The pier is done being poured.

The pier form has been filled with about 7 tons of cement. "Bones" is leveling off the top of the pier prior to placing the Pier Extension Plate anchor bolts. Only a small amount of water drained from the fresh cement out from around the pier base. The cardboard form soaked up a some water as well but everything appears to have set up well and the cardboard form dried quickly under the influence of the heat generated by the curing cement. Move to the "Pier Extension Base Plate" section to see placement of the anchor bolts.

We initially encountered a potential problem with the pier after several days of letting the cement cure when the bracing was first removed. The cement column was so long that it rocked a bit at the top when applying lateral forces. This should not be a problem for imaging under still conditions as long as the mount is not bumped but could be a stability or oscillation problem under wind loading of the telescopes and mount. Given the relatively strong prevailing winds in the area this could potentially limit imaging. Various ideas were  considered as to what might might be done to stabilize the pier. The pier currently is isolated from the structure but might require stabilizing against the structure to solve the problem. Other possibilities include putting various shock or vibration absorbing/dampening materials between the top of the pier and the attic floor that encircles the top of the pier or using some type of shock absorber between the pier top and attic floor. There is room for additional bracing or dampening material around the full length of the pier as well but it is doubtful that given the length required that this would provide a great deal of additional stability in and of itself. The main concern in placing stabilizing materials between the structure and the pier is that vibrations from the structure will then be transmitted to the pier, trading one problem for another.

After several weeks of additional curing of the cement, the pier became considerably more stable and less prone to rock than it originally was when a lateral force is applied to the top of it.  Most probably the residual flexure of the pier is not going to be a problem as the only forces that would be applied to it would be the wind  against the scopes.  The pier is very heavy and it takes more force after curing to set it to rocking than it originally did and the amount of motion that can be generated is not great.   After much thought, we have settled on a plan to provide additional stabilization of the cement pier.  First of all, four pre-stressed steel cables will be bolted to the pier every 90 degrees and extend at an angle  from about halfway up the pier to the pier base several feet from the pier's edge.  These cables will be tightened with turnbuckles and the pier position at the top will be monitored with linear dial indicators to be sure the pier is not pulled off its static position.  The indicators will also be used to assess the degree of pre and post cabling pier motion possible.  In addition, a  steel ring will be mounted around the top of the pier over an inch thick three inch wide band of Sorbothane vibration dampening material.  Four air springs between the steel ring and the observatory floor will grip the pier with equal force and resist any motion or tendency to rock or oscillate.  The air springs can be inflated up to 100 psi to achieve any level of firmness.  This will be tested at various pressures to assess how much stability can be gained at the possible expense of transmitting house vibrations to the pier.