By Andrew Cooper and Bill Lofquist
Have you ever wanted to have a place to set up your scope easily in the backyard? with instant polar alignment? no tripod legs in the way?
Even for someone with little handyman experience a pier is an easy weekend project that can be completed for around $60. Add the cost of a wedge for you scope, about $125 new, less used, and you have a usable pier. A few bags of concrete, a little rebar and a sonotube will do the job. I know, we ATM’s usually use sonotube for telescope tubes, but this is what it is really meant to do, cast concrete.A pier is also the first step in a real backyard observatory. Build the pier first, then a building around it. The process shown here works for any pier and can be scaled as needed for larger scopes. The pier shown in the photos is intended to hold an eight inch SCT.
Be sure to plan ahead carefully before you start to avoid any nasty surprises. Once concrete is being poured is a bad time to find out you missed a step.
A trip to your local home center will likely turn up most of what you need. I find that most small hardware stores do not stock rebar and sonotube, you can check, but you will most likely need to go to one of the bigger places like Home Depot for what you need. I do not support Home Base as they insist on selling illegal lighting fixtures here in Arizona, so if you like dark skies…For the stainless steel all thread rod it was necessary to go to an industrial hardware supplier. In Tucson Copper State Nut and Bolt or Hardware Metal Specialties will have what is needed.Once all the materials are on hand you are ready to proceed.
Step 1 – Purchase the Materials
|90 lb. bags concrete||10|
|Sonotube, 12 in x 4 ft||1|
|2×4’s eight feet long||3|
|Stainless all-thread rod, 3 ft x 3/8″ – 24||1|
|Stainless nuts, 3/8″ – 24||6|
|Stainless nuts, 3/8″||6|
|Carriage Bolts, 3/8″ – 16 x 4″ long||3|
|Nuts, 3/8″ – 16||3|
|Rebar, 3/9″ x 4 ft||4|
|Rebar, 3/8 x 2ft||4|
|Wood screws, 3″||6|
Step 2 – Survey the Site
A very important task that must be complete before you start digging is a proper site survey. The easiest way to do this is to setup your telescope on the site you wish to use for the pier and observe. Check your horizons and make sure you want to use exactly this site in the future. Look for any trees that might grow up into your way or other features that might interfere.There are a few other things that you need to do. One is to mark the exact site of the center of the pier with a stake directly beneath the telescope. You will also need to mark due north from your pier site so that you can align the bolts in the top of the pier. Put a stake or some other mark you can use well away from the pier at a point due north. I once used a piece of cloth tied to a chain link fence fifty feet away.
It is important to measure the height of the tripod to determine the height of the pier you want. Find a comfortable height for viewing. With a refractor or an SCT it is often possible to choose a height that will allow sitting in an observing stool. Very useful for long, relaxed observing sessions. A shorter pier is also more stable.After you have marked your spot remember that you need to dig a pit here. You now need to place four surrounding stakes. This is done in such a way that if two strings are tied to each pair of stakes around the site the strings will cross over the exact center of the pier. (see the site drawing) You can now check your center anytime you need to during construction.
Step 3 – Excavate
An often overlooked but extremely important part of the pier is the footer. It is the footer that gives the pier stability. So how big should the footer be? This depends on the size of the scope and the soil into which the footer is placed. The hard caliche soils of our desert here in Tucson do not require a very large footer. But the soft damp soils of the north would require a larger footer. Ask the builders of the Leaning Tower of Pisa about damp soils and what can happen. A good recommendation for the size of your footer would probably be at least three feet in diameter and a foot high. If in doubt concrete is only three dollars a bag, so a couple more bags are cheap insurance. On the other hand, it is amazing how fast a hole in the ground can swallow concrete when you start pouring. Don’t go too large, as you will be pouring concrete all day, and driving down to the hardware store to get more. If you are in doubt about how many bags will be needed just stack up the bags and measure. A bag of concrete will fill about the same volume after mixing so your stack of bags is the same size as the hole it will fill.The pit dug here is probably large enough for any soil and is planned for an eight inch SCT telescope. It is about three feet in diameter and about a foot and a half deep. It is deeper than the footer is high so that soil can be backfilled about the pier covering the footer.It is also important not to disturb any more soil than necessary under the pier. Loose soil will compact underneath the footer causing it to shift after completion. Dig no further than necessary and then carefully clean out the loose material and do not backfill if you think you have dug too deep, just make the column of the pier taller.
The pit will serve as the forms for the footer, so dig it with steep sides in the shape of the footer for the pier.
