Summary
Regularly scheduled maintenance includes the adjustment of the load taken by the central bearing and the wheels. Following such an adjustment on 06 Feb, inclinometry and all-sky pointing were performed and the canonical pointing performance restored.

Pointing rms's of approximately (1.5",1.5") are expected of the new models despite an observed deterioration in the symmetry behaviour of the antenna.

Inclinometry data were obtained 3 days prior to and immediately following the central bearing load adjustment (CBLA) :

    Datasets        HST     mean leg temperatures   Humidity
                   start      start   middle   end       %
 ( 20010107   cw   10:55       4.0     5.0     6.0      35 )
   20010203   cw   11:15       0.2     1.3     2.4      40
   20010206   cw   15:26      -0.5     0.2     0.3      45

The differences between the recent models and the previous ones are shown below :

20010203-20010107	20010206-20010203

The following observations regarding the inclinometry are made :

• The data prior to the CBLA differ essentially only at the noise level, while there are clear differences in the orientation of the track and/or the antenna after the CBLA.

• The formal differences in (F1,F2,F3) are described by the rms scatters of (0.26", 0.42", 0.62") and (2.07", 0.42", 0.75").

• One spike in the new data at azimuth 20 degrees has been suppressed.

• Installation of the new track model occurred at 19:30 HST 06 Feb 2001.

• The symmetry measures show an apparent deterioration - see the plot - although the increased scatter in the LY-flipped-vs-RY values would have been of more concern had the (baseline) TY -vs - RY scatter remained unchanged. The variation with azimuth of TY and RY were considered previously to be held similar by the action of the elevation servo, so a sudden change in this characteristic is itself interesting.

• The amplitude of the ripple from the right A-frame at a frequency of 8.8 degrees was added to the plot.

• The strain gauge data show little change.

• Engineer Tomas Chylek reports the following load measurements :

            Initial LVDT (average): 191
            Target LVDT (average): 182
            Achieved LVDT (average): 181.67
            Error (average): .67 LVDT = 3.7um = .00014"
  
            This puts yesterday's bearing adjustment among 
            the most precise on record.
  

  Detailed engineering results may be seen here

Despite the worrisome symmetry results, which were not in any case discovered until the following day, allsky pointing data were then obtained as follows :

      Dataset      N        mean uaz +-     mean uel +-
 
     pt010109     41          -8.0  3.3      0.1  1.7

The rate of data acquisition was slower than usual while I waited for the offline, ORACDR, data reduction to analyse each POINTING observation; ORACDR uses sensible remsky techniques that aren't done online. The ORACDR pointing offsets were those adopted and logged. The data are plotted below.

Seeing was about 0.7" throughout, varying between 0.5" and 1.0". The data show a dramatic trend of daz -vs- elevation (top row, centre), indicative of an error in the azimuth encoder zero-point of order 10 arcseconds.

After allowing all 7 of the TPOINT pointing model parameters to float to new, optimal values the performance of the antenna should appear as shown below :

RMS residuals are expected to be (1.6", 1.7") with this new model : if the seeing (0.7") is deducted in quadrature the performance is essentially expected to be our canonical (1.5",1.5").

The new model was installed at 00:40 HST on Wednesday 07 February, 2001.

Conclusion
The day's work went well - the only wrinkle being the change in the symmetry. Poor symmetry in the past was associated with underloading of the front wheels, but also with poor pointing, for which there is no evidence currently. Followup inclinometry will be requested to monitor this situation.

2001 02 09
New inclinometry were obtained this morning; the difference between the resulting track model and that of the 06^th are shown below :

• The spike from the previous data are again seen in the difference plot - they do not affect the new data. Otherwise the new data differ from the old essentially only at the level of the noise :

• The differences in (F1,F2,F3) are described by the rms scatters of (0.63", 0.23", 0.35").

• The new symmetry measures show an improvement on those taken immediately following the CBLA on the 6^th.

• The strain gauge plots have been updated.

• The new track model was installed at 17:45 HST 09 Feb 2001.