Backyard Pier
By
Members, Andrew Cooper and William Lofquist
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 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.
Step 1
-
Purchase the
Materials
Step 2
-
Survey the Site
Step 3
-
Excavate
Step 4
-
Prepare the Forms
Step 5
-
Pour the Footer
Step 6
-
Pour the Pier
Step 7
-
Place the
Hold-down Bolts
Step 8
-
Wait for Curing
Step 9
-
Remove the Forms
Step 10
-
Patch and Paint
Step 11
-
Attach the Wedge
Step 12
-
Viewing!!!
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.
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Materials List |
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|
Item |
Qty |
Unit Cost |
TTL Cost |
|
90lb. bags concrete mix |
10 |
$2.67 |
$26.70 |
|
Sonotube, 12in x 4 ft |
1 |
$4.97 |
$4.97 |
|
2x4's eight feet long |
3 |
$3.37 |
$10.11 |
|
Stainless all-thread rod, 3ft x 3/8"-24 |
1 |
$7.08 |
$7.08 |
|
Stainless nuts, 3/8"-24 |
6 |
$0.05 |
$0.30 |
|
Stainless washers, 3/8" |
6 |
$0.05 |
$0.30 |
|
Carriage Bolts, 3/8"-16 x 4" long |
3 |
$0.65 |
$1.95 |
|
Nuts, 3/8"-16 |
3 |
$0.03 |
$0.09 |
|
Washers, 3/8" |
3 |
$0.05 |
$0.15 |
|
Rebar, 3/8" x 4ft |
4 |
$1.38 |
$5.52 |
|
Rebar, 3/8" x 2ft |
4 |
$0.69 |
$2.76 |
|
Wood screws, 3in |
6 |
$0.05 |
$0.30 |
|
Total Cost |
$60.23 |
|
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To the
Top
 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.
To the
Top
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.
To the
Top
 "Forms"
is the proper term for whatever holds the concrete in place while it
cures. This is what the sonotube and 2x4'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 2x4'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.
To the
Top
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.
To the
Top
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.
To the
Top
 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.
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.
To the
Top
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
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.
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.
 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.
To the
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