Step 4 – Prepare the Forms
“Forms” is the proper term for whatever holds the concrete in place while it cures. This is what the sonotube and 2×4’s are for.If necessary you will need to cut the sonotube for the height of pier you want. The sonotube will be held in place by three eight foot 2×4’s cut in half, forming a triangular frame. (See the photo in step 6) Three lengths will be used to form a base level with the ground by securing the lengths in a triangular pattern around the base of the tube. The lower pieces are secure to each other but not the sonotube. The remaining pieces are secured high on the tube with carriage bolts from the inside of the sonotube, the smooth head of the carriage bolt will leave a slight depression in the side of the pier. It is not important that the frame look good or even be straight, just solid. However crooked the frame it can be used to level the pier and hold it securely.
The next piece of the formwork is the bolt template. This is made from a piece of plywood or similar flat piece, any odd scrap will do. The wedge is placed on the template and the holes for the mounting bolts marked. These holes are drilled to accept the bolts so that they may be secured as shown in the photo. In this manner the bolts will be held perfectly while the concrete cures.
You need to mark a line due north with respect to the bolt pattern. Examine the wedge so you are sure how it mounts and then use a ruler or carpenters square to measure and mark a reference line on the template that can be used to align the template to your north.
Step 5 – Pour the Footer
After you have the site excavated, built the forms and double check that everything is ready it is time to mix concrete.Mixing concrete is fairly easy if done right. read the instructions that come printed on the bags and do not add to much water. Add water slowly until you reach the correct amount. The concrete should be stiff and will not readily flow. As you shovel the concrete into place tamp it to remove air pockets and to settle it into place.
Shovel or pour the concrete into the pit. After four inches or so of concrete is in the bottom of the pit lay two short sections of rebar in the concrete. After adding a few more inches place the last two crosswise to the first two. These will tie the footer together.
When the footer is in place you can insert the rebar for the column, make sure it is tall enough to overlap the mounting bolts for the wedge to insure a solid mount. Now check to be sure the rebar is in the right place by using the stakes out to the sides and use a level to check that the sonotube is straight up and down.
It is time to take a break and allow the concrete of the footer to set up a bit before you proceed. Not to long however, if the concrete is fully cured, the new concrete of the pillar will not adhere completely to the footer. An hour or so ought to do it, probably a good time for lunch.
Step 6 – Pour the Pier
First step in pouring the pillar is to place the form. It is carefully lowered over the rebar coming from the footer. The form is then leveled by placing or removing soil under the legs of the frame.Be sure the tamp the dirt down so that settling will not cause problems later. A little weight placed on the frame will keep it from moving while pouring the concrete. Note the large rocks (see photo) placed on each leg and in the center.
Step 7 – Place the Hold-down Bolts
Just take the prepared bolt template and plunk it into the concrete and center up the bolts. Remember to shake it for a few moments to allow the concrete to settle properly around the bolts. Take another look at your wedge and make sure you have the bolts aligned properly and then carefully align the bolts to true north. If you sight along the line marked on the template it should line up with your survey mark for north. A quick check with the level again is also in order.You must get this right! Otherwise you will not be able to polar align the wedge without trouble. Take care and get it right the first time.
Another method of aligning the hold down bolts is to use a compass as shown in the photo. This only works with stainless steel hardware which is nonmagnetic, otherwise the steel bolts would interfere with the reading of the compass. Remember the difference between true north and magnetic north, you have to look this up for each location. Shown in the photo is the twelve degrees difference for Tucson.
We had had a couple cloudy days and could not use a telescope to survey for true north. It had also clouded up right at local solar noon which stopped us from using a shadow line like a sundial (with appropriate longitude correction) to find true north. So we resorted to a magnetic compass, which despite care, barely worked. The wedge just aligned on this pier at the limit of the bolt slots.
Step 8 – Wait for Curing
You should allow the concrete to cure for at least 24 hours before removing the forms. And wait another few days before applying the weight of a telescope to the pier.
Step 9 – Remove the Forms
Dismantle the frame and remove the template. Sonotube is easy, the several layers just peel away like a paper towel tube if you start at an upper corner.
Step 10 – Patch and Paint
While waiting for the concrete to completely cure you can patch and paint so that your pier looks good. A little concrete patching compound can patch any voids left if the tamping wasn’t complete. A coat of exterior latex paint can blend the pier into its surroundings.
Step 11 – Attach the Wedge
Attaching the wedge is the last step. Run a set of nuts and washers down the bolts before you put the wedge on, these can be used to level the wedge before tightening the upper nuts.
Step 12 – Viewing!!!
Congratulations! if you have accomplished all of the above successfully you have something to be proud of, and a pier that will provide a stable mount for you scope for many backyard observing sessions. You can use a drift star polar alignment to properly align the wedge insuring polar alignment without hassle each time you setup.Competed Pier at Bill Lofquist’s house in Tucson. The patio is covered will Cool Deck to prevent it from being a large thermal mass and interfering with seeing. Cool Deck is a swimming pool product that actually keeps the concrete cool in Arizona sun. The pier is also mechanically isolated from the patio by a soft foam to prevent vibrations, also a swimming pool product used in concrete expansion joints.