pointing error when tiedowns lose tension



      The platform is run in an unbalanced configuration. As the dome moves up and down in za, the entire platform tilts. The elevation rails have been shimmed so that this weight imbalance causes the horns to tilt onto the paraxial surface. This should be repeatable as long as the weight in the dome and the temperature remains constant.
    If the main cables are the same length, then temperature changes should make the entire platform move up and down without any extra tilting.  We use the tiedown cables to compensate for the motion of the platform due to temperature. The distomats measure the average height of the platform every 2 minutes. If the average platform height is more than .04 inches from the nominal position, the tiedowns will pull (or release) equally on all 3 corners to bring the average height back to the nominal position.

Losing tension in a tiedown cable.

    The tiedown cables can lose tension as the dome passes in front of them. This occurs more often at high zas and high temperatures. The program controlling the tiedowns does not use the td tensions to monitor this condition. It  continues  to control the average height of  the platform by moving all 3 tiedowns the same amount. With tension lost in one tiedown this operation will tilt the platform and cause a pointing error.
    On 27may04 the source 3C48 (J0137+331) was used during a calibration run with sbw. The source was tracked from 9:00 AST to 10:30 AST. The temperature was about 84 F. The plots show the pointing error caused by the tension loss in tiedown 4 (.ps)   (.pdf) (az=122.87 degrees).     Losing tension in a tiedown causes the average height of the platform to be too low. A more serious problem is that the tiedown control algorithm will cause the platform to become tilted. The tilt is a pointing error that can be much more serious than the gain loss from being out of focus. For 3C48 the pointing error was 90 asecs (one beam at 3500 Mhz).
    This problem is occurring more frequently as we continue to add weight to the dome. Adding 4000 lbs to the dome should cause the platform to tip by .4 inches at za=20 degrees. This will move the tiedowns by .7 inches.
    We need to find a reliable way to monitor the loss of tension in a cable so that the control algorithm does not make things worse.

Some other pages:

       processing: x101/model/misc/tdLoseTension_may04.pro