2" shaft mounting Pt.43: A different kind of pier.

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In an attempt to avoid a massive, static pier, or even to have something in which to cast concrete, I have purchased some more used ventilation tubing. In 12" and 14" diameters, as shown, I also have a long tapered adapter to join the two sizes. This would allow me to have a tapered section on top of the large tube. Or, to join the two at any chosen height with a smooth taper between them. They only cost me a modest contribution to the firm's cake fund to allow me to freely select my materials.

Note how the 12" tube would suit my 10" f/8 mirror to perfection. A lining of cork or foam might be useful to reduce thermal issues.] It will need my serious mount to cope with the weight and moment. I am very tempted to put a steel OTA together for my mirror.

There were no longer 14" examples amongst the collection to allow a full height pier in that size. Hence the adapter to give me some flexibility over height. I can join the larger tube and adapter and then cut off the larger tube at the ground. Or bury it and fill it with concrete! Both tubes weigh about 23lbs or 10.5kg each.

Before you jump to conclusions and assume this [galvanized] steel tubing is far too thin for a pier I strongly beg to differ. When I was younger I built a downhill skate car. This was built from the flimsiest aluminium printing sheets. The secret to its incredible stiffness was the use of twin walls with thin, softwood laths sandwiched and glued between them. The lower hull of the car was a simple half circle [roof gutter] form built in this way. It was impossible to feel any flexure [at all] when I climbed in.

Aerospace materials are built on similar principles. Often of such thin materials that the honeycomb core can be seen imprinted through the twin skins. Flush doors are also constructed this way using flimsy paper/card honeycombs glued on edge between two plywood skins. A wooden frame provides the firm base for the hinge and lock.

I could use a single tube reinforced by simple, plywood ring formers just like an aircraft fuselage. Or I could add tensioned, threaded rods between a top and bottom plate. The rods would pass though close fitting holes in the formers. The assembly would be slid inside the tube and then the nuts tightened to stiffen the whole caboodle. The rings would stop the tube from becoming oval if subjected to bending forces. The rings could easily be locally reinforced to provide solid anchors for triangulated legs. Just like the very expensive piers found under many Astro-Physics mounts. The secret here is triangulation and a complete lack of flexibility while retaining lightness for mobility.

http://www.astro-physics.com/products/accessories/mounting_acc/eagle6.htm#pier

I have no need of a short, floating pier tube cut off at midriff nor its very light weight. I also much prefer the "anchored down to the ground" look and potential stability to be had from larger, triangulated legs. I shall use four legs rather than three and have previously illustrated the tiny tipping radius of tripods in comparison with identically spread quadruple legs before. [See image above for the glaring difference in tipping radius.] I observe mostly on lawn which gets quite soft at times. So I just need massive stability with only rather limited mobility.

I have outlined various options for my pier. I could use both tubes placed concentrically and have vertical laths between the two. Or reinforcing rings between the two for an incredibly stiff but rather heavy column. Or use one tube with the construction outlined above. It would be simplicity itself to provide a ballast water tank at the bottom of the pier for extra stability. Preferably with a drain tap to lighten the load if needed. Or, perhaps better, in a cold climate, a door provided to allow sandbags to be placed inside or removed at will. Sand is also heavier than water so will provide a lower center of gravity.

It is not a minor thing to place a 200lb mounting on top of a nearly 7' tall column without serious consideration given to stability. Another 100lbs will be added with the OTA plus its balancing counterweights.  It can be thought of as similar to placing a real human being up there on top. Is their safety guaranteed by unconditional stability in all directions? If not, then I should not risk my life [or others more innocent] by standing under that considerable load. Nor is there any point in building a massive mount if flexure is introduced by its own stand or pier.

My present pier is truly massive in terms of the weight of steel I welded together. It appears and feels rock stable in use. However, when I try to move it around on its wheels it readily wants to tip over. I spend a lot of time adjusting the screw jacks on the wheels just to keep the pier upright.

The new mounting will be at about the same height but weighs at least twice as much. It will require more stability than all that steel only pretends to provide. I will have to keep that firmly in mind. A circle of the same radius as the tripod or quadpod offers even better stability. As would bolting a quadpod down to ground anchors. The danger then is when it is released to be moved across the sloping garden. Life would be so much easier if I had a clear southern view. During the recent Mercury transit I had to drag the present mount and pier along the gravel drive just to be able to catch the afternoon sun. Never again! I badly need a permanent site for the new mounting which has reasonably clear views of the sky.

Click on any image for an enlargement. 

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