Constructing a Concrete Telescope Pier for Astrophotography - Part 2
Constructing a Concrete Telescope Pier for Astrophotography - Part 2
Digging in the Dirt
Let's get started on the pier now! You should have a pier plate on hand at this point for positioning the threaded rods in the pier - my past piers has mostly held a venerable EQ-6 with aluminum pier plate, machined at a local shop. It's pretty straightforward to pick up a round piece of aluminum from a metals dealer and have it milled and drilled to your specifications, or there are plenty of off the shelf models.
- Approximately 14 bags of regular concrete (mine were 66 lbs each) assuming you are making a 12" diameter 8' tall pier. Use this link and choose "Hole, Column, or Round Footings" for calculating the number of bags of concrete needed for other sonotube sizes. I've used fancier concrete before, both types worked well for the telescope, though the higher strength crack-resistant concrete was more challenging to remove during a pier tear-down
- An 8 foot Sonotube, of your preferred diameter. I like 12" for the sheer mass, but a smaller diameter would likely work just fine with rebar reinforcement. Plan on having at least 4 feet of the Sonotube buried to keep it below the frost line
- Digging implements - If you rent a auger post hole digger it will go a lot faster - you will likely need 2 people for using one of these. Otherwise shovels and post hold diggers work
- Somewhere to mix the concrete in (I used a wheelbarrow). You can rent a mixer as well for your local rental store to make it a bit easier
- Something to mix the concrete with if you use a wheelbarrow (A garden hoe works well)
- Access to clean water to mix with the concrete
- 3 pieces of 6 foot rebar (preferably epoxy coated as mine were, for longer life and potentially less rusting)
- Threaded stainless steel rods for the plate - In my case I used 4 pieces of 3 foot stainless steel threaded rod (1/2 inch) to anchor the pier plate in the pier. You could get away with shorter lengths. Some people will bend a rod in a 'U' shape to extend through 2 of the 4 pier plate holes, using only 2 rods. Other plates may require a different number and thickness of rods.
- 8 stainless nuts that fit the rods are also used later to hold the plate in the proper place and adjust the level of the plate. I may use the nylon type lock nuts underneath the plate this time instead of lock washers to ensure that the plate doesn't shift when tightening the top nuts down. Lock washers or nylon can be used on the top nuts, I'll likely use regular nuts and lock washers there for ease of removal - To be explored in the next post. Ensure that all exposed metals used in the project are stainless
- A way to keep the stainless steel rods parallel for the lower end of the rods (I usually use a piece of wire coat hanger wrapped around the rod ends in a square frame shape to keep the rods spaced properly, but didn't have one handy so I used some scrap wood instead. I screwed 2 10" length into a cross shape, drilled 1/2" holes and inserted the ends of the rods. In the picture below showing the wheelbarrow and bag of cement you can see the bottom half of the rig in the top left corner
- A method to find the North (close to the North star for Celestial North, or magnetic North) - See below
- Outdoor-rated wiring and outdoor junction boxes, and at least one weatherproof outlet. You will need enough wire to run either from a new circuit in your breaker box, or tie on to another existing junction box (or add one where needed along a line). You can run the wire in a conduit or without. Ensure that the circuit is GFI protected. You could run an extension cord from a GFI outdoor plug, though it may quickly become a tripping hazard or be eaten by a lawnmower. If you are not familiar with wiring I would highly recommend hiring an electrician to do the work, or at the least have them come to inspect your work. It's not nearly as expensive as the possible consequences of accidents
- If you are so inclined, you can use conduit to run your wire through your concrete pier so that you can have an outlet embedded on the side of your pier. Generally I opt for just screwing an outlet to the side of the pier and don't enclose the wires in cement.
Construction of the new pier started in early June 2019, and in keeping with past tradition all of the work was done by hand with shovels and a post hole digger, and various other manual implements of destruction. It took a little over a week of on-and-off digging to excavate the requisite 4 feet down, past the frost line and giving width clearance for a 12" Sonotube. Make sure your Sonotube extends to at least the minimum height above ground that you would like your pier plate to be, minus a few inches for the gap between the plate and the concrete, and keeping in mind the floor height of your shed will make the pier height lower, if you are building a shed. The concrete cinder block was used to prop up the front of the shed floor while I had it laying flat. This let me mark on the ground where the existing hole in the shed floor would be, 'X' marks the spot!
