Construction of a Concrete Pier
The Losmandy G11 tripod is built very sturdy, but the ground it sits on and the
rigors of temperature change cause any fine polar alignment I could achieve to shift
over the course of days/weeks. Redoing an hour long drift align is big waste of
precious sky time when the scope is housed in a dome. So I browsed the net a bit
and figured out that a very cost effective way to get permanent polar alignment
was a concrete pier. Unfortunately I knew a little less than zilch about concrete,
but lucky for me my in-law's were dropping by for a spring visit on their way back
home to Newfoundland after wintering in Florida. Since my father in law had some
know-how with matters of the concrete and was always keen for a cool project, I
took advantage and got everything ready for a week long observatory redo. As luck
would have it we had perfect weather all week for working outside.
First step was to empty the POD and dissemble it from the site exposing the floor and tripod. Next two stakes were pegged into the ground, 20' to the North and South from the tripod, to create a true north reference line. Two other stakes were set to the East and West to locate the pier position on the N-S line. The stakes I mention are not the 6' high wooden poles seen in the picture, one is to keep wires away from the lawn mower, the other is for a badminton net!
Moved the tripod out of the way and rolled the floor away in one piece. Note the three stones still set in the ground where the tripod feet were resting.
Them some big rocks in there...The frost line in my area is four feet down, fortunately I hit bedrock (or a really big boulder!) a bit below 2' feet down.
The footer is contained in a 23"x19" box of 1"x6" deck boards, the box was leveled by three 2"x2" screwed into the side of the box. A bunch of fist sized rocks were used to block off the gaps between the box and the irregular shaped bedrock floor. Then pouring began right onto the scrubbed clean bedrock. Once the box was full, three 3' long rebars were sunk into the sticky concrete and the 10" Sonotube dropped over the center of the footer. Pouring continued as we kept a close eye on the tube leveling, this made the critical leveling later on a piece of cake.
Choosing the pier diameter is always a compromise of strength versus the pier getting
in the way of the telescope, and to a lesser extent how many bags of concrete you
need to haul! I had made some calculations that showed a 6" pier was a bit too flimsy
for my liking, but was well matched to the diameter of the G11 equatorial head so
that my telescope could clear the pier tracking 3 hrs past the meridian. So what
to do? The answer I came up with was a tapered pier! I bought a 10" ID and a 6.5"
ID piece of "Builder's Tube" aka Sonotube, then with a router cut two 3/4" plywood
rings that would fix (via screws) the smaller tube in the larger one. The bottom
ring would stop the concrete from creeping up between the tubes and would end up
stuck to the pier creating a nice smooth shelf. The top ring would simply come off
when the cardboard tube was stripped. As it turned out the rings served another
purpose that I wish I would have known about before starting. The tubes came from
the store somewhat out of round (from being stacked on shelves?), I thought the
weight of the concrete would force them back into round, not in this case. The bottom
of the pier is slightly elliptical and the top, thanks to the rings, is perfectly
round. I found this out when dropping the floor back into place, with a new 10.5"
hole in the middle, and it got stuck! Jigsaw to the rescue.
The 10" OD concrete pier would extend 2' above grade, and the 6" OD section
would add another 11" of height. The black cylinder on top is a Losmandy MA tripod
adapter, it is 4.25" high with a 1.25" spacing from the 6" OD pier. (NB the adapter
actually cost more than all the construction materials put together!) The G11 equatorial
head is 9.5" from the declination axis center to the bottom mounting plate making
the declination axis height above grade 50", the walls of the observatory are nominally
4' high. The top tube section with rings attached was inserted once the 10" tube
was nearly full and the footing had a chance to cure a bit. The rings were fixed
in place with short screws.
The above worksheet predicts the largest deflection will be from the concrete pier top
section, less than 0.15 arcsec deflection for a 20 N force (~5 lbs) applied at the
top. The equation used are:
Moment of inertia of a cylinder: Icyl [m4] = 3.14152926 / 64 * (OD4 - ID4)
Deflection of a beam fixed at one end [mm] = W * L3 / (3 * E * 1e9 * Icyl) * 1000
Deflection Angle of a beam fixed at one end [arcsec] = W * L2 / (2 * E * 1e9 * Icyl) * 1000 * 206.265
where W is the force in [N], OD/ID are the cylinder outside/inside diameters in [m], L is the length of the cylinder in [m] and E is Young's modulus in [GPa]. The factors 1e9, 1000 and 206.265 are to convert from [GPa] to [Pa], [m] to [mm] and [mrad] to [arcsec] respectively.
The Losmandy tripod adapter was modified by drilling three 1/2" holes, 120 degrees apart, 1.75" from center. The 1/2" diameter, 8" long concrete anchor bolts (aka L or J-bolts) are directly fixed to the adapter bottom plate by washers and nuts outside and in. The entire unit was plunged into the concrete then simultaneously leveled and aligned to true north by aligning the edge of the green tape with the black vertical line (same occurs on the opposite side blocked from view in the picture). Leveling was much easier to do by inserting some wood blocks, which also set the gap properly, and shimming with pieces of paper! The two pieces of green tape needed to accurately mark a chord passing thru the center of the cylinder and aligned on the North bolt hole. I used a trick to do this: I measured the cylinder circumference (with another piece of green tape), cut it off and folded it in two. The fold mark and the ends are now indicate a chord thru the center of the cylinder. The black vertical lines on the 6.5" tube were scribed by sighting a 4' long level, placed on the 6" yellow tube, from behind the South stake 20' away. Aligning the 4' straight edge to a 40' baseline proved very accurate, I had to adjust the mount altitude and azimuth controls less than 1 deg from the previous polar alignment setting done on the tripod.