Simulate tiedown motion after shimming the rails.


     The tiedown positions needed to correct the current pitch, roll , focus errors are shown in:  tiedown positions to correct for pitch, roll, and focus. These plots show that the  tiedowns do not have enough range to  do the correction.  We will have to remove some of the pitch and roll error by shimming and possibly rotating the dome. Rotation of the dome is done by redrilling the mount points for the dome. Shimming can be done on the elevation rails and the azimuth rails. A simulation was done using the pitch, roll, focus model computed from the aug02 surveys. It looked at two things:
  1. Correct the pitch and roll by just rotating the dome (redrilling the mount points). Assume there are no errors in the redrilling.
  2. Redrill the mount points to just correct the average roll error, and then reshim the  elevation rails from 10 to 20 degrees za. Assume a residual error on redrilling  of .02 degrees and a shimming error of .04 degrees. Do not include the focus change caused by the shimming (note.. after the fact, jose maldonado said that they could probably redrill the dome holes to 1/16 of an inch. Over the 12 foot span this is about .025 deg).
  3. Redrill the mount points to correct the roll by .12 degrees. Shim the elevation rail to remove a linear pitch from za=10 to za=20 of .14 degrees. Assume it is done exactly. Include the focus change caused by shimming the rails.

1. The tiedown corrections after redrilling the mount points to correct pitch and roll (no shimming):

  • Figure 1 Top: This is the pitch and roll before (solid) and after (dashed) the redrilling of the mount points. The pitch correction of .04 degrees and roll correction of .12 was chosen after trying various combinations.
  • This method does not modify the focus. We can probably run between 70F and 85F. The exact range will depend on the errors in drilling (and the accuracy of the model).

    2. The  corrections after redrilling for roll and shimming for pitch above 10deg za (with residual positioning errors):

        These plots set the average roll error to +/- .02 degrees and set the average pitch error (above 10 degrees za) to +/-.04 degrees. The  azimuth dependence of the error is unchanged. All 4 permutations of the errors are plotted to show how the td range changes with the shimming/positioning error.     The model shows that adjusting the pitch is not enough, we must also remove the average roll. This is probably done the easiest by redrilling the holes (if not , you'll probably have to reshim all the way down to low za).  We can emphasize the low temperature or high temperature by leaving a residual pitch at high za. This needs to be decided.
    Using the radius of 869.781 feet the platform is too high. Adding weight to the platform could give us more range at lower temperatures. Before doing this we should verify that the shorter radius of 869.781 is correct (the focus curves we've done do not support this).
    The shimming will modify the focus by up to 1 inch at za=19.5 degrees (this has not been included here).

    3. The corrections for rotating to correct roll and then shimming a linear ramp in pitch (with no positioning errors).

        These plots show the results from rotating the dome by .12 degrees  in roll and then doing a linear ramp in pitch of 0 to .14 degrees from za=10 to 20 degrees. This is probably the corrections that we will do. No errors are included in the positioning.      These  computations  include the offset of the reflector relative to AO9. It also assumed that the focus curve measured aug01 using the theodolite was the correct focus. This would apply the measured focus errors at 72deg and the platform at 1256.35 feet. The radius of curvature used for the reflector was 869.781 feet. The theoretical focus correction using this radius does not agree with the focus curves done by moving the platform.
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