Near the beginning of the digging, a wire running through the garden was discovered. It was sheared off just past the hole (wasn't me!), running to the right top in the excavation picture avec cat below, and I'd wondered if it was a power line running to some non-functional pool lights or other use from past home owners. This would potentially make life much easier in providing power to the shed. Sadly it was soon after discovered that 1) The wire was also sheared off on the left side, only being about 8 feet long, and 2) It was a low voltage braided 12V line for garden lights and not the 120V I had been hoping for.
The Severed Mystery Wire
Starting the Dig - The Original Shed Floor is Currently Holding Up a Tree
The digging was largely uneventful, the post hole digger worked quite well further down where it could chop into the earth at the edge of the hole and pry to loosen up the soil. Several large stones were encountered, but didn't slow things down terribly. You always hope in the back of your mind that it might be some sort of buried treasure, but it rarely turns out to be. Along the journey we encountered garden soil, the afore-mentioned wire, clay-based soil, fairly heavy gravel mixed with random things left behind as presents by the house builders (such as half bricks and unidentified plastic bits), a layer of very nice clean sand (naturally deposited?), and finally some tougher clay soil. Luckily the dense clay started near the 4 foot depth and that's where the digging stopped.
Nearly Done Digging. Note the Mystery Wire to the Left of the Hole and the Trusty Post Hole Digger (+5 vs Earth!)
Finished Digging - Note the Brick in the Wall Near the Bottom of the Picture
Once the hole was finished and trimmed here and there to allow the Sonotube to stand upright and true, the real fun started - the concrete phase. The Sonotube was placed in the hole approximately vertically with some sand and soil added to the hole on the outside of the tube to give it some stability. As described below, it's a good idea to measure where North is and mark it on the pier plate and Sonotube before this point in time, or plan to be ready to have the concrete added and adjust the pier plate around noon time if you want to do it in real time. I'd recommend the former as less stressful!
Getting Ready with the Concrete - You Can See the Bottom of the Stainless Steel Rods Being Held by the MacGyvered Wood Spacer Thingy in the Top Left of the Picture, the Plate is Off-Camera to the Left
Lesson #1 (Forgotten and Re-) Learnt - Sonotube Supporting Materials
Use dry clean sand to fill the hole around the outside of the sonotube (such as sandbox sand). It will flow down the hole much better than soil and leave less voids underground where you can't see them. You can add water to the hole (outside the sonotube) to settle the sand even more, though I don't bother. Don't use the soil you dug up to fill here unless it's clean sand or unless you don't have any alternative- if you use soil, try to tamp it down as best you can with a pole or rod as you go. Likely using materials that don't pack as well as sand could contribute to a pier that will list over time - Not the end of the world but you will need to adjust your pier plate level to compensate. You will find that as time goes by you may need to add more handfuls of sand around the outside edges of the pier as it settles. The bottom line is that you want the tube to be held firmly all around as best as possible from the start, with little to no air voids.
You could potentially use a fairly runny concrete to try to fill between the tube and the hole walls, but I have not tried this. If you want to go all-out for getting the concrete to settle well inside and outside the Sonotube, you can rent this gizmo from places like Home Depot that extends into wet concrete and vibrates, making the concrete flow into voids and releasing much more air than you could do otherwise and making a smoother contact between the concrete and the Sonotube form. Tapping the tube carefully with a rubber mallet or slapping it with your hands (carefully!) will release some air pockets as well. I chose to go for chunking the below ground concrete with the shovel while adding it to try to get it to settle, and slapping the outside of the Sonotube above ground level.
Rebar Time! A Bit of Water Settling Out of the Concrete is Ok, But Try to Minimize It by Reducing the Water a Small Amount in Future Concrete Batches
Rebar is added once you have 2 to 3 feet of concrete in the Sonotube, enough to keep the rebar mostly in place. As you add concrete you may need to adjust them to be equidistant and centered. Keep adding sand to the outside of the sonotube to support it at the same height as you add concrete, and check for the Sonotube being plumb every couple of concrete bags. Concrete was added until it was at a height where it would start covering the base of the pier plate rods. The rods are then placed in the concrete, with the pier plate attached in order to keep the rods aligned on both ends, and the whole mechanism turned so that your AZ adjustment post on the pier plate faces North.
