Pier & Mounting Plates

Pier and mounting plates

I started my pier with a 3x3 foot by 1 foot deep hole in the ground, with a center hole 4 feet deep and about 12 inches in diameter. I placed several pieces of 3/8 inch rebar in the hole and tied it together with wire.

I bought a piece of 12 inch sonotube and placed it in the center of the hole going down a few inches below ground level. I then cut two pieces of 3/8 plywood to fit around the sonotube at ground level. I staked three 2x4s radially from the top of the sonotube to ground level to support it in a vertical position.

The pier top plates were prepared earlier from two 12 inch squares of 3/4 inch aluminum purchased as such from McMaster-Carr, the online hardware supplier (http://www.mcmaster.com/). The 12 inch square is a convenient size because it doesn't require any cutting and is large enough for a Losmandy G-11, Meade superwedge, or Astro-Physics 1200GTO plate. I drilled the bottom plate with three holes for the bolts going into the concrete at the top of the pier and with four holes for the bolts going to the upper plate. (I also put in several 1/4-20 tapped holes around the edges of the bottom plate for possible future mounting of accessories.) The three holes for the concrete connection were placed with 120 degree spacing on the perimeter of an 8 inch circle. The four holes for connecting to the upper plate were placed at the corners to provide clearance for the wedge (see the picture of the wedge on the plate). I placed the wedge on the top plate and marked the positions of the Meade wedge center 1/2 inch hole as well as the three 5/16 bolts. I then drilled and tapped each of these four holes for the appropriate hardware. I also marked and drilled a 1/4 inch hole for the pin of the azimuth adjusting pawl at the rear of the wedge. A sketch of the top pier plate is shown HERE.

I made three "J" bolts for connecting the bottom plate to the concrete pier top from 3/4 inch stainless-steel all-thread (also from McMaster-Carr). Bending the bottom section of the 3/4 inch stainless all-thread was the most difficult part of the job. I couldn't get it hot enough to soften with any of the torches I had on hand, so I put them in the barbecue and covered them with a generous pile of white-hot briquettes for about half an hour. This treatment got the last 3-4 inches of the all-thread to a red-to-yellow heat. By clamping the hot end in a vise and putting a 4-foot long pipe over the other end, I was able to get enough torque to bend the all-thread to between 45 and 90 degrees. The four bolts holding the top plate to the bottom were made from the same material but didn't require any bending, fortunately.

I prepared the bottom plate for mounting in the concrete before setting up the sonotube form. Using the bottom plate as the template, I drilled a scrap piece of 3/8 inch plywood to accept the three J-bolts vertically below the bottom plate. With thin nuts ("jam-nuts," also from McMaster-Carr) on the J-bolts I spaced the plywood 2 inches below the bottom plate so that the "J" ends of the three bolts stood 6, 8, and 10 inches below the plywood. After pouring the concrete into the main hole and filling up the sonotube, I worked the J-bolt assembly down into the wet concrete and adjusted it for an approximate level. I made sure there were no voids in the concrete and smoothed off the top before pushing the bolts down so that the plywood rested on the top of the sonotube. I then allowed the concrete to cure for 5 days.

After the concrete had cured, I removed the bottom plate and the jam-nuts from the J-bolts and discarded the scrap plywood. I then replaced the bottom plate on the J-bolts with washers and jam-nuts and leveled it very carefully. I then added the four corner bolts with washers and jam-nuts and topped it off with the top plate, also carefully leveled.

The final assembly is shown in Fig 2 and is very solid. I suspect it will take a jackhammer to loosen the connection into the concrete. I haven't needed to alter the angle of the plates yet (about 2 years of experience with it so far), but I believe it will be fairly easy to adjust by simply moving the jam-nuts up or down the three J-bolts. For mounting the Meade superwedge, I put a piece of 1/2-13 all-thread that is rounded-off at the top in the center hole. This pin is just for centering the wedge on the plate and providing an axis for adjusting the azimuth. I then fasten it down with the perimeter bolts, just as on the Meade tripod. After several nights of fairly good polar alignments, I drilled through the bottom of the wedge into the top plate and reamed it for a taper pin (thanks to Ric Ecker for this suggestion!). This permits me to get almost perfect realignment in azimuth even after removing the wedge and using it somewhere else (Fig 3). Since the top of the pier resembles the top of a Meade tripod, I can also mount my G-11 on this pier, using the Meade tripod adapter that Losmandy provides. The base plate for an A-P 1200GTO GEM also fits neatly on the top plate.

The finishing touch for the pier was an eyepiece holder (not shown in the pictures above or at right).

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