Not According to Plan
This is where I first went wrong. The widest tube I could find was 90mm aluminium piping. I sank this into concrete that was 60cm deep (I couldn't dig any deeper - the soil was thick compressed clay). The hole I dug was 60cm square. The tube was then filled with concrete. I custom built a mount head out of hardwood into which the telescope drive would fit. The tripod legs were removed from the field tripod assembly and the remaining section and drive was seated into the hardwood mount. Bolts secured the drive to the mount via the original fixing holes for the aluminium tripod legs.
I have to say I was very pleased with the wooden mount. Tolerances were very tight and there was no play between the drive and mount head once the bolts were tightened. The whole assembly was solid. That night was clear so I got to try out my handiwork. Well, what can I say ... what a comedown. Stars were bobbing about all over the place. I checked all the bolts were tight and still stars danced in the eyepiece. The damping down time was long too - about 7 or 8 seconds. A light tap on the aluminium tube or wooden mount head confirmed my worst fears - the mount I'd built was the source of the problem. Instead of quickly damping out vibrations it was exacerbating them. It was as if the tube was 'ringing' after being tapped.
Bringing in the Professionals
It was time to go back to the drawing board. In my case, the DIY approach hadn't worked. I began looking through the magazines for suppliers of telescope piers. I own a Vixen telescope and I found that Orion Optics; in the UK sold a pier for my scope. Unfortunately, however, they don't export Vixen equipment outside the UK and the vendor I originally bought the scope from didn't sell the pier as a separate item! I scouted about for independent pier manufacturers but, surprisingly, couldn't find one in the UK. AstroPier in the USA could have supplied a pier for a few hundred dollars but they would have had to build it to order as Vixen is not one of the manufacturers they support off-the-shelf. In any case, the $1500 shipping charge seemed a tad too expensive.
The professionally manufactured pier mounted atop the plate thatâ€™s been buried in the relaid concrete. The pier is attached to the plate by four bolts.
I'd have to look closer to home for a solution. I remembered that a friend of a friend owned a metal tooling shop. If this guy could make Klingon B'athleth's from solid titanium I figured he could manage a telescope pier. I looked at lots of pictures of custom built telescope piers before submitting a design to him. There was some to-ing and fro-ing about fine details and design clarifications. In the end I made some concessions to my design on the basis of cost. He built the pier out of scraps of tubing, steel plate and bits and bobs left over from other projects. A docking ring (to mate the telescope drive with the mount) was custom tooled. A week after submitting the final design, the finished pier was delivered to my door. The total cost was Â£250 (about $350), amazingly cheap given the materials and work involved but the price was kept low due to the use of 'offcuts' for want of a better word.
The Pier Arrives
The pier came in three pieces - a bottom plate which would be buried in concrete, the pier tubing which was connected to the base plate by four sturdy bolts (allowing adjustments to be made to the tubing so it could be set vertically) and a top plate onto which the telescope drive would be placed. The top plate was also connected to the tubing by four bolts, allowing the plate to be levelled prior to the telescope drive being attached.
There was a problem I had to contend with before installing the new pier - removing the old one. Let me tell you, hacking out 0.216 cubic metres (8 cubic feet) of concrete with nothing but a sledgehammer and stone chisel is not for the fainthearted. Several hours (and mashed fingers) later, the original hole was re-excavated and enlarged to be 1-metre in diameter.
The hole was then refilled with fresh concrete and the pier baseplate submerged just below the surface. The four attached bolts protruded from the concrete in readiness for the pier tube once everything had set solid.
Testing the Result
A couple of days later, enough time for the concrete to have hardened, the pier was mated to the baseplate and vertically aligned. The top plate was then added and levelled using a spirit level. That night the telescope drive and OTA were mounted onto the completed pier. This time, the construction proved to be rock solid. Tapping on the pier didn't result in dancing stars. A sharp rap to the pier did cause vibrations, as expected, but they died out in about a second.
With the pier construction completed, all that remained was to build the observatory around it. It was time to put my meagre skills to the test....
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