The best way I have found for this is to check when the sun is at zenith in your location (which is usually close to noon, Google will tell you when it is for your location), and stick a small piece of wood or stick (I used a popsicle stick in the past) on the centre of your plate vertically as the plate is suspended in place over your partly filled Sonotube (some bits of scrap wood will do to hold the plate in place over the tube as it will likely be smaller than the top of the sonotube). Now record where the shadow from the stick points by marking both your plate and the sonotube top and outside edge. This will be Celestial North. An old fashioned compass will generally work well, but check to make sure the metal in the pier doesn't throw it off. Alternately you could try to eyeball it if you go out at night and have access to see the North star (Polaris). Line up the AZ alignment pin/bolt/peg on your pier plate to point towards the North star and mark it on the pier plate and Sonotube outside top. If it's off by a bit it's probably not going to matter, but get as close as you can, using multiple methods to confirm if possible.
Marking North on the Pier Plate and Sonotube
Lesson #2 Learnt - Phone Compass Apps -> Bad! Or, It's Off by More Than a Little
In the past I have had some luck with phone compass apps. My last pier was build and adjusted using one, and it was dead on. Not so this time! I measured it many times with a couple of phone apps and marked where it said North was (Unfortunately I wasn't able to work on this part of the construction around noon time, and I hadn't used the sun and stick method, which I would recommend doing as it is the most accurate method). Guess what? The alignment bolt for my pier plate was nowhere near the North star after the concrete settled and hardened. Oops. So it looks like part of my final set of tasks will include drilling a new hole in the plate for a new alignment bolt (Use lots of water flow on the bit if you need to do this. I don't actually use the bolt to adjust my mount, but might as well have it in the right place. The bits get hot and stop working so well (ie: melts). We'll call that Lesson #2.5). Phone apps can be super handy, but in some cases the phones just don't have the sensitivity to show magnetic North reliably and can be greatly influenced by metal nearby as well as the initial app calibration process.
Once you've set the stainless steel rods in place and have a good reading on North, you can remove the pier plate from the top of the stainless rods and continue adding concrete until it's up to the level that you desire, using a pole or rod to tamp it down and release air bubbles every so often. You may get concrete on the threaded rod tops, but it will easily come off later. In case you're wondering, the two black squares on the pier plate are Velcro stickers, used in the past to hold a power splitter box I put together. You can never have enough 12V power sources.
Finished Adding Concrete and Ready for Final Adjustments
A couple of things to keep in mind at this stage - You will want to add concrete until you're within a few inches of the height that you would like the pier plate to end up at - Some people like the plate to hold the mount at the same height as a tripod would, others opt for it to be higher up, which can be easier on the back if you're a visual observer. You should plan to leave enough space between the bottom of your pier plate and the top of the concrete such that you can insert the bolt that will hold your mount to the pier plate, but not leave so much threaded rod exposed that it could allow unnecessary vibrations. Test the spacing to find what a comfortable gap is for your fingers to fit. If you find you have extra threaded rod above the concrete when your plate is at the minimum height from the concrete to screw the bolt in, you can always take an angle grinder to the top of the rods (I generally do to make a cleaner area around the mount).
Once you've added the concrete to a height you're happy with, replace the pier plate onto the rods and make final adjustments - centre the plate relative to the Sonotube, and ensure that your North markers still line up. Do a final check of the Sonotube to ensure that it's still perfectly vertical - you can still make minor adjustments here by nudging the pier. You may find that it has deformed a little from the weight of the concrete (some people will add dirt or gravel above the ground level to support it better), so measure in a few places on the top and sides to get a good idea of how it is oriented.
Fill in any remaining gaps around the Sonotube with sand. Soil can be used for the top half foot or foot as it should be easy to tamp and settle. Now that most of the messy work is complete, it's time to step away and let the concrete do it's thing. Leave the pier plate on the rods. Give it 2-3 days to cure before touching the pier. It can take up to a month to fully cure, but unless you're mounting a behemoth on it, it will be usable in short order.
Next Time - The Unwrapping, Leveling, and Sorting Out the Shed!