I cut a 12×12 piece of 1/8″ plywood and identified the center of the board by drawing lines from corner to oposite corner. I placed the board on the flor with the sono-tube on top. I ensured the tube was centered then drew then traced around the sono-tube. At the center point I drilled a 3/8″ hole. I turned the board over so the sono-tubes outline was on the “bottom.” I placed the concrete J-bar through the hole. Treating the board as the top of the pier, I placed the adapter on top and bolted it in place. By doing this I could judge how much of the bolt needed to remain above the surface of the pier top. I left the adapter bolted to the wood square.
At the pier site.
To make my life easier, I employed he assistance of a friend with a Kubota tractor to dig the base pit. The pit was essentially a 3x3x1 hole. I followed the Tucson website guidance on supplies and concrete calculations.
Following the guidance from the link above, we created a bracing structure of 2x4s to hold sona tube in place. The bracing system was leveled. At this point we shoveled concrete into the hole as seen in the second photo. The re-bar cage was then placed into the concrete. As described in the link, we allowed an adequate amount of time (1-2hours) for the concrete to begin curing. Don’t let the concrete cure too long because you want the concrete poured into the tube to “merge” with the curing base concrete. While the concrete was curing, I inserted the concrete J-bar into a thin piece of wood.
After the brief wait, the sono-tube was then placed over the re-bar cage, into the brace and bolted to the top 2x4s as described in the TAAA documentation.
The tube was then filled with concrete. As you can see we made our concrete a little too wet and it caused the sono-tube to rupture at the bottom. So what do you use to lug up the damn….. duct tape of course. Duct tape proved an a more the suitable solution. The concrete bump which remains isn’t too visible after painting the pier, but it otherwise gives the pier some unique character.
I let the pier cure fo several day before removing any bracing or the top. Fortunately I was too busy to do anything for almost 2 weeks so when I got back to working on it I was very certain it was properly cured.
I removed all the bracing hardware followed by the pier head adapter. I was sureto make the J-bolt with a sharpie on the bottom of the board. I had not counted on the concrete shrinking and as such there was about 1″ between the concrete top of the pier and the wood plate. This would turn out to be a unforeseen benefit later. To fill the gap at the top I decided to use a smoother concrete, the type used to cement stones for stone front facades. Filled the gap with the concrete up to the level of the sharpie mark. I let the concrete begin to set.
I would check on the consistency of the concrete from time to time. Once is reached a point that it was thick but not so cured that it couldn’t be shaped, U ised a large nail to engrave a line pointing to true north. Once the north line was in place, I too the mount head adapter and sprayed the bottom with cooking spray, like Pam, and placed it on the pier to over the J-bolt. I put the nut in place but only finger tight. The adapter had to be free to spin. In then, slowly, rotated the adapter to make an impression on the concrete. Once I made an adequate imprint, I placed a leveling tool on top of the adapter head making minor adjustments to the adapter by gently pressing here and there until it was level. I rotated the level 360° in about 10°-20° intervals until one side of the level had made a 180° rotation from its starting point. At this time I knew everything was well leveled. It was time to let the concrete cure completely.
Before packing it in for the day, I looked over the pier for holes, damage, etc. I used the remaining concrete to smooth any imperfections and repair any cracks or holes. This was primarily for aesthetics.
It was easy to let the pier cure for a week since all the work I could only work on the pier on weekends. After a week of curing, I carefully removed the adapter unit and checked out my work. All was good. The final step was to pain the pier. I purchased concrete paint from Lowe’s and gave the pier a couple of coats.
Once the paint dried I replaced the adapter head, made sure I know where the front of the adapter was and aligned it to the true north line I scribed into the concrete then bolted it down.