SUMMARY
Inclinometry, strain gauge, and central bearing measurements have been done immediately prior to the central bearing removal and replacement starting 03 May 1999. These characteristics of the telescope are well determined and should provide the requisite baselines against which data taken after the c.b. work will be compared.

Inclinometry

   Datasets     HST start      mean leg temperatures   Humidity
                               start   middle   end       %
   inc990411      10:10        -1.6    -0.6     0.3      90
   inc990501      12:12         2.3     3.2     3.8      40
   inc990502      10:04         1.7     2.7     3.8      40

The difference between consecutive models are shown below :

• The agreements are good, but better in the second case due undoubtedly to the shorter interval and more similar conditions.
• The rms scatters in the (F1,F2,F3) differences are fairly small, at (0.61", 0.23", 0.59") and (0.25", 0.34", 0.27"), respectively.
• The symmetry remains good (see the plot ).
• The new track models were installed at 17:30 HST on 19990501 and at 16:08 HST on 19990502.

Strain Gauge Data
See the report of 990416 for a brief description of the strain gauge notation.

Strain gauge data are available only from the data of the 01st of May. In the new data G1 peaks at azimuths 104.54, 187.74, 284.54 and 367.74 ( wheels 4,3,2,1 resp) - exactly as before. The (-ve) G2 peaks occur earlier, by 1-2 degrees, and it is still a little unclear whether the G2 readings are of the same sign as G1 or not. For the moment they are treated as being of -ve sign. The results below are baseline corrected, and are given in raw units :

       wheel      4           3           2          1    |   Total
               -------     -------     -------    ------- |  -------
   dataset     G1   G2     G1   G2     G1   G2    G1   G2 |  G1   G2
               -------     -------     -------    ------- |  -------
  inc990403   114  -17    299  -48    215  -45   132  -10 | 760 -120
  inc990404   104  -16    297  -48    204  -45   129  -11 | 734 -120
  inc990309   105   -4    249  -29    167  -37    94   -5 | 615  -75
  inc990311    66   -6    253  -34    187  -39   103   -4 | 609  -83
  inc990501    84  -11    260  -23    229  -21   123  -15 | 696  -70

The new data seem to 'halt' the decline postulated last time , but there is seemingly much variation from run to run.

Central bearing defect
Mars was tracked on 02 May 1999 with

• RxA3i/CBE,
• the fives routine of repeated fivepoints,
• a resolution of about 98 seconds,
• the empirical corrections disabled,
• from (az/el) = (119/34) at HST 20:27 to (132/46) at HST 21:30 :

The elevation residuals show a slight baseline trend, and have a scatter of about 0.5" rms, which is the adopted accuracy of the azimuth residuals also. Allowing for a slight baseline trend, and for the lag between the centre of the fivepoint and the data logging action, the azimuth residuals show the defect to have a peak to peak amplitude of 6.4" and to occur at (az,el) = (125.7, 41.4). TEL_EMPIRICAL currently expects the event at azimuth 125.85 degrees. The peak-to-peak error at the horizon is then 6.4"/cos(41.4) = 8.5", up again a little from the low measured on 990427 , but comparable with the current version of TEL_EMPIRICAL which has an amplitude at the horizon of 10.0". The data are added to the collection .